Golden eagle

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Golden eagle
GoldenEagle-Nova.jpg
Adult of the North American subspecies Aquila chrysaetos canadensis
Conservation status
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Accipitriformes
Family: Accipitridae
Genus: Aquila
Species: A. chrysaetos
Binomial name
Aquila chrysaetos
(Linnaeus, 1758)
Cypron-Range Aquila chrysaetos.svg
Light brown : Wintering only

Brown : Breeding only
Dark brown : All-year

Synonyms

Falco chrysaëtos Linnaeus, 1758

The golden eagle (Aquila chrysaetos) is one of the best-known birds of prey in the Northern Hemisphere. It is the most widely distributed species of eagle. Like all eagles, it belongs to the family Accipitridae. These birds are dark brown, with lighter golden-brown plumage on their napes. Immature eagles of this species typically have white on the tail and often have white markings on the wings. Golden eagles use their agility and speed combined with powerful feet and massive, sharp talons to snatch up a variety of prey (mainly hares, rabbits, marmots and other ground squirrels).[2]

Golden eagles maintain home ranges or territories that may be as large as 200 km2 (77 sq mi). They build large nests in high places (mainly cliffs) to which they may return for several breeding years. Most breeding activities take place in the spring; they are monogamous and may remain together for several years or possibly for life. Females lay up to four eggs, and then incubate them for six weeks. Typically, one or two young survive to fledge in about three months. These juvenile golden eagles usually attain full independence in the fall, after which they wander widely until establishing a territory for themselves in four to five years.

Once widespread across the Holarctic, it has disappeared from many areas which are now more heavily populated by humans. Despite being extirpated from or uncommon in some its former range, the species is still fairly ubiquitous, being present in sizeable stretches of Eurasia, North America, and parts of North Africa. It is the largest and least populous of the five species of true accipitrid to occur as a breeding species in both the Palearctic and the Nearctic.[3]

For centuries, this species has been one of the most highly regarded birds used in falconry, with the Eurasian subspecies having been used to hunt and kill prey such as gray wolves (Canis lupus) in some native communities. Due to its hunting prowess, the golden eagle is regarded with great mystic reverence in some ancient, tribal cultures. The golden eagle is one of the most extensively studied species of raptor in the world in some parts of its range, such as the Western United States and the Western Palearctic.

Description

In flight
Subadult, note white in tail and dark neck

The golden eagle is a very large, dark brown raptor with broad wings, ranging from 66 to 102 cm (26 to 40 in) in length and from 1.8 to 2.34 m (5 ft 11 in to 7 ft 8 in) in wingspan.[4][5][6][7] This species' wingspan is the fifth largest amongst extant eagle species.[5] In the largest race (A. c. daphanea) males and females weigh typically 4.05 kg (8.9 lb) and 6.35 kg (14.0 lb). In the smallest subspecies, A. c. japonica, males weigh 2.5 kg (5.5 lb) and females 3.25 kg (7.2 lb).[2] In the species overall, males may average around 3.6 kg (7.9 lb) and females around 5.1 kg (11 lb).[8] The maximum size of this species is a matter of some debate. Large races are the heaviest representatives of the Aquila genus and this species is on average the seventh-heaviest living eagle species. The golden eagle ranks as the second heaviest breeding eagle in North America, Europe and Africa but the fourth heaviest in Asia.[3][5] For some time, the largest known mass authenticated for a wild female was the specimen from the nominate race which weighed around 6.7 kg (15 lb) and spanned 2.55 m (8 ft 4 in) across the wings.[9] American golden eagles are typically somewhat smaller than the large Eurasian races, but a massive female that was banded and released in 2006 around Wyoming’s Bridger-Teton National Forest became the heaviest wild golden eagle on record, at 7.2 kg (16 lb).[10] No comprehensive range of weights are known for the largest subspecies (A. c. daphanea).[9] Captive birds have been measured up to a wingspan of 2.81 m (9 ft 3 in) and a mass of 12.1 kg (27 lb) (the latter figure was for an eagle bred for the purposes of falconry which tend to be unnaturally heavy), respectively.[9] The standard measurements of the species include a wing chord length of 52–72 cm (20–28 in), a tail length of 26.5–38 cm (10.4–15.0 in) and a tarsus length of 9.4–12.2 cm (3.7–4.8 in).[5] The culmen reportedly averages around 4.5 cm (1.8 in), with a range of 3.6 to 5 cm (1.4 to 2.0 in) and the bill length from the gape measures around 6 cm (2.4 in).[11][12] The long, straight and powerful hallux-claw (or hind claw, the equivalent to the big toe) can range from 4.5 to 6.34 cm (1.77 to 2.50 in), being about one centimeter more than the hallux-claw of a bald eagle (Haliaeetus leucocephalus) and a little more than one cm less than a harpy eagle (Harpia harpyja).[13][14][15] The sexes are similar in plumage but are considerably dimorphic in size. Females are rather larger than males with the differences increasing as the body size increases across the races. The large Himalayan golden eagles females are about 37% heavier and nearly 9% longer in wing length than the males of the race compared with the small Japanese golden eagles where females are a relatively modest 26% heavier and around 6% longer in wing length than males.[2][16]

The formidable foot and talons of a golden eagle

Adults are primarily dark brown in color, with a paler, typically golden color (the source of the species’ common name) on the back of the crown and nape, and some grey on the inner-wing and tail.[17] There are subtle differences in coloration among the races, described below. Unlike in other Aquila species, where the tarsal feathers are typically of a similar color to the rest of the plumage, the tarsal feathers of golden eagles tend to be paler, ranging from light golden to white.[2] In addition, some full-grown birds (especially in North America) have white "epaulettes" on the upper part of each scapular feather tract.[3][18] The bill is dark at the tip, fading to a lighter horn color, with a yellow cere.[19] As in many acciptrids, the bare portion of the feet are yellow.[5] This species moults gradually beginning in March or April until September or October each year. Moulting usually decreases in winter. Moult of the contour feathers begins on the head and neck region and process along the feather tracts in a general anterior-posterior direction. Feathers on head, neck, back and scapulars may be replaced annually. With large feathers of the wing and tail, moult beginning with innermost feather and proceeds outwards in a straightforward manner known as "descendant" moult.[18]

The juvenile golden eagle is similar to the adult but tends to be darker, appearing black on the back especially in East Asia. Compared to adults, juveniles have a more unfaded color. Young birds are white for about two-thirds of their tail length ending with a broad, black terminal band.[16] Occasionally, juvenile eagles have white patches on the remiges at the bases of the inner primaries and the outer secondaries, forming a crescent marking on the wings which tend to be divided by darker feathers.[2][20] Rarely, juvenile birds may have only trace amounts of white on the tail. Compared to the relatively consistently white tail, the white patches on the wing are extremely variable and some juveniles have almost no white visible. Juveniles of less than 12 months of age tend to have the most extensive amount of white to the plumage.[18] By their second summer, the white underwing coverts are usually replaced by a characteristic rusty-brown color. By the third summer, the upper-wing coverts are largely replaced by dark brown feathers, however not all feathers moult at once giving many juvenile birds a grizzled pattern. The tail also follows a similar pattern of maturation.[18][20] Due to the amount of variability in different individuals, juvenile eagles cannot be reliably aged on sight alone.[21] Many golden eagles still have white on the tail during their first attempt at nesting.[22] The final adult plumage is not fully attained until the birds are between 5 and a half and 6 and a half years old.[21]

Voice

While many accipitrids are not known for their strong voices, golden eagles have a particular tendency for silence, even while breeding.[23] Some vocalization, however, has been recorded, and these normally are centering around the nesting period. The voice of the golden eagle is considered weak, high and shrill, even being emphatically described as “quite pathetic” and “puppy-like”, and as somewhat incongruous considering the formidable size and nature of the species.[5] Most known vocalization seem to function as contact calls between eagles, sometimes adults to their offspring, occasionally territorial birds to intruders and rarely between a breeding pair. In Western Montana, nine distinct calls were noted: a chirp, a seeir, a pssa, a skonk, a cluck, a wonk, a honk and a hiss.[24]

Flight physiology

Golden eagle flying in dihedral with food

Golden eagles are sometimes considered the most superlative fliers among eagles and perhaps among all raptorial birds.[23] They are equipped with broad, long wings with somewhat finger-like indentations on the tips of the wing.[5] Golden eagles are unique among their genus in that they often fly in a slight dihedral, which means the wings are often held in a slight, upturned V.[5] When they must engage in flapping flight, golden eagles appear at their most labored but this flight method is generally less common than soaring or gliding flights.[2] Flapping flight usually consists of 6–8 deep wing-beats, interspersed with 2 to 3 second glides.[2] While soaring the wings and tail are held in one plane with the primary tips often spread.[25] A typical, unhurried soaring speed in golden eagles is around 45–52 kilometers per hour (28–32 mph).[26] When hunting or displaying, the golden eagle is capable of very fast gliding, attaining speeds of up to 190 km/h (120 mph).[2][25] When diving (or stooping) in the direction of prey or during territorial displays, the eagle holds its wings tight and partially closed against its body and the legs up against its tail. In a full stoop, a golden eagle can reach spectacular speeds of up to 240 to 320 kilometers per hour (150 to 200 mph) when diving after prey. Although less agile and maneuverable, the golden eagle is apparently quite the equal and possibly even the superior of the peregrine falcon’s stooping and gliding speeds.[3][27] This places the golden eagle as the one of the two fastest moving living animals on earth.[9] Although most flight in golden eagles has a purpose (i.e. territoriality, hunting, etc.), some flights (such as those by solitary birds or between well-established breeding pairs) seems to function merely as acts of playfulness.[3][23][28]

Confusion species

Golden eagles are readily distinguished by their brown plumage, paler than most other Aquila, and pale nape patch

Size readily distinguishes this species from most other raptors when it is seen well. Most other raptors are considerably smaller. Buteo hawks, which are perhaps most similar to the golden eagle in structure among the species outside of the “booted eagle” group, are often amongst the larger very common raptors. However, a mid-sized Buteo is dwarfed by a golden eagle, as an adult female eagle has a wingspan of about twice the width and weighs around five times more. Buteos are also usually distinctly paler below, although some species occur in “dark morphs” which can be even darker than a golden eagle.[5][29] Only some Old World vultures and the California condor (Gymnogyps californianus) (among the other raptorial birds this eagle co-exists with) are distinctly larger than the golden eagle, with longer, broader wings, typically held more evenly in a slower, less forceful flight and often have dramatically different color patterns. The turkey vulture (Cathartes aura) is a potential confusion species in North America from a great distance, as it is a large species that (like the golden eagle) often flies with a pronounced dihedral but is easily separated by its less controlled, forceful flying style and its smaller, thinner body, much smaller head and, at closer range, its slaty black-brown color and silvery wing secondaries.[30] Compared to Haliaeetus eagles, the golden eagle has wings that are only somewhat more slender but are more hawk-like and lack the flat, plank-like wing positioning seen in the other genus.[30] Large Northern Haliaeetus usually have a larger bill and larger head which more distinctly protrudes than that of a golden eagle’s in flight. The tail of the golden eagle is longer on average than those of Haliaeetus eagles, appearing to be two or three times the length of the head in soaring flight as opposed to the other eagles where the head is often more than twice the length of the tail.[30] Confusion is most likely between juvenile Haliaeetus and golden eagles since the adult golden has a more solidly golden-brown coloration and all Haliaeetus eagles have obvious distinctive plumages as adults. Haliaeetus eagles are often heavily streaked in their juvenile phase. Juvenile golden eagles can show large patches of white to the wings and tail that are quite different than the random, sometimes large and splotchy-looking distribution of white typical of juvenile Haliaeetus.[5][30]

Distinguishing the golden eagle from other Aquila eagles in Eurasia is potentially a greater identification problem. This identification may rely on the golden's relatively long tail and patterns of white or gray on the wings and tail. Other Aquila eagles do not generally fly in a pronounced dihedral as do golden eagles. At close range, the golden to rufous nape-shawl of the golden eagle is distinctive from other Aquila. Most other Aquila eagles are darker looking in plumage, although the smaller tawny eagle (A. rapax) is often paler than the golden eagle (overlap in range verified only in Bale Mountains, Ethiopia). Among Eurasian Aquila, the adult Eastern Imperial (A. heliaca) and Spanish imperial eagle (A. adalberti) come closest to attaining similar sizes as golden eagles but this species pair are distinguished by their relatively longer neck, flatter wings in flight, white markings on their shoulder forewing-coverts, paler cream-straw colored nape patch and generally darker coloration. Juvenile imperial eagles are much paler overall (caramel-cream in the Spanish; cream and tawny streaks in the Eastern) and are not likely to be confused.[5] Steppe eagles (A. nipalensis) can also be nearly golden eagle-sized but are more compact and smaller headed than a golden eagle with little color variation to their dark earth-brown plumage but for juvenile birds which have distinctive cream-colored bands running through their coverts and secondaries.[3] Verreaux's eagle (A. verreauxii) are most similar in size and body shape to the golden, with the Verreaux's being slightly longer overall but marginally less heavy and long-winged than the golden eagle.[5] The plumage is very distinctly different, however, as Verreaux's eagles are almost entirely jet-black but for some striking, contrasting white on the wing primaries, shoulders and upper-wing.[5] This closely related species is known to co-occur with the golden eagle only in the Bale Mountains of Ethiopia.[2] Other booted eagles in the golden eagle’s range are unlikely to be confused, due to the differences in size and form. Among the Aquila genus, only the long-winged and tailed wedge-tailed eagle (A. audax) of Australasia notably exceeds the golden eagle in average wingspan and length. However, the wedge-tailed eagle is a slightly less heavy bird.[9]

Taxonomy and systematics

This species was first described by Linnaeus in his 1758 Systema naturae as Falco chrysaetos.[31] Since birds were grouped largely on superficial characteristics at that time, many species were grouped by Linnaeus in the Falco genus. The type locality was given simply as "Europa"; it was later fixed to Sweden. It was moved to the new genus Aquila by French ornithologist Mathurin Jacques Brisson in 1760.[32]

The golden eagle is part of a broad group of raptors called “booted eagles” which are defined by the feature that all species have feathering over their tarsus, unlike many other accipitrids which have bare legs. Included in this group are all species described as “hawk eagles” including the genera Spizaetus and Nisaetus, as well as assorted monotypical genera such as Oroaetus, Lophaetus, Stephanoaetus, Polemaetus, Lophotriorchis and Ictinaetus. The genus Aquila is distributed across every continent but for South America and Antarctica. Up to 20 species have been classified in the genus but the taxonomic placement of some of the traditional species has been questioned as of late. Traditionally, the Aquila eagles have been grouped superficially as largish, mainly brownish or dark-colored booted eagles that vary little in transition from their juvenile to their adult plumages. Genetic research has recently indicated the golden eagle is included in a clade with Verreaux's eagle in Africa as well as the Gurney's eagle (A. gurneyi) and the wedge-tailed eagle (clearly part of an Australasian radiation of the lineage). This identification of this particular clade has long been suspected based on similar morphological characteristics amongst these large-bodied species.[3] More surprisingly, the smaller, much paler-bellied sister species Bonelli's eagle (A. fasciatus) and African hawk-eagle (A. spilogaster), previously included in the Hieraaetus genus, have been revealed to be genetically much closer to the Verreaux's and golden eagle lineage than to other species traditionally included in the Aquila genus.[2][33][34] Other largish Aquila species, the eastern imperial, the Spanish imperial, the tawny and the steppe eagles, are now thought to be separate, close-knit clade, which attained some similar characteristics to the prior clade via convergent evolution.[33][34] Genetically, the “spotted eagles” (A. pomarina, hasata & clanga), have been discovered to be more closely related to the long-crested eagle (Lophaetus occipitalis) and the black eagle (Ictinaetus malayensis), and many generic reassignments have been advocated.[33][35] The Hieraaetus genus, including the booted eagle (H. pennatus), little eagle (H. morphnoides) and Ayres's hawk-eagle (H. ayresii), consists of much smaller species, that are in fact smallest birds called eagles outside of the unrelated Spilornis serpent-eagle genus. This genus has recently been eliminated by many authorities and are now occasionally also included in Aquila, although not all ornithological unions have followed this suit in this re-classification.[2][34][36] The small-bodied Wahlberg's eagle (H. wahlbergi) has been traditionally considered a Aquila species due to its lack of change from juvenile to adult plumage and brownish color but it is actually genetically aligned to the Hieraaetus lineage.[33][37] Cassin's hawk-eagle (H. africanus) is also probably closely related to the Hieraaetus group rather than the Spizaetus/Nisaetus “hawk-eagle” group (in which it was previously classified) which is not known to have radiated to Africa.[38]

Subspecies and distribution

A member of the largest golden eagle race (A. c. daphanea) mantles over carrion in winter
A captive Aquila chrysaetos canadensis shows the typical rusty coloration of the race

There are six extant subspecies of golden eagle that differ slightly in size and plumage. Individuals of any race are somewhat variable and the differences between subspecies are clinal, especially in terms of body size. Other than these characteristics, there is little variation across the range of the species.[5] Some recent studies have gone so far as to propose that only two subspecies be recognized based on genetic markers: Aquila chrysaetos chrysaetos (including A. c. homeyeri) and A. c. canadensis (including the races A. c. japonica, A. c. daphanea and A. c. kamtschatica).[39]

  • Aquila chrysaetos chrysaetos (Linnaeus, 1758) – sometimes referred to as the European golden eagle.[23] This is the nominate subspecies. This subspecies is found almost throughout Europe including the British Isles (mainly in Scotland), a lion’s share of Scandinavia, southern and northernmost France, Italy and Austria. In Eastern Europe, the race is found from Estonia to Romania, Greece, Serbia and Bulgaria in southeastern Europe. It is also distributed through European Russia, reportedly reaching itself eastern limit around the Yenisei River in Russia, also ranging south at a similar longitude into western Kazakhstan and northern Iran.[2][5] Male wing length is from 56.5 to 67 cm (22.2 to 26.4 in), averaging 62 cm (24 in), and female wing length is from 61.5 to 71.2 cm (24.2 to 28.0 in), averaging 67 cm (26 in). Males weigh from 2.8 to 4.6 kg (6.2 to 10.1 lb), averaging 3.69 kg (8.1 lb), and females weigh from 3.8 to 6.7 kg (8.4 to 14.8 lb), averaging 5.17 kg (11.4 lb).[2][5] The male of this race has a wingspan of 1.89 to 2.15 m (6 ft 2 in to 7 ft 1 in), with an average of 2.02 m (6 ft 8 in), with the female’s typical wingspan range is 2.12 to 2.2 m (6 ft 11 in to 7 ft 3 in), with an average of 2.16 m (7 ft 1 in).[23] This is a medium-sized subspecies and is the palest race. As opposed to golden eagles found further east in Eurasia, the adults of this race are a tawny golden-brown on the upper-side. The nape patch is often gleaming golden in color and the feathers here are exceptionally long.[3][5][40]
  • Aquila chrysaetos homeyeri Severtzov, 1888 –This race occurs in almost the entirety of the Iberian peninsula as well as the island of Crete, though is absent from the rest of continental Europe. It also ranges in North Africa in a narrow sub-coastal strip from Morocco to Tunisia. A completely isolated population of golden eagles is found in Ethiopia’s Bale Mountains, at the southern limit of this species range worldwide. Although this latter population has not been formally assigned to a race, the probability that it belongs with A. c. homeyeri seems high. This subspecies also ranges in much of Asia Minor, mainly Turkey, spottily through the Middle East and the Arabian Peninsula into northern Yemen and Oman to its eastern limits throughout the Caucasus, much of Iran and north to southwestern Kazakhstan.[2][5] Male wing length is from 55 to 64.3 cm (21.7 to 25.3 in), averaging 59 cm (23 in), and female wing length is from 60 to 70.5 cm (23.6 to 27.8 in), averaging 64 cm (25 in). Weight is from 2.9 to 6 kg (6.4 to 13.2 lb) with no known reports of average masses.[2][5] This subspecies is slightly smaller and darker plumaged than the nominate race, but is not as dark as the golden eagles found further to the east. The forehead and crown are dark brownish with the nape patch being short-feathered and a relatively light rusty color.[41]
  • Aquila chrysaetos daphanea Severtzov, 1888 – known variously as the Asian golden eagle, Himalayan golden eagle or berkut.[9] This race is distributed in central Kazakhstan, eastern Iran and the easternmost Caucasus distributed to Manchuria and central China and along the Himalayas from northern Pakistan in the west to Bhutan in the east (rarely ranging over into northernmost India) discontinuing in northeastern Myanmar.[2][5][42] This subspecies is the largest race on average. Male wing length is from 60 to 68 cm (24 to 27 in), averaging 64 cm (25 in), and female wing length is from 66 to 72 cm (26 to 28 in), averaging 70 cm (28 in). No range of body weights are known but males will weigh approximately 4.05 kg (8.9 lb) and females 6.35 kg (14.0 lb).[2][5] Although the wingspan of this race reportedly averages 2.21 m (7 ft 3 in), some individuals apparently are much longer-winged.[9] One aforementioned female “berkut” had an authenticated wingspan of 2.81 m (9 ft 3 in), although she was a captive specimen.[9] It is generally the second darkest race, being blackish on the back. The forehead and crown are dark with a blackish cap near the end of the crown. The feathers of the nape and top-neck are rich brown-red. The nape feathers are slightly shorter than in the nominate race and are similar in length to A. c. homeyeri.[43][44]
  • Aquila chrysaetos japonica Severtzov, 1888 – the common name is the Japanese golden eagle. This race is found in northern Japan (the islands of Honshu, Hokkaido and discontinuously in Kyushu) and undefined parts of Korea.[2][5] Male wing length is from 58 to 59.5 cm (22.8 to 23.4 in), averaging 59 cm (23 in), and female wing length is from 62 to 64.5 cm (24.4 to 25.4 in), averaging 63 cm (25 in). No range of body weights are known but males will weigh approximately 2.5 kg (5.5 lb) and females 3.25 kg (7.2 lb). This is by far the smallest bodied subspecies. It also the darkest with even adults being a slaty-grayish black on the back and crown and juveniles being similar but with darker black plumage contrasting with brownish color and white scaling on the wings, flank and tail. This race has bright rufous nape feathers that are quite loose and long. Adult Japanese golden eagles often maintain extensive white mottling on the inner-webs of the tail that tend to be more typical of juvenile eagles in other races.[2][5]
  • Aquila chrysaetos canadensis (Linnaeus, 1758) – Commonly known as the American golden eagle. Occupies the species’ entire range in North America, which comprises the great majority of Alaska, western Canada and the Western United States. The species is found breeding occasionally in all Canadian provinces but for Nova Scotia. It is currently absent in the Eastern United States as breeding species east of a line from North Dakota down through westernmost Nebraska and Oklahoma to West Texas. The southern limits of its range are in central Mexico, from the Guadalajara area in the west to the Tampico area in the east. It is the subspecies with the largest breeding range and is probably the most numerous subspecies, especially if A. c. kamtschatica is included.[2][5] Male wing length is from 59.1 to 64 cm (23.3 to 25.2 in), averaging 61 cm (24 in), and female wing length is from 60.1 to 67.4 cm (23.7 to 26.5 in), averaging 65 cm (26 in).[2][5][13] The average wingspan in both sexes is about 2.04 m (6 ft 8 in).[3] Males weigh from 2.5 to 4.47 kg (5.5 to 9.9 lb), averaging 3.48 kg (7.7 lb), and females typically weigh from 3.6 to 6.4 kg (7.9 to 14.1 lb), averaging 4.91 kg (10.8 lb).[2][5] The race does not appear to follow Bergmann’s rule (the rule that widely distributed organisms are larger-bodied further away from the Equator), as specimens of both sexes from Idaho had a mean weight of 4.22 kg (9.3 lb) and where slightly heavier than those from Alaska, with a mean weight of 3.76 kg (8.3 lb).[2][3] It is medium-sized, being generally intermediate in size between the nominate and A. c. homeyeri but with much overlap. It is blackish to dark brown on the back. The long feathers of nape and top-neck are rusty-reddish and slightly narrower and darker than in the nominate race.[5][45]
  • Aquila chrysaetos kamtschatica Severtzov, 1888 – sometimes is referred to as the Siberian golden eagle or the Kamchatka golden eagle. This race ranges from Western Siberia (where overlap with A. c. chrysaetos is probable), across most of Russia, including the Altay (spilling over into Northern Mongolia), to the Kamchatka Peninsula and the Anadyrsky District. This subspecies is often included in A. c. canadensis.[2][5] Male wing length is from 61.8 to 70.5 cm (24.3 to 27.8 in), averaging 64 cm (25 in), and female wing length is from 65 to 72 cm (26 to 28 in), averaging 69 cm (27 in).[46] No weights are known in this race.[2] The coloration of these eagles is almost exactly the same as in A. c. canadensis. The main difference is that this race is much larger in size, being is nearly the equal of A. c. daphanea going on wing-length.[5]

The larger Middle Pleistocene golden eagles of France (and possibly elsewhere) are referred to a paleosubspecies Aquila chrysaetos bonifacti, and the huge specimens of the Late Pleistocene of Liko Cave (Crete) have been named Aquila chrysaetos simurgh (Weesie, 1988).[47][48] Similarly, an ancestral golden eagle, with a heavier, broader skull, larger wings and shorter legs when compared to modern birds, has been found in the La Brea Tar Pits of southern California.[49]

Habitat

Beinn Mhor on the Isle of Mull, Scotland is typical golden eagle habitat: rugged and mountainous.

Golden eagles are fairly adaptable in habitat but often reside in areas with a few shared ecological characteristics. They are best suited to hunting in open or semi-open areas and search them out year-around. Native vegetation seems to be attractive to them and they typically avoid developed areas of any type from urban to agricultural as well as heavily forested regions. In desolate areas (i.e. the southern Yukon), they can occur regularly at roadkills and garbage dumps.[50] The largest numbers of golden eagles are found in mountainous regions today, with many eagles doing a majority of their hunting and nesting on rock formations. However, they are not solely tied to high elevations and can breed in lowlands if the local habitats are suitable. Below are more detailed description of habitats occupied by golden eagles in both continents where they occur.[2]

Eurasia

In the Arctic fringe of the great continent, golden eagles occur along the edge of the tundra and the taiga from the Kola peninsula to Anadyr in eastern Siberia, nesting in forests and hunting over nearby arctic heathland. Typical vegetation is stunted, fragmented larch woodland merging into low birch-willow scrub and various heathland. In the rocky, wet, windy maritime countries of the British Isles and western Scandinavia, the golden eagle is a mountain-dwelling bird. These areas include upland grasslands, blanket bog and sub-Arctic heaths but also fragmented woodland and woodland edge, including boreal forests. In Western Europe, golden eagle habitat is dominated by open, rough grassland, heath and bogs, in places enlivened by rocky ridges, spurs, crags, scree, slopes and grand plateaux. In Sweden, Finland, the Baltic States, Belarus and almost the entire distribution in Russia all the way to the Pacific Ocean, golden eagles occur sparsely in lowland taiga forest. These areas are dominated by stands of evergreens such as pine, larch and spruce, occasionally supplemented by birch and alder stands in southern Scandinavia and the Baltic States. This is largely marginal country for golden eagles and they occur where tree cover is thin and abuts open habitat. Golden eagle taiga habitat usually consists of extensive peatland formations caused by poorly drained soils. In central Europe, golden eagles today occur almost exclusively in the grand mountain ranges, such as Pyrenees, Alps, Carpathians and the Caucasus. Here, the species nests near the tree line and hunt subalpine and alpine pastures, grassland and heath above. Golden eagles also occur in moderately mountainous habitat along the Mediterranean Sea, from Iberia and the Atlas Mountains in Morocco, to Greece, Turkey and Kurdistan. This area is characterized by low mountains, Mediterranean maquis vegetation and sub-temperate open woodland in various stages of degradation. The local pine-oak vegetation, with a huge variety of Sclerophyllous shrubs are well-adapted to prolonged summer draughts. From Kurdistan and the southern Caspian Sea to the foothills of the Hindu Kush Mountains in Afghanistan, the typical golden eagle habitat is temperate desert-like mountain ranges surrounded by steppe landscapes interspersed with forest. Here the climate is colder and more continental than around the Mediterranean. Golden eagles occupy the alpine ranges from the Altai Mountains and the Pamir Mountains to Tibet, in the great Himalayan massif, and northwestern China, where they occupy the Tien Shan range. In these mountain ranges, the species often lives at very high elevations, living above tree line at more than 2,500 m (8,200 ft), often nesting in rocky scree and hunting in adjacent meadows. In Tibet, golden eagles inhabit high ridges and passes in the Lhasa River watershed, where it regularly joins groups of soaring Himalayan Vultures (Gyps himalayensis).[51] One golden eagle was recorded circling at 6,190 m (20,310 ft) above sea-level in Khumbu in May 1975.[52] In the mountains of Japan and Korea, the golden eagle occupies deciduous scrub woodland and carpet-like stands of Siberian dwarf pine (Pinus pumila) that merge into grasslands and alpine heathland. The golden eagle occurs in mountains from the Adrar Plateau in Mauritania to northern Yemen and Oman where the desert habitat is largely bereft of vegetation but offers many rocky plateaus to support both the eagles and their prey. In Israel, their habitat is mainly rocky slopes and wide wadi areas, chiefly in desert and to a lesser extent in semi-desert and Mediterranean climates, extending to open areas.[53] In Northeastern Africa, the habitat is often of a sparse, desert-like character and is quite similar to the habitat in Middle East and the Arabian peninsula. In Ethiopia's Bale Mountains, where the vegetation is more lush and the climate is clearly less arid than in Northeastern Africa, the golden eagle occupies verdant mountains.[2]

North America

The ecozones occupied by golden eagles are roughly concurrent with those of Eurasia. In western and northern Alaska and northern Canada to the Ungava Peninsula in Quebec, the eagles occupy the Arctic fringe of North America (the species does not range into the true high Arctic tundra), where open canopy gives way to dwarf-shrub heathland with cottongrass and tussock tundra. In land-locked areas of the sub-Arctic, golden eagles are by far the largest raptor. From the Alaska Range to Washington and Oregon, it is often found in high mountains above the tree line or on bluffs and cliffs along river valleys below the tree line.[54][55] In Washington state, golden eagles can be found in clear-cut sections of otherwise dense coniferous forest zones with relatively little annual precipitation.[56] From east of the Canadian Rocky Mountains to the mountains of Labrador, the golden eagle is found in small numbers in boreal forest peatlands and similar mixed woodland areas. In the foothills of the Rocky Mountains in the United States are plains and prairies where golden eagles are widespread, especially where there's a low human presence. Here, grassland on low rolling hills and flat plains are typical, interrupted only by cottonwood stands around river valleys and wetlands where the eagles may build their nests. Golden eagles also occupy the desert-like Great Basin from southern Idaho to northern Arizona and New Mexico. In this habitat, trees are generally absent other than junipers with vegetation being dominated by sagebrush (Artemisia) and other low shrub species. Although the vegetation varies a bit more, similar habitat is occupied by golden eagles in Mexico.[57][58][59] However, golden eagles are typically absent in North America from true deserts like the Sonora Desert, where annual precipitation is less than 20 cm (7.9 in).[60] Golden eagles occupy the mountains and coastal areas of California and Baja California in Mexico where hot, dry summers and moist winters are typical. The golden eagles here often nest in chaparral and oak woodland, oak savanna and grassland amongst low rolling hill typified by diverse vegetation.[61] In the Eastern United States, the species once bred widely in the Appalachian Plateau near burns, open marshes, meadows, bogs and lakes.[62][63] In Eastern North America, the species still breeds on the Gaspe Peninsula, Quebec. Until 1999, a pair of golden eagles were still known to nest in Maine but they are now believed to be absent as a breeding bird from the Eastern United States.[2] The golden eagles who breed in eastern Canada winter on montane grass and heath fields in the Appalachian Plateau region, especially in Pennsylvania, New York, West Virginia, Maryland and Virginia. Most sightings in the Eastern United States recently are concentrated within or along southwestern border of the Appalachian Plateau (30% of records) and within the Coastal Plain physiographic region (33% of records).[64]

Though they do regularly nest in the marsh-like peatland of the boreal forest, golden eagles are not generally associated with wetlands and, in fact, they can be found near some of the most arid spots on earth. In the wintering population of Eastern United States, however, they are often associated with steep river valleys, reservoirs, and marshes in inland areas as well as estuarine marshlands, barrier islands, managed wetlands, sounds, and mouths of major river systems in coastal areas. These wetlands are attractive due to a dominance of open vegetation, large concentrations of prey, and the general absence of human disturbance.[64] In the midwestern United States, they are not uncommon during winter near reservoirs and wildlife refuges that provide foraging opportunities at waterfowl concentrations.[65]

Dietary biology

John James Audubon's painting of a golden eagle carrying a snowshoe hare

The golden eagle is one of the most powerful predators in the avian world. One author described it as "the pre-eminent diurnal predator of medium-sized birds and mammals in open country throughout the Northern Hemisphere".[45] Golden eagles usually hunt during daylight hours, but were recorded hunting from one hour before sunrise to one hour after sunset during the breeding season in southwestern Idaho.[66] The hunting success rate of golden eagles was calculated in Idaho, showing that, out of 115 hunting attempts, 20% were successful in procuring prey.[67] An fully-grown golden eagle requires about 230 to 250 g (8.1 to 8.8 oz) of food per day but in the life of most eagles there are cycles of feast and famine, and eagles have been known to go without food for up to a week and then gorge on up to 900 g (2.0 lb) at one sitting.[3] The powerful talons of the golden eagle ensure that few prey can escape them once contact is made. The talons of this species exert approximately 440 pounds per square inch (3 MPa) of pressure, around 15 times more pressure than is exerted by the human hand, although some opine that the largest individual females may reach a pressure of 750 psi (5.2 MPa).[68] Few other large raptors have been tested in their foot strength, though the huge Harpy Eagle, when tested, exerted a psi approximately 40 kg (88 lb) greater than the tested golden eagle.[69] It has been claimed that the golden eagle can lift up to 8 kg (18 lb) in flight.[70] However, other sources claim that a hare, marmot or deer calf half of that weigh (4 kg (8.8 lb)) is a struggle for even a large female to carry and that prey much over 2 kg (4.4 lb) would need favorably high windy conditions to be lifted.[71][72][73]


Hunting methods

A golden eagle lands on carrion during a snowstorm.

At least seven main hunting techniques are known to be utilized by the species, with many individual variations and the ability in most mature eagles to quickly (and sometimes cleverly) vary back and forth between methods depending on the circumstance. The first described is “high soar with glide attack”, where the golden eagle soars at least 50 m (160 ft) above the earth. Once it spies a prey item, the eagle partially closes its wings and enters a long, low-angled glide which can carry it over distances of 1 km (0.62 mi) with the speed increasing as the wings close more. Just prior to impact, the wings are opened, the tail fanned and feet thrust forward to grab the prey, creating a booming sound, causing by the wings whipping against the wind, in the instant before the strike that sounds like a clap of thunder.[2] This technique is used for solitary or widely dispersed prey found in openings, such as hares or ptarmigans. A variation of the high soar where a lofty perch is used instead of soaring flight has been observed.[74] The next major hunting methods is the “high soar with vertical stoop”, which is used to attack birds in flight. Since they are outpaced and out-maneuvered by swift-flying birds, they can only usually attack slower-flying species such as geese and cranes. The golden eagle also requires a height advantage over their prey for this rarely observed technique to succeed. In one observed case, some Canada geese (Branta canadensis) in Montana were able to avoid predation by a golden eagle hunting them in this way by collectively gaining flying height.[75] In another observation, a golden eagle was successful in knocking a ptarmigan out of a flying covey in Scotland.[76] The next hunting method is the “contour flight with short glide attack”, which is considered the most commonly utilized hunting method for golden eagles. This consists of a low-level quartering flight often at only 5 to 15 m (16 to 49 ft) above the ground so they do not break the sky-line when observed from the ground and they can hug the contours of the earth below. This method is useful for hunting colonial (often burrowing) prey such as ground squirrels, densely populated leporids or birds found in concentrations, such as breeding grouse or even seabirds. The individual prey item is apparently selected in a manner of seconds before the strike.[75][77] If the first attempt fails, the eagle may fly around and attempt to ambush the prey again. The next hunting method is the “glide attack with tail-chase”, which commences with a low-angled stoop some distance from the quarry. The prey is then chased closely, whether a hare running evasively or a grouse in flight. The key to success is eagle's agility and the prey's inability to find cover. In one case, a flying greater sage-grouse (Centrocercus urophasianus) was caught by a pair of eagles using this technique.[78] The next major hunting method is “low flight with slow descent attack”. In this, the golden eagle quarters low below the earth and then gradually swoops down on the prey. This is used for slow-moving prey, such as tortoises and hedgehogs, or any prey item with a general absence of escape behavior. This includes any potentially dangerous prey items, such as rattlesnakes and foxes.[79] When hunting mammalian carnivores, the eagle may hover for some time over the potential prey and not press the attack unless the mammal looks down.[75][77] The next is the “low flight with sustained grip attack”, which is used for hunting ungulates. Here, the golden eagle flies over a herd of ungulates which in turn often huddle or break into run. The eagle then selects it prey (typically young animals, though sometimes infirm or exceptionally healthy grown animals) and lands on prey's back or neck, talons gripping firmly attempting to pierce vital organs or cause shock via a crushing grip to bone and cartilage. The hunting eagle typically rides its prey for several minutes with wings outstretched and flapping to maintain balance until the prey collapses, either as result of exhaustion, shock or internal injury.[2] The final major hunting method is the “walk and grab attack”, in which the eagle walks on the ground and attempts to pull its prey out of cover. This has been used for pulling jackrabbits out of brush but has even been utilized to grab the young of large prey (i.e. deer, sheep and badgers) literally right out from under their mother’s legs.[2][28][80][81][82] Tandem hunting may be done regularly, especially with larger prey items.[83][84] Reportedly, while hunting in pairs, the male golden eagle flies in front of the female at a higher elevation and usually initiates the attack.[25] Breeding pairs have been recorded hunting jackrabbits cooperatively with one individual following the other at different elevations above the ground. The initial pursuer diverts the prey's attention by stooping while the second flies in unseen to make the kill.[85][86] A study in Idaho showed that the success rate was lower during tandem hunting (9%) than during solo hunting (29%), but this may have been due to the more difficult nature of the prey targeted during tandem hunts.[67] As a whole birds are reportedly more difficult prey to capture than mammals, so hunting success rates may be lower where birds outrank mammals in importance as prey.[2] Golden eagles are not above scavenging for carrion. In fact, it makes up a significant portion (sometimes a majority) of the diet in winter, when ground squirrels are in hibernation and rabbits and hares tend to be at population lows.[2] In the Greater Yellowstone area, the golden eagle was one of the most frequent scavengers to attend wolf kill-sites in winter but, unlike common ravens (Corvus corax) and bald eagles, were not frequent at kills left out by human hunters.[87] In many parts of the range (i.e. Alberta, Scotland, Spain, etc.) carrion was readily fed to the young during the breeding season.[2][88][89]

Prey

Golden eagle with a freshly caught brown hare

Golden eagles are opportunists and virtually any animal of a reasonable size may be predated if encountered. Well over 400 species of vertebrate have been recorded as prey.[2] Prey selection is largely determined by the local availability and abundance of prey species. They normally prefer wild, native prey but can easily adapt to domesticated and/or exotic animals, most often when the prey returns to a feral state. This is most apparent on islands that have very few (or no) native land mammals, such as Corsica in Italy, Santa Rosa and Santa Cruz Islands in California and many of the Inner and Outer Hebrides of Scotland.[2][90][91] Most analysis of dietary habits of raptors results from examining the prey remains around an active eyrie at the end of the breeding season (September or October), based on pellets, skeletons and skins found. This method is not comprehensive for prey analysis, as particularly small prey may leave no trace and it cannot take into account the prey selected by wintering adults and highly nomadic juvenile eagles (both of which are believed to include a higher level of carrion and large prey items). However, since eye-witness accounts of hunting and close monitoring of prey brought to the nest are both rare and likely to disturb the eagles, this is the best known method of studying the eagle’s prey.[2] Studies have revealed that the average golden eagle nest contains 3.57 species of prey, although there is considerable variation in the dietary breadth across the range, ranging from an average of 11.2 species in the French Pyrenees to a mere 1.4 in Alaska.[92][93] In general, the dietary breadth is greater in Eurasia than it is in North America, where eagles frequently only need to hunt two or three species throughout the nesting cycle.[2] This is a moderate dietary breadth by the standards of the Aquila genus,[2] and not remarkable when compared to some other raptors.[94] Overall prey weight has ranged from 10 g (0.35 oz) to at least 114 kg (251 lb),[94][95] although most prey taken are around half the weight of the predating eagle, with a typical prey weight range of 0.5–4 kg (1.1–8.8 lb) but mostly in the lower half of that range.[2] Studies have shown an estimated mean prey weight of 1.614 kg (3.56 lb) for golden eagles across their range.[2] Only 15.8% of prey weighs over 4 kg (8.8 lb).[2] At a nest studied in Mongolia, prey items were found to be heavier than those known elsewhere in the range, with an estimated mean weight of around 3 kg (6.6 lb).[96] The average estimated weight of prey taken by most other Aquila species is generally much lower but the Wedge-tailed Eagle seemingly takes prey of a similar weight (both in average and range of prey weights) while the Verreaux's Eagle average prey weight is higher, probably over 2 kg (4.4 lb).[2][97]

The most significant group of prey for golden eagles is mammals. In 59 studies of the breeding season diet across the range, 63.2% of prey remains were of mammals.[2] The diet in North America is particularly skewed towards mammals with roughly 84% of prey items being mammalian.[98] After mammals, other birds were most significant, comprising about 26.8% of prey.[2] Reptiles comprised about 7% of prey from across the range, with other prey groups comprising the remaining 3%.[2] All the diverse groups of prey are examined below, using mainly the aforementioned 59 dietary studies. The Golden Eagle occationally takes dogs as prey [99][100]


Leporids

Painting depicting a golden eagle that has caught a Mountain Hare

The family Leporidae (rabbits and hares) is easily the most significant prey group, comprising about 32% of prey known to be taken by golden eagles.[2] Twelve out of sixteen dietary studies in North America showed leporids as the most common prey group, in some areas comprising over 75% of prey remains.[101][102] In the northern Rocky Mountains and Northern Plains areas, the main prey species are the white-tailed jackrabbit (Lepus townsendii) and the mountain cottontail (Sylvilagus nuttallii). In the Southwest, the Great Basin, and most of California, the main prey species are the black-tailed jackrabbit (Lepus californicus) and the desert cottontail (Sylvilagus audubonii), the former being especially important, comprising about a quarter of North America prey items in a 1976 study.[98] In two studies in southern Idaho, the Black-tailed Jackrabbit and the two cottontails dominated the diet, comprising over 70% of nest remains.[103][104] More locally other jackrabbits and cottontails may be taken in smaller numbers. The snowshoe hare (Lepus americanus) is thought to be the dominant prey species in the boreal forests of Canada but there have been no dietary studies conducted there. The Snowshoe and the larger Arctic hare (Lepus arcticus) are some of the most significant prey species in Alaska, though in one central Alaskan nest they were secondary to ptarmigan and nests in Denali National Park included only 0.4% leporid remains.[92][105] Nests in Alberta and Washington had almost no leporid remains as well.[56][88][106]

19 out of 45 studies in the Palearctic listed leporids as the most common family of prey for golden eagles. Important prey species include the mountain hare (Lepus timidus) (in the Scottish Highlands and the French Alps), the European rabbit (Oryctolagus cuniculus) (in the Scottish Highlands, Sicily, the Apennine Mountains, Switzerland, Spain, the Central Massif of France and the French Alps), the brown hare (Lepus europaeus) (in Switzerland, the Apennine Mountains, the French Alps and Armenia), the Japanese hare (Lepus brachyurus) (in Japan), the Cape hare (Lepus capensis) (in Mali and Niger) and the Ethiopian highland hare (Lepus starcki) (in the Bale Mountains)[107][108][109][110][111][112][113][114][115][116][117][118] In some areas leporids are secondary prey, such as Scandinavia, where the Mountain Hare comprised about 20% of prey items, the maximum known being 28.4% in Northern Finland.[119] The Tolai hare (Lepus tolai) was a secondary prey item in Mongolia, making up around 21% of prey.[96] The European Rabbit appears to be secondary in Hungary, where it comprised 14.3%.[120]

The estimated typical range of weights of hares hunted by golden eagles is 1.36 to 3.7 kg (3.0 to 8.2 lb), whereas rabbits hunted weigh a little under 1 kg (2.2 lb) on average.[23] Except for the occasional Arctic hare, white-tailed or antelope jackrabbit (Lepus alleni), most leporids hunted in North America weigh 2 kg (4.4 lb) or less, including the smallest hare in the world, the 1.45 kg (3.2 lb) snowshoe hare, and the smallest rabbit in the world, the 400 g (0.88 lb) pygmy rabbit (Brachylagus idahoensis).[98][121][122] In Europe, where the large Brown and Mountain Hares, both sometimes weighing over 5 kg (11 lb), are habitually hunted, the prey may be dismembered before being brought to the nest.[2] Across the golden eagle’s range, hares and rabbits are generally known to be hunted via either “high soar with glide attack” or “contour flight with short glide attack”. Leporids are basically solitary animals but they can occur at high densities, such as when jackrabbits are at their peak numbers in brushy areas of Western North America or (historically at least) rabbits in similar brush habitats on the Iberian peninsula. The goal of the golden eagle while hunting hares and rabbits is to catch the prey when it is foraging out in the open. If the leporid becomes aware of the eagle’s descent upon them more than a few seconds before the strike, they invariably break into a fast, hopping run. Once contact is made, rabbits and hares will often kick or try to bite the eagle but escape is unlikely once they are pinned to the ground. In some case they are able to reach vegetative coverage and, if the said cover is sufficiently deep and dense, the eagle cannot pursue them any further. Although rare, golden eagles have been known to use the “walk and grab attack” to pull a leporid out of its cover. Many hunts in Scotland of Mountain Hare involve a somewhat lengthy tail chase.[2] If the golden eagle is able to intercept a rabbit or hare far out in the open, a twisting and turning tail chase frequently occurs but the odds of survival for the prey are smaller the farther they are from coverage. On occasion, breeding pairs have been observed hunting jackrabbits together, where in one will stoop on a gathering of them while the other waits out of sight and assaults one of the jackrabbits being made to run.[67]

Ground squirrels

Yellow-bellied marmots are the favored prey for golden eagles in forested areas of Washington state

After the leporids, the next most significant group in the diet of golden eagles is the ground squirrels. They comprise about 11.2% of the prey taken by this species across the range. In areas where leporids are scarce, this group of rodents seems to become more prominent. In many parts of the golden eagle’s range, ground squirrels are rare or absent, such as the British Isles, the Iberian Peninsula and some portions of central Eurasia. Nonetheless, about half of the large Spermophilus group, all species of prairie dog, three out of four species of antelope squirrel and nearly all the species of marmot has been found as golden eagle prey. Nests in Denali National Park in Alaska included, among a sampling of 690 remains, contained 84.2% Arctic Ground Squirrels (Spermophilus parryii).[92] Studies in Alaska have shown that the other major prey species in Alaska, the Willow Ptarmigan and the Snowshoe Hare, follow an 8 to 11 year population cycle. The locally abundant ground squirrels have less dramatic population variations and come out of hibernation in mid to late May (shortly after the eagles return from migration). The Alaskan Arctic Ground Squirrel appears to be eaten supplementally in peak years but then fill the primary niche when the ptarmigan and hare populations crash.[123][124] In Alberta, Columbian ground squirrels (Spermophilus columbianus) (occasionally supplemented by other species like yellow-bellied marmots) were the primary prey species, making up 84% of the diet of eagles nesting there.[88] In the French Alps, the most significant prey species is the Alpine marmot (Marmota marmota) at 68.9% out of a sampling of 198 from 6 nests.[125] This is also the main prey species in the Italian Alps above the tree line, comprising 49.7% of a sample size of 79 from 12 nests.[126] In the Caspian Sea area of Kazahkstan, yellow ground squirrels (Spermophilus fulvus) are the most important mammalian prey at 23.4% of a sampling of 468 from 36 nests.[127] Along Coastal California, California ground squirrels (Spermophilus beecheyi) replace Black-tailed Jackrabbits (which are scarce there) as the primary prey species.[61] In Washington state, where the golden eagle inhabits mainly forest openings made by humans, the Yellow-bellied Marmot (Marmota flaviventris) is the main prey species, making up 41.8% of a sampling of 47 from 2 nests and 40.3% of a sampling of 315 from 74 nests.[56][106] In many other areas, ground squirrels are secondary prey, usually ranking after lagomorphs in dietary significance. This is especially true in the contiguous Western United States outside of Washington state and coastal California where they often register as second, third or even fourth most represented family by remains. Prairie dogs feature only as supplemental prey for breeding golden eagles but wintering eagles reportedly prey heavily on colonies of black-tailed prairie dogs (Cynomys ludovicianus).[128][129] Although rare, tree squirrels have turned up as prey in some eagle nests in both North America and Eurasia.[2][98]

Ground squirrels are generally quite social animals and some species even live in cohesive colonies. All ground squirrels live in burrows, which in species like prairie dogs can be quite elaborate. The “contour flight with short glide attack” is essentially the one and only hunting technique used by golden eagles on ground squirrels. Ground squirrels usually forage within running distance of one of their burrow entrances, so it is essential for the golden eagle to have a surprise factor. Eagles will not enter burrows, unlike some mammalian carnivores that eat ground squirrels. When they hunt ground squirrels, golden eagles seem to choose their victim in the last few seconds before they strike. Most of the ground squirrels targeted by the eagles are quite small, with the entire Spermophilus group and some prairie dogs weighing under 1 kg (2.2 lb). Marmots are a more formidable catch since most species weigh at least 1.5 to 3.5 kg (3.3 to 7.7 lb) in spring and about twice that by fall, which is roughly the highest weight feasible for a flying eagle to carry. Although they may locally target large numbers of juveniles, in other cases they may dismember an adult marmot after a kill so they are able to carry it to the nest.[2][106] In the Alps, it is believed to be advantageous for eagles to nest below the meadows that host their Alpine Marmot prey so they can fly downhill, a much easier flying method while carrying a heavy load than flying uphill.[126]

Grouse

The cock Western Capercaillie, often favored in the golden eagle's diet

Although absent or rare in some areas where the golden eagle occurs, this is by far the most significant family of birds in the diet of golden eagles, making up 10.3% of their known prey.[2] In Scandinavia and the Baltic States, grouse are the primary prey for these eagles, comprising between 47.6% and 63.3% of their diet,[119][130][131][132][133] and displacing mammals altogether. The main prey species in most of this sizeable area is the largest species of grouse, the Western Capercaillie (Tetrao urogallus), supplemented by its cousins, the black grouse (Tetrao tetrix) and the hazel grouse (Tetrastes bonasia). However in the northernmost parts of Scandinavia, these are replaced by the smaller willow ptarmigan (Lagopus lagopus) and rock ptarmigan (Lagopus muta). A study in Belarus showed grouse were the most common family of prey at 47.8% of the eagle diet.[134] In Estonia, the prey base is more varied, but grouse were still the most represented bird family, comprising 18.1%.[135] The red grouse, a race of willow ptarmigan (Lagopus lagopus scotica), and the rock ptarmigan (also called just ptarmigan) occur heavily in the golden eagle’s diet in parts of Scotland, up to 47.8% in one study.[2] One paper in Alaska showed that the two ptarmigan species (primarily the more abundant Willow) made up 49.1% of the remains in a golden eagle pair’s nest, making it the only known study in North America where birds were more prevalent in the diet than mammals.[105] Other reports from Alaska show the Willow Ptarmigan as secondary prey.[123] In some other areas, grouse are taken only as supplemental prey. A study in the Italian Alps revealed grouse made up 19.4% of nest remains and were the most common avian prey.[126] In eastern Washington state, sooty grouse (Dendragapus fuliginosus) was the second most significant prey species (at 13%) after Yellow-bellied Marmot.[56] Considered globally, it is likely that every species of grouse is prey of the golden eagle at least somewhere.[2]

Golden eagles generally hunt grouse using the “contour flight with short glide attack” method, since grouse are often found in small groups. The usual response of the grouse is to take off in flight. If it is able to surprise the grouse, the eagle has a split second to grab the prey before it flies off. The eagle will frequently abandon the hunt after the grouse has flown, since it is unlikely to overtake such prey in the air unless it gains height over its quarry very quickly, although it may be able to gain the appropriate height to tail chase a grouse or even catch it while flying high using the “high soar with vertical stoop attack”. In Idaho a juvenile eagle was observed dropping a rock near a dusky grouse (Dendragapus obscurus), possibly attempting to spook it into flight so it could be overtaken from a flying height advantage.[136] Although eagles equally attack both male and female grouse, when cock grouse display on “leks” in spring, they may be more distracted and easily found. Grouse tend to be prey of modest size for golden eagles, many species weighing little more than 1 kg (2.2 lb).[137] An exception is the adult cock of the Western Capercaillie, which typically weighs over 4 kg (8.8 lb).[137] In addition to their large size, cock capercaillie are exceptionally aggressive while on a lek and golden eagles have been observed to use different, bolder tactics while hunting them, even landing at the lek to confront and overpower feuding grouse.[138] In Sweden, most of the capercaillie remains appeared to be those of the more modestly sized adult females.[131]

Pheasants

The red-legged partridge is the golden eagle's favored avian prey in Spain

In generally warmer areas of the Eurasian continent, the diverse pheasant family replaces the grouse as the significant group of birds in the golden eagle’s diet. Across the range, pheasants make up 5.7% of the diet of this species. Unlike grouse, pheasants are not known to comprise more than half of the nest remains in any study, but they can be important nonetheless. This is especially true in Spain, where most studies show that the pheasants at 22.8-28.7% of the diet, mainly the red-legged partridge (Alectoris rufa), are the second most prominent prey species after the European rabbit, with the red-legged partridge and the rabbit collectively making up to more than 90% of the prey remains in several nests there.[107][108] The red-legged species, along with the rock partridge (Alectoris graeca) and grey partridge (Perdix perdix), are the leading prey group in the French Pyrenees at 15.8% of a sampling of 114 from 10 nests.[93] In Hungary, the common pheasant (Phasianus colchicus) appears to be the favored prey species, making up 26.6% of the prey of a sampling of 49 from 4 nests there.[120] In Japan, the copper pheasant (Syrmaticus soemmerringii) is the second most common single prey species, making up 17.8% of the remains (sample size of 967 from 5 nests).[115] In the Caspian Sea area of Kazakhstan, the most common bird species (and third most common overall prey species) is the Chukar (Alectoris chukar), representing 19.5% of a sampling of 468 from 36 nests.[127] Chukar and Caspian snowcock (Tetraogallus caspius) are reportedly the primary avian prey species in Armenia.[114] In the Himalayan region, two very large pheasants, the Himalayan monal (Lophophorus impejanus) and the Himalayan snowcock (Tetraogallus himalayensis), are reportedly among the most significant prey for eagles.[44] Moorland Francolin (Scleroptila psilolaemus) were reportedly the most common bird prey species in Ethiopia’s Bale Mountains, comprising 10% among a sample size of 49 at 4 nests.[118] North America has fewer native pheasants species but introduced species such as grey partridge, ring-necked pheasant and, especially, Chukar are readily hunted (native quail seem to be largely or entirely ignored as prey by golden eagle, perhaps because of their small size or dense thicket-dwelling habits).[56][98][139][140] The Chukar is the most significant bird species in the diet in Nevada (at 5.4%) and the second most significant bird species in Washington state (at 11.8%).[56][101][106] One native North American “pheasant” known to be occasionally hunted is the wild turkey (Meleagris gallopavo), the largest species of galliform. The golden eagle is the only major avian predator of adult turkeys. The hunting techniques used on pheasants are probably similar to those used for grouse. Pheasants hunted by golden eagles are also similar in size in grouse. Alectoris sp. weigh around 500 g (1.1 lb) and medium-sized species weighing around 1 kg (2.2 lb). Snowcock will often weigh 2.5 to 3 kg (5.5 to 6.6 lb), whereas the female and male adult turkey weigh around 4 kg (8.8 lb) and 8 kg (18 lb), respectively.[137] On a wildlife monitoring camera in Tennessee, a golden eagle was filmed assaulting an adult turkey, appearing to use a “contour flight with short glide attack” but the quarry in this case appears to successfully avoid the strikes by running and then ultimately flying away.[141] Breeding pairs of eagles have been known to hunt turkeys cooperatively.[142]

Ungulates

John James Audubon's illustration of a golden eagle attacking a lamb

The contribution of ungulates to the diet of golden eagles has long been the most controversial and debated aspect of the species’ whole biology. In total, deer comprise about 4.97% of all golden eagle food, bovids about 4.92%, pigs about 0.71% and pronghorns less than that.[2] The occasional hunting by eagles of domestic stock and preferred game species has been used as the justification for population control. Sheep farmers in Scotland have historically claimed that up to 80% of deaths in their flocks were caused by eagles.[23][143] Leslie Brown claimed, to the opposite extreme, that it was “physically impossible” for a golden eagle to kill any ungulate scaling several times their own weight.[23] The truth lies somewhere in between. Almost all predation on live ungulates is directed at lambs, fawns or kids and these mostly in the first few months of life.[144] Once they exceed a certain size, it is not practical for breeding eagles to predate the growing ungulates, not only due to the difficult and dangerous nature of the kill but also the fact it would be too heavy to carry to the nest. Sheep are sometimes eaten in considerable numbers, especially in the Inner and Outer Hebrides of Scotland (25.9% and 26.8% of nest remains respectively). Differentiating whether a lamb has been caught and killed while alive or scavenged as a carcass is possible if the remains are in good condition. In one examination of 10 such remains in nests in Scotland, it was found that 3 lambs had been taken alive and 7 after they had died.[23][145] This suggests the majority of lambs are taken as carrion, which is reinforced by the fact that much ungulate carrion found around active nest sites in Scotland is already in a malodorous and putrid state.[2] Domestic goats (Capra aegagrus hircus) are occasionally predated as well. Goats slightly outnumbered sheep in the diet of eagles breeding on Corsica, with both domesticated animals making up 20.5% of the diet there and being the most important food source.[90] In North America, lambs and goats were found to comprise less than 1.4% of all prey items.[146] In Montana, it was found that most predation on lambs by golden eagles was committed by juvenile eagles or eagles that failed to breed (which have no need to carry prey to a nest). There it was found that the hunting of domestic stock peaks during wet, cold springs that seemed to adversely effect the local abundance of jackrabbits.[147] Pigs (including feral and domestic varieties) are occasionally taken, especially in insular populations. Studies showed pigs (most certainly taken as carrion) made up to 13.3% of the diet in Corsica and 43.1% on Santa Cruz Island, California.[90][91] Few wild pigs co-exist with golden eagles but wild boar (Sus scrofa) (likely only as piglets or carrion) have been taken in Bulgaria.[148]

Of wild species of ungulate, deer are the preferred prey family. In a study in the Italian Alps, the most common prey species of all was the European roe deer (Capreolus capreolus), which made up 32.2% of nest remains.[126] The Roe Deer was also a prominent prey item in the French Pyrenees (14.9%) and Switzerland (14.6%), respectively.[93][149] In several parts of the Scottish Highlands, red deer (Cervus elaphus) are eaten with some regularity, the maximum representation there being 22.3% of prey remains in the Northwest Highlands.[2] In North America, several deer species, but mainly mule deer (Odocoileus hemionus) fawns, were significant in the diet in Washington state (14.3%) and California (12.7%).[74][150] Around the Himalayas, the small white-bellied musk deer (Moschus leucogaster) is reportedly a favorite prey item (though no quantitative analysis are known).[44] In the near Arctic regions of Scandinavia, reindeer (Rangifer tarandus) of both semi-domestic and wild stock are fed on with some regularity; one study in northern Sweden found reindeer formed 11.4% of prey items.[130] In the northernmost stretches of Alaska, golden eagles are more scarce than in equivalent areas of Scandinavia and only occasionally hunt wild reindeer.[151] However, golden eagles were reportedly the most prolific predator of neonatal calves from the porcupine caribou (Rangifer tarandus granti) herd of central Alaska.[152] Partial moose (Alces alces) remains have been found at a nest in Sweden.[131] Live fawns and carrion probably comprise most of the golden eagle’s consumption of deer (certainly in larger-bodied species such as red deer). Among wild bovids, eagles are reportedly the main predator of Saiga antelope (Saiga tatarica) calves in Mongolia and are regularly reported to take chamois (Rupicapra rupicapra) and Alpine ibex (Capra ibex) calves in Europe.[153][154][155] Mongolian gazelles (Procapra gutturosa) comprised about 15% of the remains found at a nest in Mongolia.[96] Up to 7 wild species of goat, 5 other wild species of sheep and at least 2 wild gazelle species are confirmed golden eagle prey.[2] In North America, pronghorn (Antilocapra americana) are occasionally hunted.[84]

The main method of predation used on ungulates is the “low flight with sustained grip attack”, which can take between a few seconds to at least 15 minutes to kill the prey.[2][156] In studies, the average estimated weight of ungulate prey found in golden eagle nests varied between 2.5 kg (5.5 lb) and 5 kg (11 lb), depending on the location and species involved.[2][56][91][126][131][157] In each case, the weight of the ungulate prey is similar to the average newborn weight for that respective species, and most ungulates taken are about the same weight as the eagle.[2] The taking of larger ungulates is exceptional but has been verified in several cases, and is most likely to happen in late winter or early spring, when other available prey is scarce and (in most of the range) eagles are not concerned with carrying prey to a nest. In Scotland, golden eagles have been confirmed to kill red deer calves up to 20 kg (44 lb) in mass and have been captured on film attacking an adult red deer but not carrying through with the hunt.[158][159] Both adult and kid chamois and ibex have been confirmed as prey and, in some cases, have been forced off cliff edges to fall to their deaths, after which they can be consumed.[154][157][160] At a Mongolian nest, an apparently live-caught, dismembered adult Mongolian gazelle was observed.[96] Adult pronghorns weighing 27 to 32 kg (60 to 71 lb) have been successfully attacked and killed.[84][161] Unsuccessful attacks on both adult mule[162] and white-tailed deer (Odocoileus virginianus)[163] have been recently filmed but there is only a single account that mentions predation on an adult white-tailed deer.[78] Adult roe deer, being relatively modestly sized, are possibly taken with some regularity unlike adults of other deer species.[2] A handful of confirmed attacks on relatively large sheep, exceptionally including healthy adults, estimated to weigh around 52 to 70 kg (115 to 154 lb) have occurred in Scotland.[2] One study in Finland found reindeer calves, of an estimated average weight of 12 kg (26 lb), were routinely killed.[164] Adult female reindeer weighing 60 to 70 kg (130 to 150 lb) have been killed in 3 cases in central Norway.[165] Few records of predation on domestic cattle are known but a detailed examination of calf remains has shown that golden eagles in New Mexico, mainly wintering migrants, killed 12 and injured 61 weighing from 41 to 114 kg (90 to 251 lb) from 1987 to 1989.[95] No other living bird of prey has been verified to kill prey as heavy as this, although wedge-tailed, martial (Polemaetus bellicosus) and crowned eagles (Stephanoaetus coronatus) have been confirmed to kill prey estimated to weigh up to 50 kg (110 lb), 37 kg (82 lb) and 30 kg (66 lb), respectively.[5][166]

Other mammals

Golden eagle feeding on a red fox, Nasavrky, Czech Republic

Mice, rats and related species occasionally comprise an important portion of the golden eagle’s diet, making up 3.05% of prey throughout the range.[2] Most mouse-sized mammals are too small to serve as regular prey, but in marginal habitats they can become more important, with studies revealing they sometimes form more than 10% of nest remains.[2][91][130] Species preyed on include the North American deer mouse (Peromyscus maniculatus) on Santa Rosa Island and the Santa Cruz Islands, the Norway lemming (Lemmus lemmus) in northern Sweden, the brown rat (Rattus norvegicus) in the northern highlands of Scotland, great gerbils (Rhombomys opimus) in the Caspian Sea region of Kazahkstan,[127] and grass rats (especially Blick's grass rat, Arvicanthis blicki) and the big-headed mole rat (Tachyoryctes macrocephalus) in the Bale Mountains.[118]

Mammalian carnivores can compete for some of the same prey as golden eagles but can also themselves become prey, the main prey family of carnivores being the canids (dog family). 12 species of canid are known to have been hunted by golden eagles and they collectively make up about 2.8% of the diet from all surveyed golden eagle nests. For the most part, foxes are the preferred prey, usually as juveniles though foxes of any age or condition, including adult male red foxes (Vulpes vulpes) heavier than the eagles themselves, may be hunted.[167] Across much of both species’ ranges, red fox co-exist with golden eagles and eagles hunt them in small numbers. Red foxes make up 13.5% of nest remains in Sicily and 5.9% in the Republic of Macedonia.[168] In a nest in Mongolia, Corsac fox (Vulpes corsac) were surprisingly the main prey species, making up 38% of sampled remains.[96] kit fox (Vulpes macrotis), swift fox (Vulpes velox), gray fox (Urocyon cinereoargenteus) and island fox (Urocyon littoralis) are readily predated in North America.[91][98][169][170] The other family of carnivores that turns up with some regularity in the golden eagle’s diet are the mustelids, at about 2.3% of the diet.[2] Everything in this diverse family from least weasels (Mustela nivalis), the smallest mammalian carnivore, to wolverines (Gulo gulo), the largest terrestrial mustelid, have turned up as prey in golden eagle nests.[131][171] The members of this family which are most regularly hunted are probably the martens, such as the American marten (Martes americanus), pine marten (Martes martes) and beech marten. The young of larger species are also occasionally hunted in Scotland and Ireland, including Eurasian otters (Lutra lutra) and Eurasian badgers (Meles meles).[2] Similarly, amongst wolverines, “young and inexperienced” specimens are usually targeted as prey by golden eagles.[172] In North America, full-grown large mustelids, including American badgers (Taxidea taxus) and fishers (Martes pennanti), have also turned up as prey.[98][173] The largest percentage of mustelids was 13.2% in a nest in central Alaska.[105] Mustelids, mainly martens, turned up in large numbers in the French Alps where they made up 10.1% of nest remains.[111]

Cats are rarer in the eagle diet, forming 3.4% of nest remains in the French Alps and 2.2% in the Republic of Macedonia.[168] Mainly domestic cats (Felis catus) are taken but wildcats (Felis silvestris) are also known to be hunted.[2] Rarely, the bobcat (Lynx rufus) have been reported as prey in North America[98] and the remains from the much larger Eurasian lynx (Lynx lynx) (age undetermined) have been found in a nest in Sweden.[131] A handful of accounts from North America and Scandinavia describe golden eagles hunting and flying off with small cubs of American black (Ursus americanus) and brown bears (Ursus arctos), of an estimated weight of 3 to 5 kg (6.6 to 11.0 lb).[174] Similarly, the pups of spotted seals (Phoca largha) and harbor seals (Phoca vitulina) have reportedly been hunted.[175][176] In a singular event the eagle was photographed while killing prey as big as an adult sika deer.[177] The only other family of mammals to be important in the golden eagle’s diet is the hedgehogs, though these are only known as prey in Europe. The European hedgehog (Erinaceus europaeus) appears to be the most important prey species on the Swedish island of Gotland, making up 42.5% of sampled nest remains.[178] In Estonia, the southern white-breasted hedgehog (Erinaceus concolor) was the most significant prey species (28.6%).[135] The latter species was also the most important mammalian prey (13.8%) in the a small study in the Sarnena Sredna Gora Mountains of Bulgaria.[148]

Other mammals which in some locations comprise a minor diet component include dormice,[126] pocket gophers,[179] moles,[109] raccoons,[180] horses,[168] kangaroo rats, porcupines,[113] shrews, pikas, and chinchillas.[104] Over the whole of the golden eagle's range these prey items comprise less than 1% of nest remains, though this figure can be higher locally.

Other birds

Migratory and wintering flocks of Snow Geese can attract golden eagles

Although quantitatively outnumbered by mammals, birds are the most diverse class of prey in the golden eagle’s diet, as more than 200 species have been identified at eagle nests. Little analysis has gone into how regularly golden eagles will attack the nestling and fledglings of other birds, although it has been interpreted that this behavior is not uncommon.[3] Nestling-aged rock pigeons (Columba livia) have been observed as prey in golden eagle nests.[181] The first record of a golden eagle eating eggs was recorded when a golden eagle was observed consuming Canada goose (Branta canadensis) eggs in eastern Idaho.[182] After large galliforms, the next most significant group of prey among birds is the corvid family, making up 4.1% of the diet from around the range. Most prevalent among these are magpies and the large-bodied Corvus (crow and raven) genus. On Santa Rosa and Santa Cruz Islands in California, common raven became the most common prey species after feral pigs were eradicated from the latter, making up 24% of a 454 sample size from 14 nests.[91] The raven was also the most prevalent bird prey in central Arizona, in a sampling of 1154 from 119 nests.[179] The black-billed magpie (Pica hudsonia) was an important prey item in Washington state, making up 9.7% of the remains there, and was the most significant avian prey in Montana.[106][183] In 10 studies in Europe, corvids made up more than 5% of the prey remains, usually represented by carrion crows/hooded crows (Corvus corone/cornix), rooks (Corvus frugilegus), Alpine chough (Pyrrhocorax graculus), ravens or Eurasian magpies (Pica pica). Smaller species such as jays and nutcrackers only occasionally turn up as prey, mainly in North America, though the Eurasian jay (Garrulus glandarius) has also been known as prey.[2][98][131] The next best represented family of birds are the waterfowl, making up approximately 1.4% of the golden eagle’s breeding season diet. Waterfowl of all sizes from green-winged teal (Anas crecca)[131] to trumpeter swans (Cygnus buccinator),[98] tundra swans (Cygnus columbianus)[184] and mute swans (Cygnus olor)[133] have been successfully hunted by golden eagles. Full-grown swans can weigh well over 10 kg (22 lb) and are probably the largest birds habitually hunted by golden eagles.[2][137] Moderately sized species, including larger Anas ducks such as mallards (Anas platyrhynchos) and geese such as bean goose (Anser fabalis) are perhaps most often recorded. Waterfowl are mostly recorded during the nesting season in Northern Europe, comprising 15.4% of prey in Gotland and 14.4% of prey in Belarus.[134][178] In some years at Malheur-Harney Lakes Basin in Oregon, Anas ducks can make up to 20% of the prey consumed by the locally nesting eagles.[185] Mostly in the United States, wintering golden eagles may become habitually predators of wintering and migrating groups of waterfowl, with species such as Canada geese, cackling geese (Branta hutchinsii), snow geese (Chen caerulescens), and Ross's geese (Chen rossii). Since geese are found in large concentration when seasonal conditions require it, these species can be hunted with relative ease.[56][186][187] Waterfowl are typically hunted using the “contour flight with short glide attack” technique, in order to surprise the prey before it can take flight or dive. In one case, a golden eagle was able to capture a mallard as it took off in flight. Other water birds are generally less frequent prey but may become regular in the diet in marsh-like Northern regions and coastal areas. Scotland, being surrounded by coasts and possessing quite a wet climate, often hosts water birds which become prey such as colonies of petrels (largely Northern fulmar (Fulmarus glacialis)), making up to 17% of the recorded prey in 26 nests with a 119 sample size in the Outer Hebrides, migrating throngs of sandpipers and plovers (up to 5.9% and 2.8% in 25 nest in the northern Inner Hebrides) and gulls (making up a whopping 23% of prey recorded in 25 nests in the West-Central Highlands).[2] Among shorebirds, usually only larger types such as godwits, curlews, Tringa sp., stone-curlews and oystercatchers turn up as prey as smaller species are probably too flighty and agile to catch.[2][188] Eurasian cranes (Grus grus) are regularly predated in Northern Europe, turning up at 6.8% of nests in Estonia and 5.8% of nests in southern Finland.[132][135] Hooded cranes (Grus monacha) are reportedly prey for golden eagles in China.[189] Demoiselle cranes (Anthropoides virgo) have been caught in mid-air as they migrate over the Himalayas and both sandhill (Grus canadensis) and whooping cranes (Grus americana) may be hunted in North America.[190][191] The last known breeding pair of golden eagles in Maine (which did not return after 1999) were reported to hunt an unusually large number of herons, specifically great blue herons (Ardea herodias) and American bitterns (Botaurus lentiginosus). Elsewhere, herons are basically negligible in the diet.[2] Other water birds recorded as prey include cormorants (up to 8.6% of the recorded prey in Santa Rosa and Santa Cruz Islands), auks, grebes and loons.[2][91]

Other raptorial birds can sometimes become semi-regular prey, such as various hawks which are recorded largely in North America at locations such as Oregon (8.8% of prey remains) and Arizona. Owls may be hunted occasionally across almost the entire range (maximum being 2.9% in Oregon) and, more rarely, so may the falcons.[2] Rock pigeons may be hunted regularly in some parts of the golden eagle’s range (other pigeons and doves have been recorded as prey but are typically rare in the diet). The common feral pigeon was the second most prevalent prey species in Sierra Espuña Regional Park in Spain, making up 18.8% of a sampling of 99 from 5 nests.[89] The species was also prevalent in Slovakia, making up 7.4% of remains at nests there.[2] Non-corvid passerines are usually ignored as prey but large thrushes such as song thrush (Turdus philomelos), mistle thrush (Turdus viscivorus) and common blackbird (Turdus merula), are semi-regularly recorded prey in Europe. This family is most prevalent in Sicily, making up 8.1% of a sampling of 74 from 10 nests there,[116] 7.7% in Central Spain[107] and 7.2% in the French Alps.[111] The smallest-bodied bird family recorded as semi-regular prey are pipits. Meadow pipits (Anthus pratensis) are the most represented species and are taken mainly in Scotland, making up to 3.5% of prey in the Inner Hebrides.[2][192] Other bird families rarely recorded as golden eagle prey (making up less than 1% of prey in all studied nests) include starlings (maximum being 4.8% in the French Alps, negligible elsewhere), larks (maximum is 2.3% in the West-Central Highlands of Scotland), emberizid sparrows (up to 1.7% in central Alaska), woodpeckers (up to 1.5% in Alberta), cuckoos, bustards, icterids, shrikes and finches.[2]

Reptiles and amphibians

In arid Southeastern Europe, the Greek Tortoise is a preferred prey species

Snakes are the most common group of reptiles in the golden eagle’s diet, making up about 2.9% of the remains from all studied golden eagle nests,[2] though this figure can be much higher in parts of the range, as shown by studies in Japan (27.5%),[115] Sicily (25.7%, one snake species),[116] Arizona (14.9%, one snake species),[179] Kazakhstan (9.7%, one snake species),[127] and the French Pyrenees (9.7%).[93] Usually colubrid snakes (sometimes called “harmless snakes”) are hunted but venomous species are also occasionally taken, especially the prairie rattlesnake (Crotalus viridis) in North America and the European adder (Vipera berus) in Europe.[2]

In some areas tortoises are more important in the diet than snakes, and they displace mammals and birds as the most important prey group in most of Southeastern Europe as well as the Caucasus.[2] Hermann's tortoises (Testudo hermanni) and Greek tortoises (Testudo graeca) made up 55.4% of the prey in two nests in Bulgaria and 52.9% of the prey in 19 nests in the Republic of Macedonia.[148][168] The same species are also prevalent in the diet of eagles from Greece, but no quantitative analysis is known from that country.[193] Russian Tortoises (Agrionemys horsfieldii) made up 31.9% of the prey in 5 nests in Turkmenistan and 25.4% of the prey in 36 nests in Kazakhstan.[127][194] When hunting tortoise, golden eagles use a unique technique: they pick up the tortoise and fly up to at least 20 m (66 ft) above the rocky ground, then drop the tortoise onto the terrain below in the hope of smashing open its hard shell.[2] The only other bird species confirmed to hunt vertebrates in this way is the lammergeier (Gypaetus barbatus), although gulls may use a similar technique on hard-shelled prey such as mussels.[5] The larger desert tortoise (Gopherus agassizii) of North America has occasionally turned up as prey there.[195] Other turtles have been reported as being hunted, such as painted turtles (Chrysemys picta) in Washington state and baby loggerhead sea turtles (Caretta caretta) in Corsica, but essentially are a negligible part of the diet.[56][90]

Lizards turn up with some regularity as prey in some of the hotter, drier portions of the golden eagle’s range. In Europe, lizards made up to 13% of the prey remains in the French Pyrenees and 12.7% in the Italian Apennines.[93][113] Fairly large lizards, including the monitor lizard, make the most significant contribution to the golden eagle diet.[196] The most important lizard prey in Europe is the Lacerta genus, i.e. the Ocellated Lizard (Lacerta lepida).[197] Spiny-tailed lizards of the genus Uromastyx are also of particular note in the golden eagle’s diet; they are reportedly the single most important prey for golden eagles in Northeast Africa.[2] In the particularly sparse Negev Desert of Israel, the Egyptian spiny-tailed lizard (Uromastyx aegyptia) apparently comprised 89% of the diet, the most a single species has been reported to dominate a golden eagle’s diet locally.[198][199]

Amphibians, specifically Rana frogs, have been reported in only two locations across the golden eagle’s range: in the French Pyrenees, where they made up 2.7% of the diet, and the West-Central Highlands of Scotland, where they made up 0.7%.[2][93]

Other prey

Fish have not been mentioned as prey in Eurasia in dietary studies of nesting golden eagles. However, they have been reported in nests in North America. The two largest known reports are from coastal California and along the Porcupine River in Alaska, with fish making up 3.6% and 3.5% of the diets, respectively.[3][74] Most reported species are trout and salmon from the Salmo and Oncorhynchus genera, although other species have also been hunted, including suckers (Catostomus), Sacramento perch (Archoptiles interruptus) and the northern pike (Esox lucius).[2][3][74] Fish have apparently been captured in Scotland on rare occasions, although they have not been reported in the breeding season diet.[2] Golden eagles have been observed around the Sea of Okhotsk (especially along Northern Japan) to join large numbers of white-tailed eagles (Haliaeetus albicilla) and Steller's sea eagles (Haliaeetus pelagicus) in winter to scavenge and capture various, locally abundant fish amongst ice-floes. Perhaps the most improbable prey reported in the golden eagle’s diet are insects. They’ve been reported in small quantities in the French Pyrenees, the Italian Alps and Bulgaria.[93][126][148] There is no information on how golden eagles capture insects or what kind of insects they hunt, although slower, larger, terrestrial insects like large beetles seems likely.[2]

Interspecific predatory relationships

Peregrine Falcons are often attracted to the same cliff habitat as golden eagles

One of the most fascinating, though relatively little studied, aspects of the golden eagle’s biology is how it interacts with other predators in a natural environment, especially other large predatory birds. The golden eagle is a powerful hunter with few avian rivals in size or strength, although what it gains in these areas it loses somewhat in its agility and speed. Golden eagles are avian apex predators, meaning a healthy adult is not generally preyed upon. There are several other large birds of prey that inhabit the Northern Hemisphere that may be attracted to the same prey, habitats and nesting sites as the golden eagles. Two examples are the common raven and peregrine falcon (Falco peregrinus) as these are two fairly large-bodied, mostly predatory birds that co-exist with golden eagles in almost every part of their range, although the former occurs in much larger numbers and the latter has a much larger natural distribution in more varied habitats. Both the raven and the peregrine are often attracted to much the same precipitous habitat as the golden eagle. However, both are generally dominated by the much larger eagle and will actively avoid nesting in the same area as a golden eagle pair.[200][201][202] Ravens and peregrine falcons have both been robbed of food on a few occasions by golden eagles.[203][204][205] Both species have turned up widely in the diet of golden eagles, much more so the raven than the peregrine.[91][206] However, these conspecific species follow no set "rules". Ravens often vigorously and successfully displace golden eagles from their nesting territories.[207] Peregrine falcons have also displaced golden eagles from their nesting territories and, in one instance in Utah, even killed a golden eagle that had breached the area of their nest.[208] In another case, a peregrine falcon kleptoparasitized a willow ptarmigan from a golden eagle.[209] In a study of four cliff-nesting raptor species in Spain, the golden eagle was not observed to engage in agnostic encounters with the other species but the peregrine falcons but were dominated by and occasionally killed by a relatively small race of Eurasian eagle-owls (Bubo bubo hispanus).[210] Both ravens and peregrine falcons appear to easily outmaneuver golden eagles in flight.[2]

Other raptors, large and small, are often outright dominated by golden eagles. In Eurasia, the relative paucity of mid-sized Buteo species is thought to be due to the radiation of large Aquila eagles that already occupied the large raptor niche and outcompeted them. In North America, where there are no Aquila eagles other than the golden eagle, Buteo species are more diverse with at least 3 large hawks (the red-tailed, Swainson's (Buteo swainsoni) and ferruginous hawks (Buteo regalis)) that occur in similar habitats and sometimes eat similar prey as both each other and golden eagles.[2] On occasion, golden eagles will kleptoparasitize or even predate these three Buteos.[77][185][211] Other small to medium-sized diurnal raptors are known to occasionally be hunted by golden eagles, including ospreys (Pandion haliaetus),[212] Black kite (Milvus migrans),[3] hen harriers (Circus cyaneus),[183] Eurasian sparrowhawks (Accipiter nisus),[2] Cooper's hawks (Accipter cooperii),[98] northern goshawks (Accipiter gentilis),[131] red-shouldered hawks (Buteo lineatus),[213] common buzzards (Buteo buteo),[2] long-legged buzzards (Buteo rufinus),[196] upland buzzards (Buteo hemilasius),[96] rough-legged buzzards,[131] Eurasian kestrels (Falco tinnunculus),[126] American kestrels (Falco sparverius), merlins (Falco columbarius),[214] prairie falcons[74] and gyrfalcons (Falco rusticolus).[45] Occasionally, golden eagles may even boldly predate these smaller species during migration in mid-flight, as was recorded at Hawk Mountain Sanctuary with a red-shouldered hawk.[215] It is thought that on rare occasions, more nimble, smaller raptors like rough-legged buzzards and harriers can kleptoparatisize golden eagles.[216] Interestingly, northern goshawks in Scandinavia rely on grouse to about the same extent as golden eagles there and even hunt in similar forest edge habitata there. However, the goshawks primarily hunt neonatal and fledgling grouse (30% and 53% of their take of grouse) rather than adult grouse (17% of their prey) in late summer when grouse dominate other birds in their diet, whereas the eagles seem to mainly adult grouse and hunt them aseasonally.[217] Similarly, hen harriers may become somewhat specialized at taking young red grouse, whereas the eagles are more likely to take adults.[218]

Due to the potential danger to themselves and their offspring, almost every other type of raptorial bird readily mobs golden eagles.[45] Gyrfalcons, skuas, and Buteos like rough-legged buzzards, which are normally fierce competitors with each other, have worked together to group-mob golden eagles that have passed their adjacent nesting areas.[45] Occasionally smaller raptors, especially large falcons, will drive eagles to the ground. The eagle typically ignores attacks from smaller species or at least leaves their home ranges, but will occasionally roll and extend talons toward chasing individual often without displaying active predatory behavior. If physical contact occurs, it sometimes ends in injury, death and/or consumption of the attacking species.[219] Remarkably, the nest defense by golden eagles themselves is often rather passive towards other raptorial birds, perhaps since other species may fear predation on themselves if they approach a golden eagle territory. There are exceptions to this, however, when particularly bold, large raptorial birds seemingly range too far into a pair’s home range. One pair of golden eagles killed but did not eat 3 great horned owls (Bubo virginianus), implying that they were killed in nest defense and not in predation.[28] In one of the few cases of a golden eagle killing but not eating another large raptor, an adult golden eagle was observed killing a ferruginous hawk which apparently came too close to its nest.[220]

Golden eagles, being habitual scavengers at carrion, come into conflict with scavenging birds as well. They may encounter corvids at carrion sites and the large passerines are often highly cautious, either feeding some distance from the eagle or waiting until the eagle is done eating, lest they be predaceously grabbed.[2] Occasionally, however, corvids may behave more boldly around golden eagles. In one case, a group of three black-billed magpies were observed to rob a golden eagle of its prey.[221] Golden eagles also usually dominate both New World and Old World vultures. Smaller species like black vulture (Coragyps atratus) in North America and Egyptian vulture (Neophron percnopterus) in Eurasia are known to be occasionally hunted by golden eagles.[222][223] Besides the Steller's sea eagle, the larger species of Old World vulture and the California condor (Gymnogyps californianus) are the only noticeably larger raptorial birds that a golden eagle may encounter. The relationship between these large vultures and golden eagles can be highly contentious, with most sources favoring victory for the eagle due to its more aggressive disposition, much stronger feet and sharp talons. In one case, a colony of griffon vulture (Gyps fulvus) was exceptionally able to displace a pair golden eagles from a cliff where they were trying nest so they could nest there themselves.[224] On the other hand, a 5-year-old griffon vulture reintroduced into the wild in Bulgaria was thought to have been killed by a golden eagle.[225] The golden eagle is a potential predator of the lammergeier which is often attracted to much the same habitat and prey as the eagle but often feeds largely on bone marrow from carcasses.[226] It has been observed that both golden eagles and lammergeiers will readily pirate food away from each other.[227] In the Southwestern United States, several conflicts between huge California Condors and golden eagles have been observed. At carrion, victory during conflicts can go either way apparently.[228][229][230] Recent observation have suggested that the critically endangered condors do not seem to actively defend their nesting territories from golden eagles but have been recorded gaining incidental protection from prairie falcons and common ravens, which both aggressively displaced the eagles from their own adjacent territories.[231] The golden eagle is considered an occasional predator of California condors (especially fledglings) but there seems to be little in the way of eyewitness accounts to confirm this. It is possible that (as in the Bulgarian griffon vultures) the California condors have lost their natural cautiousness around eagles due to having been reintroduced from captivity.[232]

Owls, which may have dissimilar periods of activity, are not entirely exempt from golden eagle predation either.[233][234] Several species have been recorded as prey, especially in Northern Europe, Slovakia (where owls make up 2.4% of prey remains) and some parts of the Western United States.[2] Species hunted have ranged in size from tiny Eurasian pygmy owl (Glaucidium passerinum) and little owl (Athene noctua) to the formidable, large great horned, Eurasian eagle and great gray owls (Strix nebulosa).[96][131] Barn Owls (Tyto alba) have been predated by golden eagles in both North America and Eurasia.[98][107] On Santa Rosa and Santa Cruz Islands, barn owls made up 4.5% of the diet, the largest known occurrence of that species in the eagles’ diet.[91] However, the great horned owl is the owl species that occurs most regularly in the diet of North American golden eagles.[98][183] In Europe, golden eagles are relatively minor predators of owls compared to Eurasian eagle-owls and northern goshawks. The primary owls hunted by European eagles are Ural owls (Strix uralensis) and short-eared owls (Asio flammeus).[235][236] Few eye-witness accounts are known of owl predation, but one violent mid-day confrontation between a golden eagle and an adult Great Horned Owl was witnessed in Jefferson County, Colorado. Although this owl was about a third of the eagle’s weight, it was able to land some damaging blows on the larger bird. In the end, despite its wounds, the eagle was victorious over the dead owl.[237] A case of kleptoparasitism by a Golden Eagle on a Great Horned Owl has been recorded.[238]

In the Palearctic, the golden eagle co-exists with several other large raptorial birds called eagles. Unlike in their relationship with smaller raptors, golden eagles are rarely reported to hunt other eagle species. Most conflicts between different eagles are over kills or carrion, though some species will defend nesting territories from each other. When it comes to carrion and kills, usually the “aggressor” (that is the eagle who initiates the agnostic behavior) is victorious over the other eagle.[239] Considering that that they overlap considerably in both distribution and prey selection, there is remarkably almost no known accounts from Eurasia of golden eagles behaving aggressively with other Aquila eagles. This is possibly due to segregated habitat preferences amongst these species, with lesser and greater spotted eagles being essentially woodland or wooded wetland-dwelling birds, Spanish and Eastern imperial eagles inhabiting fragmented, open woods and steppe eagles generally living in flat, often treeless steppe and desert-like habitats. In central Asia, where golden eagles and other Aquila species are rare or absent, Steppe Eagles sometimes nest in mountainous rock formations like golden eagles.[2] Compared to these eagles, the Bonelli's eagle appears to have a more contentious relationship with golden eagles, as they are often attracted to similar prey (largely rabbits and gamebirds) and occur in abutting habitats. The larger golden eagle is apparently dominant and, in at least one case, has killed and consumed a Bonelli's.[240][241] The two species defend their territories from one another exclusively, in the same way they defend them from others of their own species. Both will use nests built by the other species, more so the golden using Bonelli's nests than vice versa.[242] Further east, in Israel, Bonelli's and golden eagles are competitors as well. In the dry, barren Negev Desert, golden eagles nests were found 13 km (8.1 mi) apart and Bonelli's were scarce. In the Judaean Desert, which has more annual rainfall and more available prey, the distance between golden eagle nests averaged 16 km (9.9 mi) and the Bonelli's eagle easily outnumbered them. Apparently, the Bonelli's eagle exceptionally outcompeted its larger cousin here due to a subtle topographic variation in the habitat.[243] Still, in areas without unnatural human pressures, the gradation between these species is still adequate enough that they can exist without serious negative effect on either population.[244] In Japan, golden eagles were observed to hunt mountain hawk-eagles (Nisaetus nipalensis), despite the fact that the local race of hawk eagle is around the same mass as a Japanese golden eagle.[137][245] The Bale Mountains of Ethiopia is arguably the richest biosphere the golden eagle is known to inhabit and there, this species may potentially interact with more than 10 other eagle species and nearly 30 other accipitrid species, including both migratory and resident breeding species. These are often supported by an exceptionally diverse, dense population of rodents there. The Bale Mountains golden eagles were recorded kleptoparatisizing steppe eagles, augur buzzard (Buteo augur), pallid harrier (Circus macrourus) and lanner falcons (Falco biarmicus) and the eagles were themselves kleptoparatisized by steppe eagles on three occasions. The golden eagles were also observed to chase tawny and steppe eagles out of their nesting territories. In the Bale Mountains, the golden eagle overlaps with perhaps its closest living relative, the Verreaux's eagle. The golden eagles appear to vigorously defend their territories from the Verreaux's eagles and, as in its relationship with the Bonelli's, the two species appear to maintain exclusive territories. Several chases involving the golden chasing Verreaux's were witnessed but only one where a Verreaux's chased off the golden. Despite several reported agnostic interactions, no other raptors were found as prey in the Bale Mountains nests of golden eagles.[118]

Large gathering of golden eagles sharing carrion while two Steller's sea eagles look on

Perhaps the most formidable raptorial birds that the golden eagle co-exist with are the large northern Haliaeetus sea or fish eagles. Two species, the white-tailed eagle and the bald eagle, overlap in distribution frequently with golden eagles. Both are marginally heavier on average than the golden, especially the white-tailed eagle, which tends to have a slightly longer wingspan as well. There are many differences in the dietary biology of these species as they primarily eat fish, occasionally supplemented by water birds or other semi-aquatic prey, and obtain more of their food via scavenging of dead or injured animals or via kleptoparasitism than golden eagles do. They also prefer nesting in large trees by the shore of a body of water, often in lowland areas, quite different from the upland, often mountainous nesting habitat preferred by golden eagles.[246] However, conflicts between the great eagles do occur, especially at carrion or kills. In most cases, the golden eagle is reportedly dominant over white-tailed eagles at food, reportedly being more aggressive and a more agile and swift flier.[239] However, in some cases, white-tailed eagles have dominated golden eagles at carrion or kills.[247][248] Competitions between golden and bald eagles in North America can reportedly go either way, depending on the size and disposition of the eagles involved.[45][249] In Arizona, bald eagles were successful in 25% of attempts to rob golden eagles of their prey.[250] Conflicts between white-tailed eagles and golden eagles over nesting sites occur with some frequency in some areas, but are unknown in other areas. In one instance in Norway, a pair of golden eagles harassed a newly formed pair of white-tailed eagles so persistently that the White-tails abandoned their nesting attempt.[251] This relationship between the two eagle species is particularly aggressive in Scotland where the white-tailed eagle has been reintroduced recently. In the Scottish Highlands, a white-tailed eagle was found dead with talon piercings in its skull likely from a female golden eagle it had been interacting with the previous day.[252] There are at least two known instances of Scottish white-tailed eagles fiercely attacking golden eagles in apparent territorial bouts, in one instance pulling the golden down into shallow coastal waters to drown.[253] White-tailed eagles are able to survive on a great breadth of food and have longer guts so can live on less food for a longer time than golden eagles. Thus, the white-tailed may locally outcompete the golden eagle in terms of populating a given area.[239] In North America, few such contentious relations have been reported between bald and golden eagles but golden eagles were reported to avoid the vicinity of active bald eagle nests.[28] Even in areas where conflicts between white-tailed and golden eagles do occur it is thought that, since the species are very different in their preferred foods and nesting sites, they do not adversely affect the other at a population level.[254] A much more passive interspecies guild of eagles have been observed in a wooded area of Kazakhstan, consisting of white-tailed, Eastern imperial and golden eagle. This guild was seen to freely use abandoned nests built by the other two species. Unlike in Scotland and Norway (or in a guild of Buteos in the American Midwest), no aggression or conflicts were reported between the three large eagle species even though they sometimes nested within 475 m (1,558 ft) of one another. All observed conflicts in the Kazakhstani woods were amongst others of their own species.[255] On the Scottish Isle of Mull, golden and white-tailed eagles also nested in close proximity without any observed aggressive encounters.[256] The Steller's sea eagle co-exists with golden eagles in coastal Japan and Russia. This species is the heaviest living eagle and may weigh twice as much as the small-bodied Japanese golden eagles.[137] Occasionally, both golden and white-tailed eagles join Steller's sea eagles to feed on fish caught along ice floes in the Sea of Okhotsk. The Steller's, being more aggressive in disposition than its bald and white-tailed cousins,[3] appears dominant over golden eagles here and has been photographed repeatedly displacing them from fish.[257][258] The smaller Pallas's sea eagle (Haliaeetus leucoryphus) also overlaps in range with golden eagles but there is no published information on their relationship.[3]

An inaccurately rendered but striking golden eagle in battle with a red fox

Mammalian carnivores are potential competitors with golden eagles for food. Foxes may prey on the same species in a given area but, as they are occasionally hunted by the eagles and are mainly nocturnal, they tend to avoid direct conflicts.[2][91][126] Though rare, golden eagles may sometimes pirate prey from various species of fox.[157][259] In North America, coyotes (Canis latrans) may engages in agnostic interactions with golden eagles. Despite being around three times as heavy, lone coyotes are seemingly dominated by golden eagles at many carrion or kill sites.[260][261][262] In the Greater Yellowstone, golden eagles were ranked as the second most dominant scavenger at wolf kills during winter (when bears are hibernating) after coyote packs but above bald eagles.[87] In one instance, a healthy adult male coyote was attacked and killed by a golden eagle. This coyote weighed 13.5 kg (30 lb) after the eagle had consumed some of its internal organs.[263] Conflicts between golden eagles and Scottish wildcats have been reported on rare occasions. In one instance, a golden eagle trying to hunt some wildcat kittens was intercepted by the mother wildcat. In the ensuing violent battle, the wildcat mother and the eagle both died.[253] In Northern California, golden eagles have pirated lambs killed by bobcats, a species which has itself occasionally turned up in the eagle’s diet.[98][264] Per one anecdote from the Himalayas, a golden eagle reportedly tried to predaceously attack a snow leopard (Panthera uncia) and was killed by the much larger felid, although this could have actually been a case of an overly bold eagle trying to displace a large predator from its own home range.[265] Adult wolverines appear to be one of the few conspecific mammal carnivores to actively pose a threat to golden eagles. Wolverines were observed to prey on nestling golden eagles in Denali National Park.[55] During incubation in Northern Sweden, an incubating adult golden eagle was killed on its nest by a wolverine.[266] In Yellowstone National Park, one prey item amongst 473 recorded for Cougars (Puma concolor) was a golden eagle, although no details of the attack or of the age of the bird were reported.[267] grizzly bear (Ursus arctos horribilis) have also reportedly killed and eaten golden eagle nestlings in Denali National Park.[45] The grizzly is reportedly one of the few mammals to trigger a strong aggressive reaction from the parent eagles when spotted close to the nest, as eagles have been observed to strike the bruins about the head and neck with their talons.[45][268]

Activity and movements

In Spain, golden eagles such as this one in the Province of Ávila are sedentary.

Despite the dramatic ways in which they attain food and interact with raptors of their own and other species, the daily life of golden eagles is often rather uneventful. In Idaho, adult male golden eagles were observed to sit awake on a perch for an average of 78% of daylight, whereas adult females sat on nest or perched for an average of 85% of the day.[269] During the peak of summer in Utah, hunting and territorial flights occurred mostly between 9:00 and 11:00 am and 4:00 and 6:00 pm, with the remaining 15 or so hours of daylight spent perching or resting.[23] When conditions are heavily anticyclonic, there is less soaring during the day.[2] During winter in Scotland, golden eagles soar frequently in order to scan the environment for carrion.[2] In the more wooded environments of Norway during autumn and winter, much less aerial activity is reported, since the eagles tend to avoid detection by actively contour-hunting rather than looking for carrion.[270] Golden eagles are believed to sleep through much of the night. Although usually highly solitary outside of the bond between breeding pairs, exceptionally cold weather in winter may cause eagles to put their usual guard down and perch together. The largest known congregation of golden eagles was observed on an extremely cold winter’s night in eastern Idaho when 124 individuals were observed perched closely along a line of 85 power poles.[271]

Migration

Most populations of golden eagles are sedentary, but the species is actually a partial migrant. Golden eagles are very hardy species, being well adapted to cold climates, however they cannot abide declining available food sources in the northern stretches of their range. Eagles raised at latitudes greater than 60° N are usually migratory, though a short migration may be untaken by those who breed or hatch at about 50° N.[272] During migration, they often use soaring-gliding flight, rather than powered flight.[272] In Finland, most banded juveniles move between 1,000 and 2,000 km (620 and 1,240 mi) due south, whereas adults stay locally through winter. Further east, conditions are too harsh for even wintering territorial adults.[2] Golden eagles that breed from the Kola peninsula to Anadyr in the Russian Far East migrate south to winter on the Russian and Mongolian steppes, and the North China Plains. The flat, relatively open landscapes in these regions hold relatively few resident breeding golden eagles.[273] Similarly the entire population of golden eagles from northern and central Alaska and northern Canada migrates south. At Mount Lorette in Alberta, approximately 4,000 golden eagles may pass during the fall, the largest recorded migration of golden eagles on earth.[274] Here the mountain ranges are relatively moderate and consistent, thus being reliable for thermals and updrafts which made long-distance migrating feasible.[274] Birds hatched in Denali National Park in Alaska traveled from 818 to 4,815 km (508 to 2,992 mi) to their winter ranges in western North America.[272] These western migrants may winter anywhere from southern Alberta and Montana to New Mexico and Arizona and from inland California to Nebraska. Adults who bred in northeastern Hudson Bay area of Canada reached their wintering grounds, which range from central Michigan to southern Pennsylvania to northeastern Alabama, in 26 to 40 days, with arrival dates from November to early December.[275] The departure dates from wintering grounds are variable. In southwestern Canada, they leave their wintering grounds by April 6 to May 8 (the mean being April 21); in southwestern Idaho, wintering birds leave from March 20 to April 13 (mean of March 29); and in the Southwestern United States, wintering birds may depart by early March.[2][272][276] Elsewhere in the species' breeding range, golden eagles (i.e. those who breed in the contiguous Western United States, all of Europe but for Northern Scandinavia, North Africa and all of Asia but for Northern Russia) are non-migratory and tend to remain within striking distance of their breeding territories throughout the year.[5] In Scotland, among all recovered, banded golden eagles (36 out of 1000, the rest mostly died or disappeared) the average distance between ringing and recovery was 44 km (27 mi), averaging 63 km (39 mi) in juveniles and 36 km (22 mi) in older birds.[2] In the dry Southwestern United States, golden eagles tend to move to higher elevations once the breeding season is complete.[28] In North Africa, populations breeding at lower latitudes, like Morocco, are mostly sedentary, although some occasionally disperse after breeding to areas outside of the normal breeding range.[277]

Territoriality

Two golden eagles in an aerial conflict over their home ranges, the upper bird clearly a juvenile.

Territoriality is believed to be the primary cause of interactions and confrontations between non-paired golden eagles. Golden eagles maintain some of the largest known home ranges (or territories) of any bird species but there is much variation of home range size across the range, possibly dictated by food abundance and habitat preference. Home ranges in most of the range can vary from 20 to 200 km2 (7.7 to 77.2 sq mi).[278] In San Diego County in California, the home ranges varied from 49 to 137 km2 (19 to 53 sq mi), with an average of 93 km2 (36 sq mi).[279] However, some home ranges have been much smaller, such as in southwestern Idaho where, possibly due to an abundance of jackrabbits, home ranges as small as 4.85 km2 (1.87 sq mi) are maintained.[280] The smallest known home ranges on record for golden eagles are in the Bale Mountains of Ethiopia, where they range from 1.5 to 9 km2 (0.58 to 3.47 sq mi).[118] 46% of undulating displays in Montana occurred shortly after the juvenile eagles left their parents range, suggesting that some residents defend and maintain territories year-round.[281] Elsewhere it is stated that home ranges are less strictly maintained during winter but hunting grounds are basically exclusive.[2] In Israel and Scotland, aggressive encounters peaked from winter until just before egg-laying and were less common during the nesting season.[2][243] Threat displays include undulating flight and aggressive direct flapping flight with exaggerated downstrokes.[24] Most displays by mature golden eagles (67% for males and 76% for females) occur, rather than around the nest, at the edge of their home ranges.[269] In Western Norway, most recorded undulating flight displays occur during the pre-laying period in late winter/early spring.[270] Display flights seem to be triggered by the presence of other golden eagles.[2] The use of display flights has a clear benefit in that it lessens the need for physical confrontations, which can be fatal.[269] Usually, non-breeding birds are treated aggressively by the golden eagle maintaining their home range, normally being chased to the apparent limit of the range but with no actual physical contact.[2] The territorial flight of the adult golden eagle is sometimes preceded or followed by intense bouts of undulating displays. The invader often responds by rolling over and presenting talons to the aggressor. Rarely, the two eagles will lock talons and tumble through the air; sometimes fall several revolutions and in some cases even tumble to the ground before releasing their grip.[282][283] In some parts of the Alps, the golden eagle population has reached the saturation point in appropriate habitat and apparently violent confrontations are more common than in other parts of the range.[284] Golden eagles may express their aggression via body language while perched, typically the adult female when confronted by an intruding eagle: the head and body are upright, feathers on head and neck are erect; the wings may be slightly spread and beak open; often accompanied by intense gaze. They then often engage in a similar posture with wings spread wide and oriented toward the threat; sometimes rocking back on tail and even flopping over onto the back with talons extended upward as defense. Such behavior may be accompanied by wing slap against the threatening intruder.[24] When approached by an intruder, the defending eagle turns away, partially spreads tail, lowers head, and remains still; adults on the nest may lower head and “freeze” when approached by a person or a helicopter.[24] Females in Israel displayed more than males and mostly against interspecific intruders; males apparently displayed primarily as part of courtship.[243] Five of 7 aggressive encounters at carcasses during winter in Norway were won by females; in 15 of 21 conflicts, the younger bird dominated an older conspecific.[239] However, obvious juvenile eagles (apparent to the adult eagles due to the amount of white on their wings and tail) are sometimes allowed to penetrate deeply into a pair’s home range and all parties commonly ignore each other.[104][285] In North Dakota, it was verified that parent eagles were not aggressive towards their own young after the nesting period and some juveniles stayed on their parents territory until their 2nd spring and then left by their own accord.[286]

Reproduction

Eyrie (in hollow at left center) in the Valley of the Siagne de la Pare, Alpes-Maritimes, France

Golden eagles usually mate for life. A breeding pair is formed in a courtship display. This courtship includes undulating displays by both in the pair, with the male bird picking up a piece of rock and dropping it only to enter into a steep dive and catch it in mid-air, repeating the maneuver 3 or more times. The female takes a clump of earth and drops and catches it in the same fashion. Small sticks may also be used in this display.[2] Compared to the bald eagle, golden eagles do not repeat courtship displays annually (which is believed to strengthen pair bonds) and rarely engage in talon-locking downward spirals.[45] Golden eagles typically build several eyries within their territory and use them alternately for several years. Their nesting areas are characterized by the extreme regularity of the nest spacing.[287] In 9 studies of annual nest spacing, the average minimum distance between nests range from 16 km (9.9 mi) apart in Norway to 8 km (5.0 mi) apart in Switzerland.[149][288] Nests in Scotland may found at anywhere from 10 to 65 pairs per 1,000 km2 (390 sq mi), with an average of over 20 pairs found per area.[2] In much of continental Europe, densities of less than 10 pairs per 1,000 km2 (390 sq mi) are typical.[2] In the United States different areas had from 10 to more than 20 pairs on average per 1,000 km2 (390 sq mi). Wyoming had the greatest densities of breeding golden eagles of any complied study, though numbers were comparable to western Scotland as there were an average of just over 20 pairs per 1,000 km2 (390 sq mi), with greatest estimated densities of possibly 125 per area.[289] In Wyoming, the distance between nests ranged from 3.1 to 8.2 km (1.9 to 5.1 mi), averaging 5.3 km (3.3 mi).[289] In the wooded peatlands of Sweden and Belarus, a maximum of 5 pairs appear to occur per 1,000 km2 (390 sq mi).[2] In Quebec, the distance between nests ranged from 8 to 44.7 km (5.0 to 27.8 mi).[290] In the Snake River canyon in Idaho, nests are 5 to 8 km (3.1 to 5.0 mi) apart, while two other nearby studies in Idaho found the average distance were 4.3 km (2.7 mi) and 4.39 km (2.73 mi), respectively.[291][292] The nesting density for a breeding population near Livermore, California and the Altamont Pass Wind Farm is among the highest in the world for golden eagles, with at least 44 pairs in 1997, a density of one pair per 19 km (12 mi).[293][294] Due to the consistency of use by golden eagle pairs, population densities change generally happens only quite gradually.[2]

Nest requirements

The cliff nest of a golden eagle

Golden eagles seem to prefer to build their nests on cliffs where they are available.[149] Nests are generally located at around half of the maximum elevation of the surrounding land. This height preference may be related to having the ability to transport heavy prey downhill rather than uphill. A massive benefit to cliff nests is that they tend to be largely or entirely inaccessible to mammalian predators on foot (including humans).[2] In Spain, studies revealed the preferred sites of golden eagle nests were on inaccessible cliffs at a great distance from tracks, roads and villages.[295] 95.6% of 410 nests built in Scotland were on cliffs.[296] Similarly, in Bulgaria, Italy, Switzerland, France and Yugoslavia, more than 90% of golden eagles nests were located on cliffs.[111][168][297][298] Greek nests are mostly on cliff ledges, but in the Evros District, nearly 30% of the population nests on trees, mostly pines.[193] Rural, arid areas of Europe such as the Iberian peninsula, Provence in France and the Apennines in Italy, fire combined with pastoral activity has maintained suitable nesting sites at relatively low elevations.[2] In the Spanish Pyrenees, however, nest were found at up to 1,500 m (4,900 ft) in elevation.[108] 80% of nests in Spain are on rocky cliffs, the remaining 20% being in trees.[299] Rocky, low mountainous areas are used for nesting in Israel.[53] All known nests in Iraq have been on cliffs.[300] In Norway, the mean elevation of nests was 500–600 m (1,600–2,000 ft).[288] Throughout the Baltic States and eastern Fennoscandia, golden eagles inhabit relatively flat wooded peatlands and, here, the local golden eagles generally nest in trees. Tree nests are known to be used exclusively in Estonia and Belarus.[301][302] On the island of Gotland in Sweden, the trees holding nests had an average trunk diameter of more than 55 cm (1.80 ft) and an average height of 17.2 m (56 ft) with the nest being located at an average of 11.7 m (38 ft) above the ground.[303] Cliff nests are preferred as nesting sites in most of North America.[304] In Northern California, tree nests were apparently mainly used.[61] In the forested landscape of western Washington, where large clear cut areas having provided suitable hunting habitat, golden eagles almost exclusively nest in Douglas firs (Pseudotsuga menziesii) at forest edge. There, the nest-hosting tree were 38 to 72 m (125 to 236 ft) with the nest being located at a height of 20 to 64 m (66 to 210 ft).[305] In a study of 170 eyries in the state of Wyoming, 111 were on deciduous trees, 36 in ponderosa pines (Pinus ponderosa) and 23 on sides of buttes or bluffs along river. Cottonwood and willow trees dominated the trees selected, averaging 73 cm (2.40 ft) in diameter and 13.4 m (44 ft) in height.[305] In Wyoming, the tree nests are often the tallest tree in a stand and are in a small or isolated woodlot less than 500 m (1,600 ft) away from large clearcuts or fields.[305] Ground nest are rare in Scotland but not uncommon in the United States, especially in arid areas of states such as Nevada, North Dakota and Wyoming. Ground nests typically occur on lofty hills where have little ground vegetation and require adults to have a good protective all-around view.[2][305][306][307] Other exceptional nest sites known in North America have included river banks,[308] abandoned gold dredges[55] and electrical transmission towers.[61]

Both heavy rain and excessive heat can potentially kill nestlings, so golden eagles often place their nests to suit the local climate.[2] In Northern areas, such as Alaska, sun exposure (Southern orientation) may help nesting success, while those nesting in hot, lowland Utah and arid regions of Israel were mainly northerly facing to keep the nest out of the hot glare of the sun.[243][309] Almost all established breeding golden eagles build more than one nest.[2] A typical range of nests per pair is between 2 and 5. In Sweden, pairs on average built 2.4 of them.[303] There is an average of about 4.5 nests per pair in Scotland.[2] In Idaho, there was an average of 6 nests per pair.[291] In more exceptional cases, up to 13 nests have built by a single pair in Scotland and up to 12 by a single pair in Idaho.[2][291] Some pairs utilize alternate nest sites every year, others apparently rarely use alternate nests.[2] Some golden eagle pairs may not use a nest for up to six years after its construction.[2] Golden eagle nests usually consist of heavy tree branches, upholstered with grass when in use. As is typical of a large accipitrid, the nests of golden eagles are very large. However, they are smaller on average than bald eagle nests.[310] In Kazakhstan, golden eagle nests were similar in size to white-tailed eagle nests.[255] In the Isle of Mull in Scotland, however, white-tailed eagle nests were slightly larger as they used thicker, larger sticks lined with grass that make a more massive nest than the isle's golden eagle nests, which used thinner sticks or heather and lined their nest with woodrush.[2] In Arizona, golden eagle nests averaged 175.7 cm (69.2 in) in length (ranging from 121.9 to 264.2 cm (48.0 to 104.0 in)), 119.8 cm (47.2 in) in width (range of 83.8–203.2 cm (33.0–80.0 in)), and 65.0 cm (25.6 in) in height (range of 12.7–200.7 cm (5.0–79.0 in)). The sticks used in the Arizona nests ranged up to a length of 185.4 cm (73.0 in), a diameter of 5.3 cm (2.1 in) and a weight of 980 g (2.16 lb).[310] Nests in Scotland average 133 cm (52 in) in length, 106 cm (42 in) in width and 79 cm (31 in) in depth.[2] In Washington state, tree nests averaged 90 cm (35 in) in depth and 1.2 to 1.5 m (3.9 to 4.9 ft) in diameter.[310] Nests in Sweden averaged 140 cm (55 in) in length, 140 cm (55 in) in width and 110 cm (43 in) in depth.[311] Nests in Kazakhstan averaged 148.3 cm (58.4 in) in length, 115.7 cm (45.6 in) in diameter and 48 cm (19 in) in depth.[255] The nest of the golden eagle may weigh well over 250 kg (550 lb).[312] As the eagles use a nest repeatedly, they repair their nests whenever necessary and enlarge them during each use. If the eyrie is situated on a tree, supporting tree branches may break because of the weight of the nest. The largest known golden eagle nest, located on a steep river butte along Sun River in Montana, was 6.1 m (20 ft) deep and 2.59 m (8.5 ft) in width.[313] Certain other animals—too small to be of interest to the huge raptor—sometimes use the nest as shelter or even as a nest for themselves, for the incidental protection offered. This has mainly been associated with Eurasian wren (Troglodytes troglodytes) in Europe, though has also been recorded remarkably with ring ouzels (Turdus torquatus) and pine martens, both of which have been recorded as prey for golden eagles elsewhere in Europe.[2][126] In a similar situation, little curlews in northeastern Siberia apparently gain some protection from predators by nesting close to golden eagle eyries.[314] In each case, the natural predators of these animals are just the right size for golden eagle prey, and therefore avoid active eyries. Related species such as eastern imperial and Bonelli’s eagles have been observed to nest in abandoned golden eagle nests, as well as distantly related or unrelated raptors including white-tailed eagles, lammergeiers, gyrfalcons and peregrine falcons.[202][255][315][316]

Mating and egg-laying

Mating and egg-laying timing for golden eagle is variable depending on the locality. Copulation normally lasts 10–20 seconds. Mating seems to occur around 40–46 days before the initial egg-laying.[2] Unusual mating systems of three birds have been recorded in Scotland and Sweden, usually the third being an immature of either sex. These may simply reflect a relatively unstable period of population adjustment.[2] Occasionally, pairs have been recorded copulating outside of the context of fertilization, possibly for pair maintenance and displacement activity. In Japan, a pair was seen mating 46 days before egg laying and in the United States 55 days after egg-laying.[317][318] The date of egg laying has been directly correlated by the latitude where the pair lives.[2] In the U.S., egg laying can be from anywhere between January and September, though is usually in March or later. In Scotland, egg-laying occurs in March to mid-April.[2] The earliest median laying date in 25 international studies was December 3 in Oman; the latest median date of egg-laying was May 7 in sub-Arctic Alaska.[2][319] Egg laying starts in the dry season as early as late November around Mali and Niger while the median egg laying date in southwestern Morocco is January 15.[118][320] In Ethiopia, the estimated range of egg-laying dates ranged from October 24 to January 5.[118] The median egg laying day in Arctic Russia was May 1.[273] In the Kilbuck and Ahklun Mountains area, three pairs had eggs hatched from May 14 to 23, and young fledged from July 8 to August 10 with a median date of July 23 for 11 nests.[55] Two nests on the Kisaralik River still contained young on July 24, and four nests contained young from 16 to 21 July.[321] Clutches have been recorded range in size from 1 to 4 eggs, though two is the norm around the range. Records of single egg clutches are fairly common in Europe, with 3 being rare there but seemingly more common in North America, where up to 12% of clutches include 3 eggs. A 4 egg clutch is considered exceptional.[2][5] The lowest mean clutch size known from surveys is 1.82 eggs in the Altai Mountains of Russia.[322] A mean of 48 clutches from Algeria and Tunisia showed an average of 1.89 eggs per nests, with a range of 1 to 3.[320] The female laid an average number of eggs of 1.99 in 332 clutches from 8 studies in 5 of the Western United States. The largest mean clutch size across the range was 2.1 in Montana.[183] In the wild, eggs are typically laid at 3 to 5 day intervals, with records in captivity of intervals of up to 7 or 10 days.[253][323][324] The eggs vary from all white to white with cinnamon or brown spots and blotches. Scottish eggs averaged 75 by 59 mm (3.0 by 2.3 in) in size and weigh about 145 g (5.1 oz).[2] In Armenia, the average egg measured 76.5 by 59.1 mm (3.01 by 2.33 in) and weighed 123 g (4.3 oz). In North America, eggs average 74.5 by 58 mm (2.93 by 2.28 in), with a range of lengths from 67.5 to 85.7 mm (2.66 to 3.37 in) and a range of width of 49.4 to 63.2 mm (1.94 to 2.49 in).[78][325] Eggs of golden eagles in California weighed from 113.9 to 176.6 g (4.02 to 6.23 oz), averaging 141.4 g (4.99 oz).[326] The incubation period lasts from 41 to 45 days, averaging about 42.4 days, with previously reported claims of as low as 33 days from North America now known to be erroneous.[2][5] Females do a majority of, but not all, of the incubating and largely attain their own food up to the stage of egg-laying, after which they are typically fed by the male. The female may continue to grab most of their own food in areas where carrion is easily accessible.[253] If made to forage excessively during the incubation stage by a non-attentive mate, the female may abandon the nesting attempt.[327] In Idaho, the female did 84% of the incubation during the day, with the male doing about 16%.[327] At night, the female generally appears to do all of the incubating.[328] In Japan, the female did 90% of the observed incubating.[317]

Early life

A few day-old golden eagle nestling with its unhatched sibling's egg

The golden eagle chick may be heard from within the egg 15 hours before it begins hatching. After the first chip is broken off of the egg, there is no activity for around 27 hours. After this period, the hatching activity accelerates and the shell is broken apart in 35 hours. The chick is completely free in 37 hours.[24] Upon hatching, the chicks are covered in fluffy white down. One day after hatching, chicks will weigh 105 to 115 g (3.7 to 4.1 oz), with an average of 110.6 g (3.90 oz).[329][330] In the first 10 days, chicks mainly lie down on the nest substrate.[24] The eagles are capable of preening on their second day but are continually thermoregulated via brooding by their parents until around 20 days.[2] Within 10 days, the hatchlings grow considerably, weighing around 500 g (1.1 lb).[327] Around this age, they also start sitting up more.[24] Around 20 days of age, the chicks generally start standing, which becomes the main position over the course of the next 40 days.[24] The whitish down continues until around 25 days of age, at which point it is gradually replaced by dark contour feathers that eclipse the down and the birds attain a general piebald appearance.[2] After hatching, 80% of food items and 90% of food biomass is captured and brought to the nest by the adult male. The adult male golden eagle perch away from the nest for about 74% of the nestling period, whereas females mainly stay on the nest for about 45 days after the first chick hatches.[327] The voice of nestlings advances from a soft chirp to a disyllabic seeir at around 15 days of age and then to a louder, clearer and conspicuously harsher psaa call given from about 20 days of age to as late as several weeks after fledging.[24] From 10 days to 45 days of age, the nestlings eat much more food and grow considerably. Meal size increases throughout the nesting season, the estimated morsel of flesh fed to the young ranged from 6 mm (0.24 in) at hatching to 15 mm (0.59 in) at fledging.[327] The nestling golden eagles start “mantling” over food at around 10–20 days old: when given a food object, they stand over it, wings partially open, tail fanned and head bowed, covering the food item completely. This is believed to be a competitive behavior as it seen only in nests with more than one chick.[2] Within a matter of days, the chicks try to defecate over the edge of the nest but are not competent at it until they are around 20 days old.[2] The average daily food consumption for female nestlings tends to be greater than those in male nestlings. The average food consumption of the nestling sexes averages 691 g (24.4 oz) and 381 g (13.4 oz) per day, respectively, with male nestlings weighing about 500 to 600 g (1.1 to 1.3 lb) less than the equivalent-aged females.[2] Despite this, males typically tend to develop sooner and fledge more quickly than the females.[331] The average amount of food brought to the nest daily was notably higher in Idaho and Montana, where an average of 1,417 g (3.124 lb) and 1,470 g (3.24 lb) of prey were brought to the nest, respectively, than in Texas, where an average of 885 g (1.951 lb) was brought.[327][332][333] For the first 30 days or so, the nestlings are fully dependent on their parents to feed them but after that period, they start standing around the edge of the nest and practice food tearing.[2] When the first chick is an average of 29 days old, the female first start perching off the nest and at 40 days of age, she stops perching on the nest platform.[327] From 50 days onward, dark brown plumage sprouts from the same sockets as the down and plumage changes become more subtle. The feet go from flesh-colored at hatching to grey to black, thence finally to yellow.[2] When they are around 20 days or so old, as the structure of their wing develops, the nestlings start wing flapping and the frequency and intensity of this behavior increases considerably by 40 days old.[24] Fledging occurs at 66 to 75 days of age in Idaho and 70 to 81 days in Scotland.[2][327]

Cainism

Cainism”, as it is sometimes called, or siblicide is inarguably the most controversial and confusing aspect of the golden eagles’ reproductive biology.[334] This is the habitual behavior in the nest of the oldest hatchling to attack and usually kill their young siblings. In fact, this behavior is quite common, not only in large accipitirids but also in unrelated raptorial birds such as skuas and owls.[331] Cainism is frequent, even typical in species of the Aquila genus. The traditionally classified genus can be broken down into two groups: facultative cainists (wherein fewer than 90% of known nests do the oldest nestling attack and kill their younger siblings) and obligate cainists (wherein more than 90% of nests do the older kill the younger siblings). The golden eagle is part of the facultative cainists group, along with the wedge-tailed, eastern imperial, steppe and greater spotted eagles. The obligate cainists are two tropical species, the Verreaux's and the tawny eagle, and one temperate-climate-dwelling species, the lesser spotted-eagle.[2][331] Other Aquila eagles seem to roughly fall into the tropical species being obligate cainists vs temperate species being facultative cainists categories.[3][5][335] Sometimes called “biologically wasteful”,[23] this strategy is most commonly explained as useful for the species because it makes the parents' workload manageable even when food is scarce, while providing a reserve chick in case the first-born dies soon after hatching. Golden eagles invest much time and effort in bringing up their young; once able to hunt on their own, most golden eagles survive many years, but mortality even among first-born nestlings is much higher, in particular in the first weeks after hatching.[2][334] This theory is borne out by the fact that the tropical species which are obligate cainists invariably have a longer average nesting period than species which nest in temperate zones.[2][3][334] In southwestern Idaho, sibling aggression occurred in all nests with 2-chick broods observed from blinds, and resulted in 1 death in 3 (43%) of 7 broods.[327] In another study, siblicide accounted for 7% of 41 nestling mortalities in southwestern Idaho.[291] 6 (40%) of 15 nestling losses in nests in Central Europe were from cainism.[336] In Scotland, there may be a weak link between food supply and cainism. On Skye, where carrion and rabbits are quite abundant, the younger sibling survives to fledge in about 20% of nests, whereas in the West-Central Highlands, where food is more scarce, the second sibling fledged in only about 4% of nests.[2] However, many nest where food availability was seemingly much higher than what all nestlings would need for food still experience siblicide.[23] The nestlings hatch in approximate 3 to 5 day intervals. If it is a three egg clutch, the mean estimated weight of the three hatchlings at the time the final egg hatches is 367 g (12.9 oz), 252 g (8.9 oz) and 98 g (3.5 oz), making the largest chick easily dominant and giving the youngest practically no chance of survival.[23] The oldest golden eagle hatchling may start acting aggressively to its younger sibling(s) as soon as it or they hatch. Within the first two days, this often escalates into “bill-stabbing” wherein the younger sibling is jabbed around their neck or the middle of their body until a gapping, fatal wound is created. If it is not directly killed, the younger sibling may starve to death, which may be an even more common occurrence. It is apparently common for the younger, weaker siblings to stop begging for food after sibling aggression starts and the parent eagles do not feed the nestlings unless they beg.[23] In some cases, the young nestlings are physically forced from the nest entirely by their older sibling.[337] It is possible that cainism is more common when the older hatchling happens to be a female but, in many cases of males hatching first, they are still larger than the younger siblings and often do dominate and kill them whether male or female.[338] In one nest in Idaho and one in Montana, the oldest sibling was reported to eat their younger siblings, the only verified instances of cannibalism in golden eagles.[45][103] Although the brooding mothers, otherwise famous for the high level of their parental care, is fully aware of the sibling aggression, in no raptorial bird species are they known to intervene when cainism occurs.[5] After the young are about 20 days old, the amount of aggression between siblings (if both survive) decreases and both chicks can usually fledge, though aggression may again increase shortly before fledging.[2]

Fledging and ensuing years

The first attempted flight departure can be abrupt, with the young jumping off and using a series of short, stiff wing-beats to glide downward or being blown out of nest while wing-flapping. The initial flight often includes a short flight on unsteady wings followed by an uncontrolled landing.[339] Young eagles stay within 100 m (330 ft) of the nest in the first few weeks after fledging. They typical have a favored perch where food is brought by the parents and the fledglings only rarely need to take to the wing.[2] 18 to 20 days after first fledging, the young eagles will take their first circling flight but they cannot gain height as efficiently as their parents until approximately 60 days after fledging.[340] Around the time they are 4 months old, the juveniles start to shun the parents attention, even if offered shelter from rain.[24] In Cumbria, young golden eagles were first seen hunting large prey 59 days after fledging and 75 to 85 days after fledging the young were largely independent of parents.[2] In Israel, at 60–70 days old after fledging, the juveniles were still close to nest and quite dependent on parents for food. At the next stage, at around 120 days old, the Israeli juvenile eagles hunting attempts increased and at 160–180 days old they moved further and further away from the nests. The juveniles in Israel first settled over 12 km (7.5 mi) away from the nests.[341] From their first winter until their fourth or fifth winter is the least well-known of the golden eagle’s life. Juveniles disperse widely during their first year, with males remaining closer to the natal area than the more highly exploratory females.[342] In North Dakota, radio-tagged juvenile golden eagles stayed within 5 km (3.1 mi) of the nest for the first 100 days after fledging but then dispersed over 15 km (9.3 mi) in the following 40 days. The study in North Dakota focused on juveniles from six different nests which successfully produced two fledglings and the behavior of the sibling-pairs was surprisingly gregarious as they flew together, perched together and mutually preened for months after independence.[286] In the juvenile stage, most Idaho non-breeders stayed within 100 km (62 mi) of their place of hatching, although some birds distributed more than 1,000 km (620 mi) away from their natal range.[343] The movements of first-year eagles from Denali National Park averaged more than 5,500 km (3,400 mi), with surviving individuals migrating south to western Canada and the Western U.S. in autumn then moving back north to western Yukon and Alaska in spring.[272] In Switzerland, juvenile birds traveled an accumulated range of 2,000 to 15,000 km2 (770 to 5,790 sq mi) whereas the adults never left their home ranges of 75 to 191 km2 (29 to 74 sq mi).[149] A radio-tagged juvenile in Spain travelled a range of more than 16,000 km2 (6,200 sq mi) in its first three years of independence, then ultimately settled in a vacant territory 26 km (16 mi) from its hatching place.[344] There is a handful of records of pairs of sub-adult golden eagles (based on their plumage) nesting, sometimes even successfully producing fledglings.[345][346][347] The first attempt of nesting by six banded golden eagles in southwestern Idaho occurred when they were from four to seven years of age, with five years appearing to be the average internationally.[3][343] Once they attempt to nest for the first time, golden eagles will often return to the vicinity of the natal zone, regularly within 7 to 65 km (4.3 to 40.4 mi) of their original nest site, sometimes attacking and even killing older golden eagles pairs if they occupy the area.[348]

Reproductive success

Golden eagle - nest scene reconstitution

Generally breeding success seems to be greatest where prey is available in abundance.[2] In Central Utah, 57% of eggs successfully hatched out of 87 eggs from 44 clutches.[349] In Idaho, 65% of 282 eggs from 145 clutches successfully hatched, and in Montana, 86% of 28 eggs from 14 clutches hatched.[45][350] In Scotland, the highest breeding success of golden eagles was in the Eastern Highlands where heather moorland still abundant, bearing plentiful red grouse and mountain hare.[2] The jackrabbit follows a 10-year cycle where it peaks and crashes. In Idaho, 100% of observed nests produced at least one fledgling when the jackrabbits peaked in the late 1970s through the early 1980s and then at the low point in the mid to late 1980s, the nests produced on average only 0.2 fledglings.[94] Similarly, in central Utah jackrabbit numbers were correlated with average number of young reared by 16 golden eagle pairs. Here the average number of young ranged from 0.56 in 1967 to 1.06 in 1969 to 0.31 in 1973.[351] In Sweden, nesting success rose noticeably in years where the hunting bag (estimated quantity of prey) rose.[352] After viral hemorrhagic pneumonia (VHP) killed off many rabbits in Spain, the average breeding success of golden eagles in Northern Spain dropped from 0.77 in 1982-1989 to 0.38 by 1990-1992.[353] In Belarus, the average brood size is reportedly 1.8 whereas the average number of fledglings is 1.1.[302] As previously mentioned, adverse weather can affect breeding success. In the exceptionally stormy and cold spring of 1984 in Montana, 71% (10 out of 14) studied nests failed.[354] A long period of exceptionally rainy, cold springs in Scotland reportedly resulted in a 25% reduction of fledging pairs being produced. Rather than directly killing the nestlings, stormy, wet weather probably causes the most harm to productivity due to the hampering of the parents ability to hunt.[355] In arid areas, heavier rainfall has the opposite effect and actually improves nesting success. Rainy years in the deserts of Israel, which provide more brown hares and chukars to hunt, were more successful years for breeding.[356] While siblicide is estimated to claim about 80% of second hatchlings among golden eagles, in some parts of the range, the survival rate in successful nests is higher. In ideal habitats in North America (Northwestern United States and Alaska), 38 to 56% of nests produce a second fledging.[272] Populations at northern end of range have smaller broods (brood size being about 12% smaller) and produce fewer fledglings (population productivity being about 25% smaller) than those in temperate areas, as has been revealed by contrasting studies of Alaska to Idaho.[269][272]

Longevity

Golden eagles are fairly long-living birds in natural conditions. The survival rate of raptorial birds tends to increase with larger body size, with a 30-50% annual loss of population rate in small falcons/accipiters, a 15-25% loss of population rate in medium-sized hawks (i.e. Buteos or kites) and a 5% or less rate of loss in eagles and vultures. The oldest known wild golden eagle was a bird banded in Sweden which was recovered 32 years later.[2] The longest-lived known wild golden eagle in North America was 23 years and 10 months.[357] The long-lived known captive golden eagle, a specimen in Europe, survived to 46 years of age.[253] The estimated adult annual survival rate on the Isle of Skye in Scotland is around 97.5%.[358] When this extrapolated into an estimated lifespan this results in 39 and half years as the average for adult golden eagles in this area, which is probably far too high an estimate.[2] Survival rates are usually much lower in juvenile eagles than in adult eagles. In the western Rocky Mountains, 50% of golden eagles banded in the nest died by the time they were 2 and a half years and an estimated 75% died by the time they were 5 years old.[359] Near a wind turbine facility in west-central California, estimated survival rates, based on conventional telemetry of 257 individuals, were 84% for first-year eagles, 79% for 1- to 3-year-olds and adult floaters and 91% for breeders; with no difference in survival rates between sexes.[61] Survival rates may be lower for migrating populations of golden eagles.[360] A 19-34% survival rate was estimated for juvenile eagles from Denali National Park in their first 11 months.[360] The average life expectancy of golden eagles in Germany is 13 years, extrapolated from a reported mere 92.5% survival rate.[361]

Natural mortality

Natural sources of mortality are largely reported in anecdotes. On rare occasions, golden eagles have been killed by competing predators or by hunting mammalian carnivores, including the aforementioned wolverine, snow leopard, cougar, brown bear and white-tailed eagle attacks. Most competitive attacks resulting in death probably occur at the talons of other golden eagles. Nestlings and fledglings are more likely to be killed by another predator than free-flying juveniles and adults. It has been suspected that golden eagle nests may be predated more frequently by other predators (especially birds, which are often the only other large animals that can access a golden eagle nest without the assistance of man-made climbing equipment) in areas where golden eagles are regularly disturbed at the nest by humans. Jeff Watson believed that common raven occasionally eats golden eagle eggs but only in situations where the parent eagles have abandoned their nesting attempt.[2] However, there are no confirmed accounts of predation by other bird species on golden eagle nests.[2] Occasionally, golden eagles may be killed by their prey in self-defense. There is an account of a golden eagle dying from the quills of a North American porcupine (Erethizon dorsatum) it had attempted to hunt.[362] On the Isle of Rùm in Scotland, there are few cases of red deer trampling golden eagles to death, probably the result of a hind having intercepted a bird that was trying to kill a fawn.[363] Although usually well out-matched by the predator, occasionally other large birds can put up a formidable fight against a golden eagle. An attempted capture of a great blue heron by a golden eagle resulted in the death of both birds from wounds sustained in the ensuing fight.[364] There is at least one case in Scotland of a golden eagle dying after being “oiled” by a northern fulmar, a bird whose primary defense against predators is to disgorge an oily secretion which may inhibit the predator's ability to fly.[365] Of natural sources of death, starvation is probably under-reported. 11 of 16 dead juvenile eagles which had hatched in Denali National Park had died of starvation.[360] Of 36 deaths of golden eagles in Idaho, 55% were possibly attributable to natural causes, specifically 8 (26%) from unknown trauma, 3 (10%) from disease and 6 (19%) from unknown causes.[366] Of 266 golden eagle deaths in Spain, only 6% were from unknown causes that could not directly attributed to human activities.[367] Avian cholera caused by bacteria (Pasteurella multocida) infects eagles that eat waterfowl that have died from the disease. The protozoan Trichomonas sp. caused the deaths of 4 fledglings in a study of wild golden eagles in Idaho.[291] Several further diseases that contribute to golden eagle deaths have been examined in Japan.[368] A captive eagle died from two malignant tumors - one in the liver and one in the kidney.[369]

In human culture

1870s illustration of burkut falconers in Kazakhstan

As early as recorded history, mankind was fascinated by the eagle. Most early recorded cultures regarded the golden eagle with reverence. It was only after the Industrial Revolution, when sport-hunting became widespread and commercial stock farming became internationally common, that humans started to widely regard golden eagles as a threat to their livelihoods. This period also brought about the firearm and industrialized poisons, which made it easy for humans to kill the evasive and powerful birds. The following are various reportages of the significance of eagles, many likely pertaining to the golden eagles, in early cultures and older religions as well as national and military insignias.

Falconry

Russian depiction of Mongolian falconers fox-hunting with a golden eagle.

Golden eagles can be trained to be highly effective falconry birds, though their size, strength, and aggressiveness require careful handling to control the risk of injury to the falconer.[370] They have been used in this practice at least since the Middle Ages. In Asia, they were reportedly used in teams to hunt such animals as deer, antelope and wolves. Concurrently in Europe, their use for falconry was typically reserved for Emperors and Kings, which is why there are several common names for the golden eagle in various languages that roughly translate as the “royal eagle”.[371] In the United States falconers seldom use golden eagles, as the similarly aggressive Ferruginous hawk is more available and provides a similar hunting experience with most of the same game species with lower risk of injury to the falconer.[372] The most common interaction of American falconers with golden eagles is in trying to avoid them in order to reduce golden eagle attacks on their trained birds.[373] The very athletic golden eagle is approximately as swift as the large falcons, is quite willing to attack smaller raptors when the opportunity is available, and is often capable of flying down a falcon or hawk. Experienced falconers will consequently not fly their birds if golden eagles are spotted, and usually prefer to fly later in the day when the golden eagles have typically already fed.

The culture in which falconry with golden eagles is prominent today is amongst the Kyrgyz people of the Tien Shan Mountains of southeastern Kazakhstan and Kyrgyzstan.[374] This practice is also culturally prominent in western Mongolia and Xinjiang.[375][376] There are around 250 active eagle hunters in Bayan-Ölgii Province of Mongolia, and 50 in Kazakhstan.[377][378] In these cultures, the golden eagle is considered a highly-valued working animal which will be used for 15 years or more. Falconers carry their bird on a gloved right hand, usually with a wooden brace to support its considerable weight. In the Tien Shan Mountains, falconry mostly occurs in late fall and early winter. It is possible for up to 30 to 50 foxes to be caught by a single eagle during this season.[2]

Full-grown wolves are not believed to be viable prey for wild golden eagles; they are too dangerous due to their large size and large, powerful bite.[23] Despite this, falconers occasionally use golden eagles to hunt wolves. The steppe wolf (Canis lupus campestris), a relatively small-bodied race of wolf at around 35 kg (77 lb), is the main wolf reportedly hunted by golden eagles in falconry.[2] There is little solid evidence that even a well-trained golden eagle in the possession of falconers can normally overtake a healthy adult wolf without assistance, either from other eagles or from their human “owners”.[379] Some wolves prove particularly challenging quarry: there is the tale of one that was injured by 11 successive eagles but foiled their attempts – killing each one – until it was finally dispatched thanks to the efforts of a twelfth eagle.[265]

Heraldry and myth

Main article: Eagle (heraldry)

The golden eagle is the most common national animal in the world, with five nations—Albania, Germany, Austria, Mexico and Kazakhstan—making it the national animal. It is also a common motif in the national symbols of countries that have not officially made it the national animal or national bird. The reasons for this are various, but among the nations that use the golden eagle as or in a state symbol, there are two clear traditions that help explain the modern usage. Among European countries, the golden eagle was the model for the aquila, the most prominent symbol of the Roman legions and more generally the Roman civilization that had such a powerful impact on Western culture; furthermore, some Roman traditions were carried on by the Byzantine Empire in the Southern and Eastern of Europe and the Holy Roman Empire in Central and Western Europe, transmitting the use of the golden eagle to several modern states. This association of the golden eagle with Rome has also led to the adoption of similar symbols in other countries; for instance, the adoption of the related and physically similar bald eagle as the national bird of the United States was inspired by the conception of the United States as a modern reincarnation of the Roman Republic, a theme that recurs in other elements as well (including the prevalence of neoclassical architecture in American public buildings and the use of Roman terminology—such as naming the upper house of Congress the Senate—to hark back to the Roman model). In perhaps one of the golden eagle's more unfortunate representations in human culture, Adolf Hitler used a golden eagle regularly as a symbol for the Nazi Party, including large monuments or statues on buildings and bridges as well as the pins worn on lapels of Nazi officers. In this case, the eagle was used as to stir up nationalism, since it was a longtime symbol (Hoheitszeichen) for the German Empire dating back centuries in use for the Coat of arms of Prussia.[2][380]

Another large tradition of using the golden eagle can be found in the Arab world, where the eagle is historically a symbol of power in Arabic poetry, and was according to legend the personal emblem of Saladin. The specific depiction of golden eagle legendarily considered to be Saladin's was adopted by the Arab nationalist movement, and currently appears on the arms of Egypt, Iraq, and Palestine; it had previously appeared on the arms of the People's Democratic Republic of Yemen (1967–1990) and on the arms of the Libyan Arab Republic (1970–1972). The current emblem of Yemen displays a golden eagle, but it is not that of Saladin. In Japanese culture, the main representation of eagles in mythology is Tengu, a monster human-bird possibly modeled on an eagle and given its mountain dwelling habits, perhaps partially inspired by golden eagles. Tengu is described as a bipedal creature with a long nose-beak, wings and a fan with 12 wooden ribs, resembling tail feathers, though is sometimes is illustrated as far more human-like than bird-like. Mischievous and powerful, Tengu is mainly the protector of the mountains, is able to foretell the future, creates winds and even attack people who try to harm the mountainous landscape.[2]

Religion

Eagles are often prominent in The Bible, though are sometimes mixed with carrion birds and are not specifically identifiable to species. As the most widespread eagle in the Middle East and Eurasia, certainly many said references must pertain to the golden eagle. The use of eagles seems generally heavier in the Torah or the Old Testament than in the New Testament. In biblical times, eagles and other meat-eating birds were banned from being eaten since their diet was considered unsavory.[381] However, eagles are mentioned in the Bible as being admired for their swiftness,[382] great physical power[383] and their seemingly endless endurance.[384] Eagles are one of four dimensions of creation,[385] as a messenger of God,[386] and a skilled predator.[387] Eagles are also widespread in the Bible for symbolism.[2] For example, due to the perceived high level of parental care, eagles were associated with protection and even paralleled to God carrying the Israelites out of Egypt.[388]

Earlier Eurasian cultures and faiths also feature eagles quite prominently. In Hellenistic religion, the golden eagle is the signature bird of the god Zeus, a connection most notable in the myth of Ganymede, where the god adopted the form of a golden eagle to kidnap the boy, as well as the eagle-like daimon Aetos Dios Theoi: EAGLE OF ZEUS . At least a few sources also associate it with Helios. In Norse mythology, the golden eagle sits atop Yggdrasil, the great ash tree that runs through the universe. A squirrel, Ratsatosk, carries messages and insults between the eagle at the crown and a serpent gnawing at the tree roots.[389] In many cultures, eagles were viewed as a link between terrestrial mankind and celestial deities.[390] Eagles were particularly prominent in Roman culture. Many banners, coins and insignias from Rome feature eagles. In Roman religion, the eagle was both the symbol and the messenger of the Roman sky-god, Jupiter. When an emperor died, his body was burned in a funeral pyre and an eagle was released above his ashes to carry his soul to the heavens. Somewhat surprisingly, eagles play a small role in Celtic mythology.[391] In Celtic stories, eagles are often characters who have transformed from human to bird form. One of the most prominent eagle-related Celtic myths is that of Llew Llaw Gyffes who escaped death at the hands of a hunter who spears him by taking an eagle’s form and killing the hunter.[392] When the Roman Empire blended with Celtic tradition, many symbols feature both eagles and the Celtic sun gods.[2]

In North America

A ceremonial head-dress made by the Nez Perce people from the tail feathers of juvenile golden eagles in what is now Idaho

Because of the nature-based culture, religions and lifestyles of the people living there, eagles were often even more prominent in ancient North America than in Eurasia. The eagle is a sacred bird in some of the cultures there and the feathers of the eagle are central to many religious and spiritual customs, especially among some Native Americans in the United States and First Nations in Canada, as well as among many of the peoples of Mesoamerica. Some Native American peoples revere eagles as sacred and the feathers and other parts of bald and golden eagles were prominent possessions amongst tribes. Feathers are often worn on Native American headdresses and have been compared to the Bible and crucifix of Christianity. Eagle feathers were also used to construct prayer sticks, doctors’ rattles and medicine pipes. Per Thomas E. Mails: "in the mind of the Plains warrior in the 18th and 19th century, the male golden eagle flew above all the creatures of the world and saw everything. Nothing matched his courage and swiftness, and his talons had the strength of a giant's hand. The eagle was very holy."[393] The tail feathers of juvenile golden eagle feathers have often been used in various Native ceremonies and are used to honor noteworthy achievements, great strength and healing abilities and qualities such as exceptional leadership and bravery.[393] The golden eagle is thought to be the origin of the Thunderbird legends of the southwestern United States,[371] In the Cheyenne, golden eagles are referred to, in addition to "Thunderbird", as "bird father", "war eagle", "striped eagle" and "prairie eagle".[2] The “eagle dance” is an important part of Sioux tradition, wherein the participants would dance around, mimicking the motions of a flying eagle while chanting the equivalent of “The eagles are flying, the eagles are flying. The eagles are here, the eagles are here…”[393] No Native warrior would be allowed to kill an eagle for his feathers until he had proven his bravery in battle with other tribes. Per Mails, “It has been claimed that the Indian warrior would rather part with his horse, his tipi or even his wife, than lose his eagle feathers in battle, since to have this happen would be to make dishonor and loss of status in the eyes of his entire tribe”.[393] The warriors were not allowed to use weapons when capturing eagles for their feathers. The most common method to capture golden eagles was to lay a trap, consisting of an earth-colored tarp, with a dead jackrabbit on it, over a pit containing a crouching warrior. When the eagle came to land on the tarp, the warrior would grab the eagle by the top of its feet to pull it in and kill it. Apparently, if the warrior failed to grab the eagle properly it sometimes resulted in serious injury to the human.[393] In the Hopi tribe, one of two eagle nestlings (it was strictly forbidden to capture one unless there was two nestlings in the nest) would be captured and raised with great care, until it reached an age where it could be killed and harvested for its feathers.[394]

Current United States eagle feather law (50 CFR 22) stipulates that only individuals of certifiable Native American ancestry enrolled in a federally recognized tribe are legally authorized to obtain eagle feathers for religious or spiritual use. Thus, the supply of eagle material for traditional ceremonial use can be guaranteed and ceremonial eagle items can be passed on as heirlooms by their traditional owners without the restrictions that would usually apply. Commercial trade in golden eagles or their feathers or body parts is not legalized by these exceptions.[395]

Postage stamps

Golden eagle logo on the tail fin of a Cessna 421B Golden Eagle twin-engine light aircraft

The golden eagle is the eighth-most common bird depicted on postage stamps with 155 stamps issued by 71 stamp-issuing entities.[396][397]

Other

J.R.R. Tolkien used an image of an immature golden eagle from T. A. Coward's 1919 work The Birds of the British Isles and Their Eggs for an illustration depicting Bilbo Baggins awaking next to Gwaihir, a giant eagle of Middle-earth.[398]

American aircraft manufacturer Cessna manufactured its Model 421 series of aircraft between the late 1960s and mid-1980s, calling the type the Golden Eagle.

Status and conservation

In flight in Czech Republic

At one time, the golden eagle lived in a great majority of temperate Europe, North Asia, North America, North Africa, and Japan. Although widespread and quite secure in some areas, in many parts of the range golden eagles have experienced sharp population declines and have even been extirpated from some areas. The total number of individual golden eagles from around the range is estimated to range somewhere between 170,000 and 250,000 while the estimated total number of breeding pairs ranges from 60,000 to 100,000.[2][399] Few other eagle species are as numerous, though some species like tawny eagle, wedge-tailed eagle and bald eagle have total estimated populations of a similar size to the golden eagle’s despite having distributions which are more restricted.[3][5] The world’s most populous eagle may be the African fish eagle (Haliaeetus vocifer), which has a stable total population estimated at 300,000 individuals and is found solely in Africa.[3][400] On a global scale, the golden eagle is not considered threatened by the IUCN.[1]

Eurasia

In Europe, there are an estimated 6,000 to 10,000 breeding pairs. There was a great decline in Central Europe where they are now essentially restricted to the Apennine, Alps, and Carpathian Mountains. The strongholds in the continent are Spain, which holds an estimated 1,300 breeding pairs, Norway, which holds an estimated 860 to 1,040 breeding pairs and European Russia, which holds an estimated 500 to 1,000 breeding pairs.[401] Other European countries with stable and sizable populations include Italy, with an estimated 476-541 pairs, Switzerland, with 300-310 pairs and Romania, with an estimated 85-130 pairs.[401] The following nations are thought to have golden eagle populations that are increasing: Bulgaria with 150-170 pairs, Denmark with 1 known breeding pair, Finland with an estimated 300-350 pairs, France with approximately 390-460 pairs, Hungary with 3-5 known pairs, Ireland with 2 current pairs and Poland with approximately 35-40 pairs.[401][402] The following nations are thought to have decreasing golden eagle populations: Albania with about 50-200 pair, Croatia with approximately 90-110 pairs, England with no known current pairs, Greece with an estimated 100-200 breeding pairs and Latvia with somewhere around 5-10 pairs.[193][401][403] Several other European countries have a small number of golden eagles with less than 50 breeding pairs but with populations that are generally considered stable.[401] Despite their large population there, the golden eagle was considered near threatened in Spain in a 2003 report.[404] One of the authors of the previous study asserted that the population had increased in 2008 by perhaps 20% in Spain since the last survey in the late 1990s.[405] In Belarus, the population has reportedly declined considerably due to trapping, poisoning and the draining and development of upland bogs.[302] The golden eagle is considered Critically Endangered in the Czech Republic, where it was once quite common in the Beskydy and Krkonoše Mountains until logging hit the area hard around the time of World War II. All recent breeding attempts by the species in the Czech Republic are believed to have been unsuccessful.[406]

In Britain, the last comprehensive survey of golden eagles took place in 2003, and found 442 occupied territories.[407] A less thorough survey in 2007 showed that in addition to large numbers of territories in the Scottish Highlands and the Inner and Outer Hebrides, there were a handful of birds in southern Scotland and northern England.[408] The population is higher today in Scotland than it was in the 19th century, due to the heavy persecution at that time by sheep farmers, gamekeepers, and collectors.[2] There may have been as few as 190 pairs in the 1950s, though this survey may have not been complete.[409] Between 1969 and 2003 they nested in the Lake District, Cumbria.[407] In Ireland, where it had been extinct due to hunting since 1912, efforts are being made to re-introduce the species. In April 2007, a pair of golden eagles produced the first chick to be hatched in the Republic of Ireland in nearly a century. Forty-six birds were released into the wild in Glenveagh National Park, County Donegal, from 2001 to 2006, with at least three known female fatalities since then. It is intended to release a total of sixty birds, to ensure a viable population. The reintroduced golden eagles at the park produced a pair of fledglings for the first time in 2011.[410] The golden eagle is classified as bird of “High Conservation Concern” in Ireland.[411]

Fewer estimates are known from Asia and North Africa. A stronghold population is in mountainous Turkey, where the large population included an estimated 2,000-3,000 breeding pairs persist. In Japan, there is an estimated 175-260 breeding pairs, with a total population of approximately 500 individuals. One study stated that food shortages and decreases in suitable foraging habitat are assumed to be responsible for an observed decline in population size and reproductive success in Japan.[412] In the Koreas, the golden eagle is known to be rarely observed and, in 2010, only 10 were seen in South Korea during winter birding censuses.[413] Little is known in terms of population numbers elsewhere in Eurasia, with the IUCN estimating between 100 and 10,000 individuals each in China and in Russia, numbers that suggest the species occurs very sparsely in these massive countries.[414] In North Africa, the main occurrence is in Morocco, which is estimated to hold 200 to 500 breeding pairs. There appear to much fewer in other North African countries, with small, scattered populations in Algeria, Tunisia and Egypt, areas where no immature-plumaged eagles were observed in 2005.[415]

North America

In North America the situation is not as dramatic. It appears the greatest density of golden eagles anywhere in the world occurs in the Western United States and Western Canada. In total, the contiguous Western United States may hold up to 30,000 individual golden eagles, with a total of 70,000 to 100,000 individuals estimated across all of North America.[25][416][417] One estimate of the number of breeding pairs in the contiguous Western United States that excluded California, South Dakota, Montana and Oregon was 9,387.[418] The state with the largest known winter count of golden eagles is Montana with 13,138, followed by Wyoming with 10,072, Colorado with 7,081 and Utah with 5,993. Wyoming had the highest estimated set of breeding pairs 3,381-4,174, followed by 1,800 in Utah, 1,200 in Nevada and California and Idaho both with around 500 pairs (notably, Montana was not included in these particular studies, although the breeding population must include well over a thousand pair there).[419][420] In 6 out of 8 Canadian provinces where golden eagles breed, over 10,000 birds were observed in breeding bird surveys.[421] In 1981, it was estimated that there were 63,242 wintering individual golden eagles in the 16 Western United States (excluding Alaska).[422] However, there has still been a noticeable decline in some areas.

Threats

In modern times, almost all threats to golden eagles are attributable, directly or indirectly, to human activities.[423] In fact, all Aquila eagles in Eurasia face the same human-sourced threats as the golden eagle: habitat change, persecution, poisoning (often directed at other species) and collisions with man-made objects.[2] Certainly the most widespread unintentional threat to golden eagles by humans is urbanization and human-population growth which have made areas historically used by eagles unsuitable both in terms of habitat and prey availability. Habitat destruction in North American had, by the late 19th century, already driven golden eagles from some regions they used to inhabit.[424] In Southern California and the Colorado Front Range, this has been proved via long-term population and habitat surveys.[425][426] In Western China, the main threats to golden eagles are land development, the use of pesticides and apparent regular captures for falconry.[427] Fires since 1980 have caused large-scale losses of shrubs and jackrabbit habitat in areas used by eagles throughout the Intermountain West of North America. Wildfires that burned more 40,000 hectares of scrublands between 1981 and 1987 in the Morley Nelson Snake River Birds of Prey National Conservation Area affected nesting populations adversely. Nesting success at burned territories in Snake River Canyon declined after major fires. Abandoned burned territories have been subsumed by neighboring pairs, resulting in a decreased number of nesting pairs.[428] In a few cases, mankind has accidentally benefited golden eagles by logging previously heavily wooded areas. This has been recorded in the 1800s and 1900s in the Appalachian Mountains of the Eastern United States, where reforestation has now made the habitat unsuitable for nesting eagles, and in Washington state, which still holds breeding eagles in desolate areas that have been logged.[56][62] Afforestation, the commercial planting of non-native woodland, is a serious issue in Scotland, with the largest amount of it occurring in southwestern Scotland, especially in Argyll. During afforestation, the land is ploughed and in excess of 2,500 seedlings are planted per hectare, mainly with exotic conifers including Sitka spruce (Picea sitchensis) and lodgepole pine (Pinus contorta). The woodland canopy closes and ground vegetation dies, making these dark and gloomy places until harvesting in 40 to 50 years. More than 50% of land in Scotland at an elevation of 200–600 m (660–1,970 ft) has been planted as such. Afforestation requires removal of sheep and the fencing out or shooting deer, both important sources of carrion for golden eagles. Foraging areas of golden eagles have been confirmed to not include afforested areas.[2][429]

Death by collisions with man-made structures and objects can be serious local issue. Electrocution or collision with power lines has become an increasingly significant cause of mortality since the early 20th century. Apparently, juveniles birds are more susceptible than adults, being generally less cautious and physically adept.[430] Due to attempts to perch on or by flying into power lines, a total of 134 and 115 golden eagles were killed by power lines in the western United States in 1972 and ’73, with a peak of 66 in Utah in 1972.[431] Today, golden eagles still regularly meet their demise due to power lines in the United States and the species was considered the bird-of-prey in North America most likely to killed by them.[432] 266 examined golden eagle deaths from 1980 to 1990 in Spain showed that 14 of the deaths (5%) were from electrocution and 57 (21%) were from wire collision.[367] Compared to the United States and Spain, in some other parts of the range such as Scotland and Scandinavia, electrocutions and wire-collisions are rare due to fewer high power-poles in the desolate areas that the golden eagles occupy.[2] Controversially, the US Fish and Wildlife Service has permitted that a "wind-farmer" in central Oregon could legally cause the incidental killing of golden eagles by large wind turbines. Such turbines have set up as an alternative source of energy but are often fatal for high-flying birds such as raptors.[433] Wind turbine blades appear to move slowly from a distance but at close range move so quickly as to appear a nearly invisible blur. It is estimated that up to 70 golden eagles may be killed locally by turbines each year in west-central California, almost all of them being juveniles as opposed to adults which tend to remain on their home ranges that largely occur outside of the wind farm area.[61][434] There are mounting concerns of the effect of wind turbines in Europe as well, since construction of new ones are increasing in Scotland and Spain.[252][435] Collisions with automobiles rarely claim golden eagle lives, though instances of this can increase in desolate areas during winter, when road-side prey or carrion may attract the eagles. In the Diablo Range, California, 5% of 61 radio-tagged birds died from vehicle collisions.[61] Where disturbance is regular, breeding failure for golden eagles is significantly more frequent. This was certainly inferred in the 1982 Scottish breeding bird survey when disturbances were heavy in the Highlands.[2] Sheep farmers and egg-collectors are the leading cause of disturbances at the nests. Recreation, forest management and development projects such as road construction, mining or power generation are other potential sources of disturbances. When disturbed by humans at the nest, the parents frequently leave their nest for a period of up to two hours, reduced provisioning rates, endangered eggs or young to predation, as well as overheating, chilling or desiccation.[436] Human intrusion within 750–1,500 m (2,460–4,920 ft) of nests can cause disturbance.[437] Nesting success was found to be reduced in Norway during years where the Easter holidays fell early, apparently due to the volume of vacationers in the countryside in these years during the pivotal early stages of nesting.[438] During a study in Wrangell – St. Elias National Park and Preserve of Alaska, experimenters camped within 400 m (1,300 ft) of active nests, which led to reduced food delivery and nest attendance by the parents, then at 800 m (2,600 ft), at which distance the disturbance of nesting behavior seemed to decrease considerably.[439] The topography of the landscape and location of the nest can affect how closely the nest can be approached without disturbance.[440] Mining and various types of energy development occur in eagle nesting and wintering habitat. The practice of surface coal mining threatens the limited nesting sites in Wyoming.[441] In the Italian Apennines, high levels of nesting failures have been contributed directly to disturbance, thanks to increased tourism in remote mountain areas, construction of new roads and mining.[442] Aggressive behavior by golden eagles due to a human presence near the nest is considered exceptional and usually only result in minor injuries if any, as a particularly bold eagle may rarely attempt to dissuade a human trespasser.[253][443] Golden eagles are somewhat sensitive to human disturbance even while not nesting. Experimental studies showed that pedestrians, which caused flushing at 105–390 m (344–1,280 ft), were more likely to cause wintering golden eagles in Colorado to flush than vehicles, at 14–190 m (46–623 ft). This study showed that golden eagles were more sensitive to human disturbance during winter than several other raptor species, including Bald Eagles.[444]

The intentional killing of golden eagles has been a huge conservation hurdle for the species. While illegal in most countries today, hunting, trapping and poisoning may still occur clandestinely across almost the entire range. During 1948, in Carter County, Montana, 286 golden eagles were shot and killed in the month of March alone. At the time $5 was offered by the Montana Fish and Game Commission for each eagle killed, a bounty initiated by pressure from sportsmen and ranchers who believed that eagles were killing large numbers of sheep and “antelopes”.[445] Approximately 20,000 golden eagles were killed from light aircraft in the Southwestern United States in the 1940s, with the rate of such shooting in Texas having continued at over a thousand a year (5,000 from 1942 to 1947).[446] In the United States, the golden eagle was given federally protected status in 1963. Allegedly, in 1972, over 800 golden eagles were shot from aircraft in Wyoming and Colorado.[28][447] In Switzerland more than 100 golden eagles were reportedly shot each decade until the species was legally protected in 1953.[149] In Scotland, many estate managers would pay gamekeepers to hunt down any and kill all eagle species in the 1800s and early 1900s. On one estate in Caithness, Scotland, allegedly 295 eagles (including both golden and white-tailed eagles) were shot and trapped from the years of 1820 to 1826.[448] Occasionally, golden eagles may be caught in trap lines laid out to capture mammalian predators.[366] Today, some authorities believe many game managers still try to deliberately destroy the contents of nests in spring or intentionally disturb the eagles during their nesting process.[2] In a study conducted in Scotland, areas were broken down between low disturbance areas (with a low human presence and limited history of persecution), moderate areas (where minor disturbances occur, mainly unintentionally from hill-walkers or rarely intentionally by egg-collectors) or severe disturbance areas (where persecution, heavy disturbance and considerable egg-collecting is believed to still occur). In low disturbance areas, about 45% of nests failed, in moderate disturbance areas about 74% failed and in severe disturbance areas 93% of nesting attempts failed.[449] 73 out of 147 inaccessible nesting sites in this study (50%) produced fledglings, whereas more accessible nests produced fledglings in only 21 out of 68 nests (31%).[449] The golden eagle became legally protected in 1980 in Spain but the species is still regularly killed there today. In 266 golden eagle deaths from 1980 to 1990, 59% (157 specimens) had died from shooting or trapping and 8% (21 specimens) died from poisoning.[367]

Poisoning, both intentional and unintentional, is perhaps the most insidious man-made threat to golden eagles. The usual targets of carrion-poisoning are species such as coyotes, red fox and gray wolves, which are considered pest that threaten livestock (a charge usually vastly exaggerated by the human imagination). However, golden eagles are occasionally targeted as well for the same reasons. The main cause of mortality for golden eagles in Britain has been poisoning, with a number of 51 eagles verified to be killed by poisoning from 1980 to 2008 probably being far lower than the actual amount killed as such.[2][450] A disproportionate amount of golden eagle poisonings in Scotland from 1981 to 2000 were linked to grouse moors (where grouse are kept for the pleasure of shooting) and were probably cases of gamekeepers deliberately poisoning eagles and foxes to keep their stock of grouse high.[451] It is estimated that the adult survival rate is reduced by 3 to 5% in Scotland by intentional poisonings.[450] In the 1980s, California Ground Squirrels, due to the perception among cattle rangers that they are agricultural pests, were poisoned by the anticoagulant rodenticide, Chlorophacinone. In turn, the poisonings caused golden eagles, as one of the major natural predator of California ground squirrels, to die in turn.[452] At least 10 individuals died in 1971 from eating Thallium(I) sulfate–laced pronghorn set out by sheep ranchers in Wyoming; despite public outcries, poisoning by sheep ranchers continued into the 1980s.[447] In the 20th century, organochloride and heavy metal poisonings were also commonplace, but these have declined thanks to tighter regulations on pollution. In southern Idaho, 10 out of 17 golden eagles examined were found to have had exposure to lead.[453] As a whole, the golden eagle is less susceptible than most raptors to organochlorine pesticides because of mammal-feeding habits, as opposed to predominantly bird-eating species like peregrine falcons and fish-eating species like bald eagles, both of which had huge population declines in the 20th century due to the use of pesticides like DDT . Eggs from golden eagle nests that were collected after 1946 in North America had shell thicknesses similar to (of less than a 10% difference) those collected in earlier years.[454] However, in Scotland egg shell thickness did decrease by around 10% from 1951 to 1965.[455] A dead golden eagle collected on the island of Lewis had the highest concentration of organochlorine known from a modern bird in Scotland.[456] The higher effects of organochlorines in Scotland may be due to the fact that birds there consume a relatively high quantity of seabirds, as opposed to North America, where this practice is rare.[2]

The reason why golden eagles are intentionally killed is usually due to the fear of loss of livestock and game species. As previously described, most feeding on ungulates is probably only as a carrion. The findings of virtually every study of losses of stock to golden eagles has revealed that the actual amount of livestock killed by the eagles is negligible and financial restitution from conservation organizations should not be necessary, including those studies funded by the farming organizations or governmental programs seeking to justify the “control” of golden eagles.[2] In Scotland, studies have revealed that an estimated 0.5-8% of lamb mortality is attributable to golden eagle predation. This equates to the raptors killing about 0.15-2.4% of a given flock of sheep. Starvation and accidents are actually the primary cause of lamb deaths in Scotland, with occasional fox, crow and dog predation also occurring.[143][457][458] On the Western Isles of Scotland, a closely studied relationship between the nesting golden eagles and the local sheep flocks showed that 1 to 3% of lamb deaths were attributable to this species.[459] In North America, sheep predation is generally even rarer than in Eurasia but some flocks of sheep in Montana have experienced a relatively heavy rate of loss, more than 10% in some cases, determined to be caused by golden eagle predation in harsh-weather years that lower the numbers of jackrabbits and cottontails.[147] Some organizations have attempted to relocate golden eagles from areas where lamb predation was semi-regular. In Montana, over 400 eagles (most seemingly migratory specimens) were relocated at a cost of over $100,000. Similarly, 16 breeding eagles were relocated in Wyoming after relatively heavy levels of lamb depredation. In each case, relocation has failed as the eagles find their way back to their own home ranges or wintering grounds in a matter of a few weeks, showing that the eagles have a strong homing ability.[147][460] In northern Norway, among 36 deaths out of 263 first-year, radio-collared lambs in a free ranging sheep flock died, 17 from wolverine predation, 8 were from disease, 5 from golden eagle predation, 3 from Eurasian lynx predation and 2 from Red fox predation.[461] In Sweden, the hunting of semi-domestic reindeer by golden eagles has been analyzed. Of an estimated 120,000 reindeer born every year in that nation, golden eagles are reported to kill less than 500 of them, amounting to less than 0.4% loss.[462] Jeff Watson advocated that, instead of trying to kill the golden eagle, that sheep-farmers should encourage population growth in prey like rabbits, hares and ground squirrels, as golden eagles will generally ignore livestock if wild prey is abundant.[2]

Conservation

The golden eagle is not threatened at the species level but efforts need to be taken to prevent extinctions from many northern countries. The primary efforts undertaken to conserve the species have been, in order of prevalence from highest to lowest: conservation education and awareness, policy protection, directed land management, legislation and law enforcement and the provision of indentures.[2] In Scotland, only 3 out of 16 regions in Scotland occupied by golden eagles since 1982 have been deemed favorable for conservation status, based upon the extent of local persecution, prey abundance and habitat change.[252] The United Kingdom has put in place deterrent legislation to prevent behavior around the nest with the potential to cause harm and acts of willful harm to white-tailed eagles and some have advocated using the same policies for golden eagles.[252] Education in this region mainly is undertaken by the Royal Society for Protection of Birds (RSPB). To curb the destructive practice of afforestation, some locals have switched to planting native Caledonian pine forests instead, likely resulting in much less harm to the native fauna possibly including golden eagles.[463] While conservation efforts in Scotland have previously included the setting aside Sites of Special Scientific Interest (SSSIs), these are usually too small to benefit golden eagles. More recently, the United Kingdom government has instead taken from governmental policies enacted in Continental Europe the idea of Special Protection Areas (SPAs), which offers strong legal protection of single species. Potentially more than 7,000 km2 (2,700 sq mi) may be set aside in Scotland as SPA for golden eagles.[464] Some education of mountain-climbers in country holding breeding golden eagles has been undertaken by the Mountaineering Council of Scotland.[465] Within the United States, the golden eagle is legally protected by the Bald and Golden Eagle Protection Act.[5] In the United States, many studies of the effects of industrialization and development have been undertaken by the very companies attempting to develop near areas holding golden eagles, in order to understand and hopefully minimize harmful effects. For example, Swan Falls electric power-plant in southern Idaho has funded research into the effects of reconstruction activities on breeding raptors (including the golden eagles), the Arch Mineral Corporation has funded studies attempting to test and successfully relocate golden eagle nests and KENETECH Windpower and the National Renewable Energy Laboratory have funded research into the effects of turbine-based wind energy on golden eagles.[61][466][467] In the 1970s in the United States, bounds were made to reduce the number of golden eagles to die from electrocution and wire-collisions. The primary change has been to raise the central insulator more than 1 m (3.3 ft) above the cross-arm and to position the ground-wire at a lower height on the pole, both likely to reduce the probability of golden eagles striking the wires with their wings. Also, the power company may place an insulating tube from 1 to 2 m (3.3 to 6.6 ft) on either side of the pole attachment or, especially if the previous modifications are not feasible, install raised perches at the top of the power pole.[468] In Spain, the issue of electrocutions is more intractable because all pylons were made out of metal, which makes them much more dangerous to wildlife flying into them.[469]

The golden eagle’s effect on conservation-dependent species

On the conservation front, the golden eagle is itself unintentionally contributing to the conservation crisis of another animal, the island fox, a small insular relative of the gray fox found only in the Channel Islands of California. Golden eagles did not start nesting in these islands until the 1960s. The predominant theory on why they did not previously occupy them was that territorial bald eagles deterred them until DDT use decimated the local bald eagle population.[470] This critically endangered canid had evolved without major natural predators but the large breeding population of golden eagles in California has spilled in numbers over to the islands and is there feeding partially on the foxes, whose already declining population cannot support sustained predation.[471] Also in North America, attempts to reintroduce endangered whooping cranes by mixing them with flocks of sandhill cranes have been largely unsuccessful in part due to natural predators picking off the unnaturally unwary birds. Although the chief predator has been Bobcats, golden eagles are one of the other predators that are habitually killing the birds.[191] The golden eagle may be a competitor and, rarely, a predator of the recently reintroduced California condors in central Arizona and southern California, but the pressure exerted by the eagles on condors are seemingly minor, especially in contrast to manmade conservation issues for the species such as lead poisoning from bullets left in hunter-killed ungulate carcasses.[231][232] In Scotland, the common hen harrier has been the subject of much unfavorable attention due to the fact that it is a habitual predator of the chicks of red grouse, a subspecies considered to be near threatened in the nation. One of the methods to control harrier numbers has been proposed is to encourage a population increase of golden eagles, which may also hunt grouse but are unlikely to cull young grouse and tend to outcompete and sometimes hunt the harriers themselves.[472]

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Further reading

  • Cramp, Stanley, ed. (1979). Handbook of the Birds of Europe the Middle East and North Africa, the Birds of the Western Palearctic, Volume 2: Hawks to bustards. Oxford: Oxford University Press. ISBN 978-0-19-857505-4. 

External links