|In Serengeti National Park|
Finsch & Hartlaub, 1870
(J. F. Miller, 1779)
The secretarybird or secretary bird (Sagittarius serpentarius) is a large, mostly terrestrial bird of prey. Endemic to Africa, it is usually found in the open grasslands and savannah of the sub-Saharan region. John Frederick Miller described the species in 1779. Although a member of the order Accipitriformes, which also includes many other diurnal raptors such as kites, hawks, vultures, and harriers, it is placed in its own family, Sagittariidae. The secretarybird is instantly recognizable as a very large bird with an eagle-like body on crane-like legs which increases the bird's height to as much as 1.3 m (4.3 ft) tall. Adults have a featherless red-orange face and predominantly grey plumage, with a flattened dark crest and black flight feathers and thighs.
Breeding can take place at any time of year, but tends to be late in the dry season. The nest is built in a thorny tree, and a clutch of one to three eggs is laid. All three young can survive to fledging in years with plentiful food. The secretarybird hunts and catches prey on the ground, often stomping on victims to kill them. Rodents and grasshoppers likely form the bulk of its diet, though it does kill snakes. It appears on the coats of arms of Sudan and South Africa.
|Position of the secretarybird in the order Accipitriformes.|
The Dutch naturalist Arnout Vosmaer described the secretarybird in 1769 based on a live specimen that had been sent to Holland from the Cape of Good Hope two years earlier by an official of the Dutch East India Company. Vosmaer suggested that the species was called "Sagittarius" because its gait was thought to resemble an archer's. He also mentioned that it was known as the "Secretarius" by farmers who had domesticated the bird to combat pests around their homesteads and proposed that the word "Secretarius" might be a corruption of "Sagittarius".
In 1779 the English illustrator John Frederick Miller included a plate of the secretarybird in his Icones Animalium et Plantarum and coined the binomial name Falco serpentarius. It was assigned to its own genus Sagittarius in 1783 by the French naturalist Johann Hermann in his Tabula Affinitatum Animalium. The generic name Sagittarius is Latin for "archer", and the specific epithet serpentarius is from Latin serpens meaning "serpent" or "snake". In spite of its large range, the secretarybird is considered as monotypic: no subspecies are recognised.
The evolutionary relationship of the secretarybird to other raptors had long puzzled ornithologists. The species was usually placed in its own family Sagittariidae within the order Falconiformes. A large molecular phylogenetic study published in 2008 found that the secretarybird was sister to a clade containing the ospreys in the family Pandionidae and the kites, hawks and eagles in the family Accipitridae. The same study found that the falcons in the order Falconiformes were only distantly related to the other diurnal birds of prey. The families Cathartidae, Sagittariidae, Pandionidae and Accipitridae were therefore moved from Falconiformes to the resurrected Accipitriformes.[a] A later molecular phylogenetic study published in 2015 confirmed these relationships.
Sometimes, the enigmatic bird Eremopezus is classified as an early relative of the secretarybird, though this is uncertain as the bird is only known from a few fragmentary body parts such as the legs. The earliest fossils associated with the family are two species from the genus Pelargopappus. The two species, from the Oligocene and Miocene respectively, were not discovered in Africa but France. The feet in these fossils are more like those of the Accipitridae; it is suggested that these characteristics are primitive features within the family. In spite of their age, it is not thought that the two species are ancestral to the secretarybird. Though strongly convergent with the modern secretarybird, the extinct raptor Apatosagittarius is thought to be an accipitrid.
"Secretarybird" has been designated as the official common name for the species by the International Ornithologists' Union (IOC). The French polymath Georges-Louis Leclerc, Comte de Buffon suggested that the name secretary/secrétaire had been chosen because of the long quill-like feathers at the top of the bird's neck. More recently it has been suggested that "secretary" is from the French "secrétaire", a corruption of the Arabic saqr-et-tair meaning either "hawk of the semi-desert" or "hawk that flies". Professor Ian Glenn has dismissed this etymology on the grounds that there is no evidence that the name came through French. Instead, Buffon's etymology is almost certainly correct and the word comes from the Dutch "Secretarius" for "secretary" used by the settlers in South Africa. Glenn further suggests Vosmaer's "Sagittarius" is a misheard or mis-transcribed form of "Secretarius", rather than the other way around as proposed by Vosmaer.
The secretarybird is instantly recognizable as a very large bird with an eagle-like body on crane-like legs, which increase the bird's height to as much as 1.3 m (4.3 ft) tall. This bird has an eagle-like head with a hooked bill, but has rounded wings. Height can range from 90 to 137 cm (35 to 54 in). Total length from 112 to 152 cm (44 to 60 in) and the wingspan is 191–220 cm (75–87 in). Body mass can range from 2.3 to 5 kg (5.1 to 11.0 lb) with 20 birds from southern Africa found to weigh an average of 4.02 kg (8.9 lb). Other attempts to estimate the mean weight range for secretarybirds correspondingly lie between 3.5 and 4.2 kg (7.7 and 9.3 lb). The tarsus of the secretarybird averages 31 cm (12 in) and the tail is 57–85 cm (22–33 in), both factor into making them both taller and longer than any other species of raptor since these features are not as long in any other living raptor. The neck is not especially long, and can only be lowered down to the inter-tarsal joint, so birds reaching down to the ground or drinking must stoop to do so.
From a distance or in flight it resembles a crane more than a bird of prey. The tail has two elongated central feathers that extend beyond the feet during flight, as well as long flat plumage creating a posterior crest. The plumage of the crown, upperparts, and lesser and median wing coverts are blue-grey, and the underparts and underwing coverts are lighter grey to grey-white. The scapulars, primary and secondary flight feathers, rump and thighs are black, while the uppertail coverts are white, though barred with black in some individuals. It has a large wedge-shaped tail with alternating white and black banding at its ends. Sexes look similar to one another as the species exhibits very little sexual dimorphism, although the male has longer head plumes and tail feathers, and tends to have paler plumage. Adults have a featherless red-orange face with pale brown irises and a blue-grey cere. The legs are pinkish grey.
Immature birds have yellow bare skin on their faces, more brownish plumage, shorter tail feathers and greyish irises.
Distribution and habitat
The secretarybird is endemic to Sub-Saharan Africa and is non-migratory, though it may follow food sources. Its range extends from Mauritania to Somalia and south to the Cape of Good Hope. The species is also found at a variety of elevations, from the coastal plains to the highlands. The secretarybird prefers open grasslands, savannas and shrubland (Karoo) rather than forests and dense shrubbery which may impede its cursorial existence. More specifically, it prioritises areas with grass under 0.5 m high and avoids those with grass over 1 m high. It is rarer in grasslands in northern parts of its range that otherwise appear similar to areas in southern Africa where it is abundant, suggesting it may avoid hotter regions. It also avoids deserts.
Behaviour and ecology
Secretarybirds generally roost in trees of the genus Acacia or Balanites, or even introduced pine trees in South Africa. They set off 1–2 hours after dawn. Mated pairs roost together but may forage separately, though often remain in sight of one another. They pace around at a speed of 2.5–3 km/hour, taking 120 steps on average each minute. After spending much of the day on the ground, secretarybirds return at dusk, moving downwind before flying in upwind.
Secretarybirds form monogamous pairs and defend a large territory of around 50 km2 (19 sq mi). They can breed at any time of the year, more frequently in the late dry season. During courtship, they exhibit a nuptial display by soaring high with undulating flight patterns and calling with guttural croaking. Males and females can also perform a grounded display by chasing each other with their wings up and back, much like the way they defend their territory. They either mate on the ground or in trees.
The nest is built by both sexes in a dense thorny tree, often an Acacia, at a height of between 2.5 and 13 metres (8 and 40 ft) above the ground. The nest is constructed as a relatively flat platform of sticks 1.0–1.5 metres (3–5 ft) across with a depth 30–50 centimetres (12–20 in). The shallow depression is lined with grass and the occasional piece of dung.
Eggs are laid at two to three day intervals until the clutch of 2-3 eggs is complete. The elongated chalky bluish green or white eggs average 78 mm × 57 mm (3.1 in × 2.2 in) and weigh 130 g (4.6 oz). Both parents incubate the eggs starting as soon as the first egg is laid but it is usually the female that remains on the nest overnight. The incubating parent greets its partner when it returns with a display of bowing and bobbing its head with neck extended. The tail is held upright and its feathers fanned out,and the chest feathers are puffed out. The eggs hatch after around 45 days at intervals of 2–3 days. Both parents feed the young. The adults regurgitate food onto the floor of the nest and then pick up items and pass them to the chicks. Despite the difference in nestling size due to the asynchronous hatching, little sibling aggression has been observed. Under favourable conditions all chicks from a clutch of 3 eggs fledge, but if food is scarce one or more of the chick will die from starvation.
The young are born covered in grey-white down that becomes darker grey after two weeks. Their bare facial skin and legs are yellow. Crest feathers appear at 21 days, and flight feathers by 28 days. They can stand up and feed autonomously after 40 days, although the parents still feed the nestlings after that time. At 60 days, the now fully -feathered young start to flap their wings. Their weight gain over this period is variable, from 56 g (2.0 oz) at hatching, to 500 g (18 oz) at 20 days, 1.1 kg (2.4 lb) at 30 days, 1.7 kg (3.7 lb) at 40 days, 2 kg (4.4 lb) at 50 days, 2.5 kg (5.5 lb) at 60 days, and 3 kg (6.6 lb) at 70 days. The age at which they leave the nest is very variable but is usually around 75–80 days but can be anywhere between 65 and 106 days. Fledging is accomplished by jumping out of the nest or using a semi-controlled glide to the ground. After this time, the young are quickly taught how to hunt through expeditions with their parents and are considered independent soon after.
Juveniles remain in their natal range before dispersing when they are between four and seven months of age. The usual age at which they first breed is uncertain but there is a record of a male bird breeding successfully at an age of 2 years and 9 months. This is young for a large raptor.
The first successful rearing of a secretarybird in captivity occurred in 1986 at the Oklahoma City Zoo. Although secretarybirds build their nests in the trees in the wild, the captive birds at the Oklahoma City Zoo built theirs on the ground, which left their eggs open to depredation by local wild mammals. Therefore, zoo staff removed the eggs from the nest each time they were laid to be incubated and hatched at a safer location. The species now successfully breeds in captivity around the world, including in the San Diego Zoo.
Food and feeding
Unlike most birds of prey, the secretarybird is largely terrestrial, hunting its prey on foot. Adults hunt in pairs and sometimes as loose familial flocks, stalking through the habitat with long strides. Prey may consist of insects, mammals ranging in size from mice to hares and mongoose, crabs, lizards, snakes, tortoises, small birds, bird eggs, and sometimes dead animals killed in grass or bush fires. Larger herbivores are not generally hunted, although there are some reports of secretarybirds killing young gazelles and cheetah cubs. The importance of snakes in the diet has been exaggerated in the past, although they can be locally important and venomous species such as adders and cobras are regularly among the types of snake preyed upon. Secretarybirds do not eat carrion.
Prey is often flushed out of tall grass by the birds stomping on the surrounding vegetation. It also waits near fires, eating anything it can that is trying to escape. They can either catch prey by chasing it and striking with the bill and swallowing (usually with small prey), or stamping on prey until it is rendered stunned or unconscious enough to swallow. Larger or dangerous prey, such as venomous snakes, are instead stunned or killed by the bird jumping onto their backs, at which point they will try to snap their necks or backs. There are some reports that, when capturing snakes, the secretarybirds will take flight with their prey and then drop them to their death, although this has not been verified. Even with larger prey, food is generally swallowed whole through the birds' considerable gape. Occasionally, like other raptors, they will tear apart prey with their feet before consuming it.
Young are fed liquefied and regurgitated insects directly by the male or female parent and are eventually weaned to small mammals and reptile fragments regurgitated onto the nest itself. The above foodstuffs are originally stored in the crop of the adults.
The secretarybird has a relatively short digestive tract in comparison to other large African birds such as the kori bustard. As the foregut is specialized for digesting large amounts of meat in a short amount of time, there is little need for the physical breakdown of food within the digestive tract over extended time spans. The crop of the secretarybird is dilated and the gizzard is nonmuscular in comparison to other birds. The large intestine lacks a cecum as there is little need for fermentative digestion of plant material.
Secretarybirds specialize in stomping their prey until the prey is killed or immobilized. This method of hunting is commonly applied to lizards or snakes. An adult male trained to strike at a rubber snake on a force plate was found to hit with a force equal to 5 times its own body weight, with a contact period of only 10–15 ms. This short time of contact suggests that the secretarybird relies on superior visual targeting to determine the precise location of the prey's head. Although little is known about its visual field, it is assumed that it is large, frontal and binocular.
As secretarybirds are anatomically similar (but apparently not closely related) to the extinct Phorusrhacidae, it has been hypothesized that these birds may have employed a similar hunting technique.
Secretarybirds have unusually long legs (nearly twice as long as other ground birds of the same body mass), which is thought to be an adaptation for the bird's unique stomping/striking hunting method. However, these long limbs appear to also lower its running efficiency.
Relationship with humans
The secretarybird has traditionally been admired in Africa for its striking appearance and ability to deal with pests and snakes. Africans sometimes call it the Devil's Horse. As such it has often not been disturbed, although this is changing as traditional observances have declined.
In Sudan, it is featured in the middle white strip of the Presidential Flag; it is the main object on the Presidential Seal, and features heavily in Sudanese military insignia. The secretarybird on the Presidential Flag and Seal has its head turned to the right, with its distinctive crest clearly visible and its wings spread out with a white banner between its outstretched wings reading "Victory Is Ours".
The secretarybird has been a common motif for African countries on postage stamps: over a hundred stamps from 36 issuers are known, including some from stamp-issuing entities such as Ajman, Manama, the Maldives and the United Nations where the bird does not exist.
Threats and conservation
The International Union for the Conservation of Nature (IUCN) lists the secretarybird as a vulnerable species, due to a recent rapid decline across its entire range, particularly in South Africa. Although widespread, the species is thinly spread across its range; its population has been estimated at anywhere between 6,700 and 67,000 individuals.
Long term monitoring across South Africa between 1987 and 2013 has shown that populations have declined in Kruger National Park as the vegetation cover has increased there, resulting in loss of open habitat that the species prefers.
The young are preyed upon by crows, ravens, hornbills, large owls and kites as they are vulnerable in Acacia tree tops, with no known incidents of predation on adults. As a population, the secretarybird is mainly threatened by loss of habitat and deforestation. In 1968 the species became protected under the Africa Convention on the Conservation of Nature and Natural Resources. Loss of grassland to bush encroachment driven by rising CO2 levels has also been implicated, as has a susceptibility to power line collisions.
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|Wikimedia Commons has media related to Sagittarius_serpentarius.|
- Secretary Bird videos, photos & sounds on the Internet Bird Collection
- Birdlife Species Factsheet
- Secretarybird on ARKive
- Secretary Bird on postage stamps
- Species text in The Atlas of Southern African Birds