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In Malaysia, large areas of rainforest were felled to make way for oil palm plantations; and with few tree cavities for breeding, the barn owl population, with its ability to control rodent pests, diminished. In a trial, the provision of two hundred nest boxes saw almost one hundred percent occupancy; and as the programme expanded, the plantations supported one of the densest barn owl populations in the world.<ref name="Taylor261" /> Similarly, providing nesting boxes has increased the number of barn owls in rice-growing areas of Malaysia, where the rodents do much damage to the crop. Although barn owl numbers have increased in both these instances, it is unclear as to how effective this biological control of the rats is, as compared to the baiting and trapping that occurred previously.<ref>{{cite journal |author1=Wood, B. J. |author2=Fee, Chung Gait |year=2003 |title=A critical review of the development of rat control in Malaysian agriculture since the 1960s |journal=Crop Protection |volume=22 |issue=3 |pages=445–461 |doi=10.1016/S0261-2194(02)00207-7}}</ref>
In Malaysia, large areas of rainforest were felled to make way for oil palm plantations; and with few tree cavities for breeding, the barn owl population, with its ability to control rodent pests, diminished. In a trial, the provision of two hundred nest boxes saw almost one hundred percent occupancy; and as the programme expanded, the plantations supported one of the densest barn owl populations in the world.<ref name="Taylor261" /> Similarly, providing nesting boxes has increased the number of barn owls in rice-growing areas of Malaysia, where the rodents do much damage to the crop. Although barn owl numbers have increased in both these instances, it is unclear as to how effective this biological control of the rats is, as compared to the baiting and trapping that occurred previously.<ref>{{cite journal |author1=Wood, B. J. |author2=Fee, Chung Gait |year=2003 |title=A critical review of the development of rat control in Malaysian agriculture since the 1960s |journal=Crop Protection |volume=22 |issue=3 |pages=445–461 |doi=10.1016/S0261-2194(02)00207-7}}</ref>

==== Rodent Consumption ====
Barn owls are the most widespread land bird species in the world, and can be found on every continent except Antarctica. Barn owls have a wide range of diet but in North America and Europe, voles and shrews make up the majority of their diet. Barn owls may also consume birds, lizards, insects, amphibians, and other rodents. Barn owls also have a high metabolism for their size, and typically require one or more voles per night to sustain them. This makes the barn owl one of the top consumers of rodents in the world<ref name=":0">http://revistas.unisinos.br/index.php/neotropical/article/view/nbc.2016.111.03</ref>. It is estimated that a family of five barn owls will consume around 1000 rodents in a single season<ref name=":1">{{Cite web|title=Barn owls help clean up rodents naturally|url=https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=25626|access-date=2021-12-01|website=ANR Blogs|language=en-us}}</ref>.

==== Use on Farms ====
Because of their large rodent requirements, barn owls make very effective pest control for farmers<ref name=":2">{{Cite web|date=2016-08-19|title=Are Barn Owls Nature’s Best Pest Control?|url=https://www.sierraclub.org/sierra/2016-4-july-august/green-life/are-barn-owls-nature-s-best-pest-control|access-date=2021-12-01|website=Sierra Club|language=en}}</ref>. Barn owls consume thousands of rodents every year and there can be multiple families of barn owls on a single farm or property. A farmer can install a barn owl nest box for every 10 to 20 acres of land and can be placed as close as 100 feet apart from each other. Barn owls can also be established on many different types of farms, like vineyards, rice fields, fruit orchards, and nut crops<ref name=":3">{{Cite web|last=Adler|first=Steve|date=April 27, 2016|title=Three-year study evaluates owls as rodent control|url=https://www.raptorsarethesolution.org/wp-content/uploads/2016/05/barn-owl-benefits-ag-alert_4_27_16.pdf|url-status=live|website=www.raptorsarethesolution.org}}</ref>. This is because barn owls are found in many different habitats across the world, as long as there is a stable source of prey<ref name=":0" />. Barn owls are extremely effective at controlling rodent populations, and can be a more environmentally friendly option at controlling pests than rodenticides. Rodenticides can travel up the food chain and kill predators like raptors, foxes, and coyotes. Barn owls can remove the use of rodenticides, traps, and chemicals by lowering the population of rodents naturally<ref name=":2" />. Barn owls can be attracted to a farmers field by setting up and maintaining nest boxes<ref name=":3" />. Barn owls also come back to the same breeding sites every year, have multiple young, live in their nest year-round and typically live in concentrated groups, making barn owl pest control self-sustaining<ref name=":2" />.

==== Cost Effectiveness ====
[[File:Barn Owl nest box - geograph.org.uk - 566792.jpg|thumb|Barn Owl Nest Box]]
Barn owls as pest control are also cost effective for farmers. In a two year study using barn owls as pest control it calculated that the average cost of taking a single rodent was 26 cents compared to 8 dollars and 11 cents to trap each rodent. This estimate did not include the savings the farmers would be making from a lower number of damage to equipment from rodent holes, crop damage, and reduced holes in irrigation systems and other equipment<ref name=":3" />. The same study found that the average cost of applying strychnine poison to kill rodents was 1,400 dollars per year, and that it would cost more for farmers in the long term to use strychnine poison than to install barn owls on property. The initial cost to install barn owls on the farmers property was 6,025 dollars, which means that over 10 years, strychnine poison would be about double the expenses as hosting barn owls on property<ref name=":2" />. Barn owl boxes require regular maintenance that includes cleaning once a year, annual structural inspections, and replacing mulch inside the boxes<ref name=":1" /><ref name=":3" />. Barn owl boxes do not need to be placed higher than 8 feet off the ground, making them easy to access and maintain. Farmers may also be able to make some cash selling the barn owl pellets to schools for the students to dissect them<ref name=":3" />.


==References==
==References==

Revision as of 20:06, 3 December 2021

This article is about the common barn owl species. For the entire barn owl family, see Tytonidae. For the band, see Barn Owl (band). For the place in Siberia, see Barnaul.

Barn owl
Temporal range: Quaternary–present
↓
Barn owl at the British Wildlife Centre, England
Barn Owl screeches recorded in Ceredigion, Wales
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Order: Strigiformes
Family: Tytonidae
Genus: Tyto
Species:
T. alba
Binomial name
Tyto alba
(Scopoli, 1769)
Global range in green
Synonyms

Strix alba Scopoli, 1769
Strix pratincola Bonaparte, 1838
Tyto delicatula Gould, 1837

The barn owl (Tyto alba) is the most widely distributed species of owl in the world and one of the most widespread of all species of birds, being found almost everywhere in the world except for the polar and desert regions, Asia north of the Himalayas, most of Indonesia, and some Pacific Islands. It is also known as the common barn owl, to distinguish it from the other species in its family, Tytonidae, which forms one of the two main lineages of living owls, the other being the typical owls (Strigidae).

Phylogenetic evidence shows that there are at least three major lineages of barn owl: one in Europe, western Asia, and Africa; one in southeastern Asia and Australasia; and one in the Americas; as well as some highly divergent taxa on various islands. Accordingly, some authorities divide barn owls into the western barn owl, for the group in Europe, western Asia, and Africa; the eastern barn owl for the group in southeastern Asia and Australasia; and the American barn owl for the group in the Americas. Some taxonomic authorities classify barn owls differently, recognising up to five separate species; and further research needs to be done to resolve the disparate taxonomies. There is considerable variation of size and colour among the approximately 28 subspecies, but most are between 33 and 39 cm (13 and 15 in) in length, with wingspans ranging from 80 to 95 cm (31 to 37 in). The plumage on the head and back is a mottled shade of grey or brown; that on the underparts varies from white to brown and is sometimes speckled with dark markings. The face is characteristically heart-shaped and is white in most subspecies. This owl does not hoot, but utters an eerie, drawn-out screech.

The barn owl is nocturnal over most of its range; but in Great Britain and some Pacific Islands, it also hunts by day. Barn owls specialise in hunting animals on the ground and nearly all of their food consists of small mammals, which they locate by sound, their hearing being very acute. The owls usually mate for life unless one of the pair is killed, whereupon a new pair bond may be formed. Breeding takes place at varying times of the year, according to the locality, with a clutch of eggs, averaging about four in number, being laid in a nest in a hollow tree, old building, or fissure in a cliff. The female does all the incubation, and she and the young chicks are reliant on the male for food. When large numbers of small prey are readily available, barn owl populations can expand rapidly; and globally the bird is considered to be of least conservation concern. Some subspecies with restricted ranges are more threatened.

Etymology

The barn owl was one of several species of bird first described in 1769 by the Tyrolean physician and naturalist Giovanni Antonio Scopoli in his Anni Historico-Naturales. He gave it the scientific name Strix alba.[2][3] As more species of owl were described, the genus Strix came to refer solely to the wood owls in the typical-owl family Strigidae; and the barn owl became Tyto alba in the barn-owl family Tytonidae. Tyto alba literally means 'white owl', from the onomatopoeic Ancient Greek τυτώ tyto, 'owl' – compare English "hooter" – and Latin alba, 'white'.[4] The bird is known by many common names that refer to its appearance, call, habitat, or its eerie, silent flight: white owl,[5] silver owl, demon owl, ghost owl, death owl, night owl, rat owl, church owl, cave owl, stone owl, monkey-faced owl, hissing owl, hobgoblin or hobby owl, dobby owl, white-breasted owl, golden owl, screech owl, straw owl, barnyard owl, and delicate owl.[4][6] "Golden owl" might also refer to the related golden masked owl (T. aurantia). "Hissing owl" and, particularly in the U.K. and in India, "screech owl" refer to the piercing calls of these birds.[7] The latter name is also applied to a different group of birds, the screech-owls in the genus Megascops.[4]

Description

Barn owl in flight

The barn owl is a medium-sized, pale-coloured owl with long wings and a short, squarish tail. There is considerable size variation across the subspecies, with a typical specimen measuring about 33 to 39 cm (13 to 15 in) in overall length, the length ranging from 29 to 44 cm (11 to 17 in). Barn owls have a typical wingspan of some 80 to 95 cm (31 to 37 in), with an extreme range of from 68 to 105 cm (27 to 41 in).[8][9] Adult body mass is also variable, with male owls from the Galapagos (T. a. punctatissima) weighing approximately 260 g (9.2 oz) on average, while male eastern barn owls (T. javanica) average 555 g (19.6 oz), the weight of all barn owls ranging from 224 to 710 g (7.9 to 25.0 oz).[10] Female barn owls are larger than males, with a strong female T. alba of a large subspecies weighing over 550 grams (19 oz).[11] In general, owls living on small islands are smaller and lighter, perhaps because they have a higher dependence on insect prey and need to be more manoeuvrable,[12] an exception being the largest-bodied race of barn owl, T. a. furcata, from Cuba and Jamaica, which is also an island race, albeit being found on sizeable islands with larger prey available and few larger owls competing for dietary resources.[9] The shape of the tail is a means of distinguishing the barn owl from typical owls when seen in the air. Other distinguishing features are the undulating flight pattern and the dangling, feathered legs. The pale face with its heart shape and black eyes give the flying bird a distinctive appearance, like a flat mask with oversized, oblique black eye-slits, the ridge of feathers above the beak somewhat resembling a nose.[13]

In most subspecies, the bird's head and upper body typically vary between pale brown and some shade of grey (especially on the forehead and back). Some are a purer, richer brown instead, and all have fine black-and-white speckles except on the main wing feathers and tail feathers, which are light brown with darker bands. The heart-shaped face is usually bright white, but in some subspecies it is brown.[4][14] The left ear is slightly above the eyes, whereas the right ear is slightly below. Also, the orientation of one ear's covert feathers differs, by about 15°, from those of the other.[15] The underparts, including the tarsometatarsal (lower leg) feathers, vary from white to reddish buff among the subspecies, and are either mostly unpatterned or bear a varying number of tiny blackish-brown speckles. It has been found that, at least in the continental European populations, females with more spotting are healthier than plainer birds. By contrast, this does not hold true for European males, where the spotting varies according to subspecies. The beak varies from pale horn to dark buff, corresponding to the general plumage hue, and the iris is blackish brown. The feet, like the beak, vary in colour, ranging from pink to dark pinkish-grey; and the talons are black.[4][14] Within any one population, males, on average, tend to have fewer spots on the underside, and are paler in colour, than females. Nestlings are covered in white down, but the heart-shaped facial disk becomes visible soon after hatching.[11]

Contrary to popular belief, the barn owl does not hoot (such calls are made by typical owls, such as the tawny owl, or other members of the genus Strix). It instead produces the characteristic shree scream, an eerie, long-drawn-out shriek that is painful for humans to hear at close range. Males in courtship give a shrill twitter. Both young and old animals produce a snake-like defensive hiss when disturbed. Other sounds produced include a purring chirrup denoting pleasure, and a "kee-yak", which resembles one of the vocalisations of the tawny owl. When captured or cornered, the barn owl throws itself on its back and flails with sharp-taloned feet, making for an effective defence. In such situations, it may emit rasping or clicking noises, produced probably by the beak but possibly by the tongue.[13][16]

Distribution

Barn owl, Canada, with detail of wings and tail feathers

The barn owl is the most widespread landbird species in the world, occurring on every continent except Antarctica.[17] Its range includes all of Europe (except Fennoscandia and Malta), most of Africa apart from the Sahara, the Indian subcontinent, Southeast Asia, Australia, many Pacific Islands, and North-, Central-, and South America.[17] In general, it is considered to be sedentary; and, indeed, many individuals, having taken up residence in a particular location, remain there even when better nearby foraging areas are available. In the British Isles, the young seem largely to disperse along river corridors; and the distance travelled from their natal site averages about 9 km (5.6 mi).[18]

In continental Europe the dispersal distance is greater, commonly somewhere between 50 and 100 kilometres (31 and 62 mi) but exceptionally 1,500 km (932 mi), with ringed birds from the Netherlands ending up in Spain and in Ukraine. In the United States, dispersal is typically over distances of 80 and 320 km (50 and 199 mi), with the most travelled individuals ending up some 1,760 km (1,094 mi) from their points of origin. Dispersal movements in the African continent include 1,000 km (621 mi) from Senegambia to Sierra Leone and up to 579 km (360 mi) within South Africa. In Australia there is some migration as the birds move towards the northern coast in the dry season and southward in the wet season, as well as nomadic movements in association with rodent plagues. Occasionally, some of these birds turn up on Norfolk Island, Lord Howe Island, or New Zealand, showing that crossing the ocean is within their capabilities.[4] In 2008, barn owls were recorded for the first time breeding in New Zealand.[19] The barn owl has been successfully introduced into the Hawaiian island of Kauai in an attempt to control rodents; distressingly, it has been found to also feed on native birds.[20]

Taxonomy

Male Tyto alba alba (left) and female T. a. guttata barn owls in the Netherlands, where these subspecies intergrade

The ashy-faced owl (T. glaucops) was for some time included in T. alba. Based on DNA evidence, König, Weick & Becking (2009) recognised the American barn owl (T. furcata) and the Curaçao barn owl (T. bargei) as separate species.[8] They also proposed that T. a. delicatula should be split off as a separate species, to be known as the eastern barn owl, which would include the subspecies T. d. delicatula, T. d. sumbaensis, T. d. meeki, T. d. crassirostris, and T. d. interposita.[21] The International Ornithological Committee has its doubts and has stated that the split of Tyto delicatula from T. alba "may need to be revisited".[22]

Some island subspecies are occasionally treated as distinct species, a move which should await further research into barn owl phylogeography. According to Murray Bruce in Handbook of Birds of the World Volume 5: Barn-owls to Hummingbirds, "a review of the whole group [is] long overdue".[4] Molecular analysis of mitochondrial DNA shows a separation of the species into two clades, an Old World alba and a New World furcata, but this study did not include T. a. delicatula, which the authors seem to have accepted as a separate species. Extensive genetic variation was also found between the Indonesian T. a. stertens and other members of the alba clade, leading to the separation of stertens into Tyto javanica.[23]

Twenty to thirty subspecies are usually recognized, varying mainly in body proportions, size, and colour. Barn owls range in colour from the almost beige-and-white nominate subspecies alba, erlangeri, and niveicauda, to the nearly black-and-brown contempta. Island forms are mostly smaller than mainland ones, and those inhabiting forests have darker plumage and shorter wings than those living in open grasslands. Several subspecies are generally considered to be intergrades between more distinct populations.[24][4]

In Handbook of Birds of the World Volume 5: Barn-owls to Hummingbirds, the following subspecies are listed:[4]

Subspecies Description Range Synonyms
T. a. alba (Scopoli, 1769)
Light-coloured adult
 Upperparts grey and light buff. Underparts white, with few if any black spots; males often appear entirely unspotted.[14] Western Europe from the British Isles south to the Maghreb and east along Mediterranean coastal regions to northwestern Turkey in the north and the Nile in the south, where it reaches upstream to northeastern Sudan. Also the Aïr Mountains in the Sahara of Niger, the Balearic Islands and Sicily in the Mediterranean, and the West Canary Islands (El Hierro, La Gomera, La Palma Gran Canaria and Tenerife). Intergrades with guttata from the Balkans through Hungary and along the Rhine and Lower Meuse Rivers, and with affinis around the Egypt-Sudan border.[4] common barn owl – includes hostilis, kirchhoffi, kleinschmidti and pusillus.
African populations might belong to erlangeri.
T. a. furcata (Temminck, 1827)
In Cuba
 Large. Upperparts pale orange-buff and brownish-grey, underparts whitish with few speckles. Face white.[4] Cuba, Jamaica, the Cayman Islands (rare or possibly extirpated on Grand Cayman).[25] Caribbean barn owl – might include niveicauda.
T. a. tuidara (J.E. Gray, 1829)
In Brazil
 Upperparts grey and orange-buff. Underparts whitish to light buff with little speckling. Face white. Resembles pale guttata.[4] South American lowlands east of the Andes and south of the Amazon River all the way south to Tierra del Fuego; also on the Falkland Islands.[4] Includes hauchecornei and possibly hellmayri.
T. a. guttata (C. L. Brehm, 1831)
In flight, Sandesneben (Germany)
 More grey on upperparts than alba. Underparts buff to rufous with some dark speckles (more than in alba).[14] Face whitish. Females are on average redder below than males.[4] Central Europe north of the Alps from the Rhine to Latvia, Lithuania and Ukraine, and south to Romania, northeastern Greece and the southern Balkans. Intergrades with alba at the western border of its range.[4] Includes rhenana.
T. a. pratincola (Bonaparte, 1838)
Adult in flight
 Large. Upperparts grey and orange-buff. Underparts whitish to light buff with much speckling. Face white. Resembles pale guttata, but usually more speckles below.[4] North America from southern Canada south to central Mexico; Bermuda, the Bahamas, Hispaniola; introduced to Lord Howe Island (where it was extirpated) and in 1958 to Hawaii (where it still lives).[4] North American barn owl – Includes lucayana; might also include bondi, guatemalae and subandeana.
T. a. punctatissima (G. R. Grey, 1838)
On Santa Cruz Island (the Galápagos Islands)
 Small. Dark greyish-brown above, with white part of spots prominent. Underparts white to golden-buff, with distinct pattern of brown vermiculations or fine dense spots.[4] Endemic to the Galápagos islands.[4] Galápagos barn owl – sometimes considered a separate species.
T. a. poensis (Fraser, 1842) Upperparts golden-brown and grey with very bold pattern. Underparts light buff with extensive speckles. Face white.[4] Endemic to Bioko.[4] May include affinis.
T. a. thomensis (Hartlaub, 1852) Smallish. Upperparts dark brownish grey with bold pattern, including lighter brown bands on remiges and rectrices. Underparts golden brown with extensive speckles. Face buff.[4] Endemic to São Tomé Island. A record from Príncipe is in error.[4] São Tomé barn owl – sometimes considered a separate species.
T. a. affinis (Blyth, 1862)
In Tanzania
 Similar to poensis, but supposedly lighter on average. Upperparts very grey. Underparts light buff with extensive speckles. Face white.[26] Sub-Saharan Africa, including the Comoros, Madagascar, Pemba and Unguja Islands; introduced to the Seychelles. Intergrades with alba around the Egypt-Sudan border.[4] Includes hypermetra; doubtfully distinct from poensis.[26]
T. a. guatemalae (Ridgway, 1874) Similar to dark pratincola; less grey above, coarser speckles below.[4] Guatemala or southern Mexico through Central America to Panama or northern Colombia; the Pearl Islands.[4] Includes subandeana; doubtfully distinct from pratincola.
T. a. bargei (Hartert, 1892) Similar to alba; smaller and noticeably short-winged.[4] Endemic to Curaçao and maybe Bonaire in the West Indies.[4] Curaçao barn owl – sometimes considered a separate species.
T. a. contempta (Hartert, 1898) Almost black with some dark grey above, the white part of the spotting showing prominently. Reddish-brown below.[4] Northeastern Andes from western Venezuela through eastern Colombia (rare in the Cordillera Central and Cordillera Occidental)[27] south to Peru.[4] Includes stictica.
T. a. schmitzi (Hartert, 1900) Small. Similar to guttata, but breast region light buff.[4] Endemic to Madeira and Porto Santo Island in the eastern Atlantic.[4]
T. a. ernesti (Kleinschmidt, 1901) Similar to alba; breast region always pure unspotted white.[4] Endemic to Corsica and Sardinia in the Mediterranean.[4]
T. a. gracilirostris (Hartert, 1905) Small. Similar to schmitzi but breast darker, approaching guttata. Face light buff.[4] Endemic to the East Canary Islands (Chinijo Archipelago, Fuerteventura, Lanzarote; perhaps formerly also on Lobos).[4] Canary Islands barn owl
T. a. detorta (Hartert, 1913) Similar to guttata, but less reddish. Face buff.[4] Endemic to the Cape Verde Islands.[4] Cape Verde barn owl -sometimes considered a separate species.
T. a. erlangeri (W. L. Sclater, 1921) Similar to ernesti; upperparts lighter and yellower.[4] Crete and southern Aegean islands to Cyprus; the Near and Middle East including the Arabian Peninsula coastlands, south to Sinai and east to southwestern Iran.[4] Might include African populations assigned to alba.
T. a. hellmayri (Griscom & Greenway, 1937) Similar to tuidara, but larger.[4] Northeastern South American lowlands from eastern Venezuela south to the Amazon River.[4] Doubtfully distinct from tuidara.
T. a. bondi (Parks & Phillips, 1978) Similar to pratincola; smaller and paler on average.[4] Endemic to Roatán and Guanaja in the Bay Islands.[4] Doubtfully distinct from pratincola.
T. a. niveicauda (Parks & Phillips, 1978) Large. Similar to furcata; paler in general. Resembles alba.[4] Endemic to Isla de la Juventud.[4] Doubtfully distinct from furcata.

Behaviour and ecology

Like most owls, the barn owl is nocturnal, relying on its acute sense of hearing when hunting in complete darkness. It often becomes active shortly before dusk but can sometimes be seen during the day when relocating from one roosting site to another. In Britain, on various Pacific Islands, and perhaps elsewhere, it sometimes hunts by day. The owl's daylight hunting may depend on whether it can avoid being mobbed by other birds during that time.[4] In Britain, some birds continue to hunt by day—even when mobbed by such birds as magpies, rooks, and black-headed gulls—possibly because the previous night has been wet, making night hunting difficult. By contrast, in southern Europe and the tropics, the birds seem to be almost exclusively nocturnal, with the few birds that hunt by day being severely mobbed.[28]

Being mobbed by Brewer's blackbird in California
A barn owl's talons

Barn owls are not particularly territorial but have a home range inside which they forage. For males in Scotland this home range has a radius of about 1 km (0.6 mi) from the nest site and an average area of about 300 hectares (740 acres). Female home ranges largely coincide with that of their mates. Outside the breeding season, males and females usually roost separately, each one having about three favoured sites in which to conceal themselves by day, and which are also visited for short periods during the night. Roosting sites include holes in trees, fissures in cliffs, disused buildings, chimneys, and hay sheds, and are often small in comparison to nesting sites. As the breeding season approaches, the birds move back to the vicinity of a chosen nest to roost.[29]

The barn owl is a bird of open country, such as farmland or grassland with some interspersed woodland, usually at altitudes below 2,000 metres (6,600 ft) but occasionally as high as 3,000 metres (9,800 ft) in the tropics, such as in Ethiopia's Degua Tembien mountain range.[30] This owl prefers to hunt along the edges of woods or in rough grass strips adjoining pasture. It has an effortless wavering flight as it quarters the ground, alert to the sounds made by potential prey. Like most owls, the barn owl flies silently; tiny serrations on the leading edges of its flight feathers and a hairlike fringe on the trailing edges help to break up the flow of air over the wings, thereby reducing turbulence and the noise that accompanies it. Hairlike extensions to the barbules of its feathers, which give the plumage a soft feel, also minimise noise produced during wingbeats.[31] Behavioural and environmental preferences may differ slightly even between neighbouring subspecies, as shown in the case of the European T. a. guttata and T. a. alba, which probably evolved, respectively, in allopatric glacial refugia in southeastern Europe, and in Iberia or southern France.[14][32]

Hunting and feeding

Hunting in twilight or at night, the barn owl can target its prey and dive to the ground. Its legs and toes are long and slender, which improves its ability to forage among dense foliage or beneath the snow, and gives it a wide spread of talons when attacking prey. This bird hunts by flying slowly, quartering the ground and hovering over spots that may conceal prey. It has long, broad wings that enable it to manoeuvre and turn abruptly. It has acute hearing, with ears placed asymmetrically, which improves detection of sound position and distance; the bird does not require sight to hunt. The facial disc helps with the bird's hearing, as is shown by the fact that, with the ruff feathers removed, the bird can still determine a sound source's direction, although without the disc it can't determine the source's height.[33] It may perch on branches, fence posts, or other lookouts to scan its surroundings; and this is the main means of prey location in the oil palm plantations of Malaysia.[31][34]

Skull, showing the powerful beak

Rodents and other small mammals may constitute over ninety percent of the prey caught.[35][36][37] Birds are also taken, as well as lizards, amphibians, and insects. Even when they are plentiful, and other prey scarce, earthworms do not seem to be consumed.[38] In North America and most of Europe, voles predominate in the diet and shrews are the second most common food choice. In Ireland, the accidental introduction of the bank vole in the 1950s led to a major shift in the barn owl's diet: where their ranges overlap, the vole is now by far the largest prey item.[39] Mice and rats are the main foodstuffs in the Mediterranean region, the tropics, subtropics, and Australia. Barn owls are usually specialist feeders in productive areas and generalists in areas where prey is scarce.[38] On the Cape Verde Islands, geckos are the mainstay of the diet, supplemented by birds such as plovers, godwits, turnstones, weavers, and pratincoles.[40] On a rocky islet off the coast of California, a clutch of four young were being reared on a diet of Leach's storm petrel (Oceanodroma leucorhoa).[41] On bird-rich islands, a barn owl might include birds as some fifteen to twenty percent of its diet, while in grassland it will gorge itself on swarming termites, or on Orthoptera such as Copiphorinae katydids, Jerusalem crickets (Stenopelmatidae), or true crickets (Gryllidae). Smaller prey is usually torn into chunks and eaten completely, including bones and fur, while prey larger than about 100 grams (3.5 oz)—such as baby rabbits, Cryptomys blesmols, or Otomys vlei rats—is usually dismembered and the inedible parts discarded.[26][32][42]

Compared to other owls of similar size, the barn owl has a much higher metabolic rate, requiring relatively more food. Relative to its size, barn owls consume more rodents—often regarded as pests by humans—than possibly any other creature. Studies have shown that an individual barn owl may eat one or more voles (or their equivalent) per night, equivalent to about twenty-three percent of the bird's bodyweight. Excess food is often cached at roosting sites and can be used when food is scarce.[43] This makes the barn owl one of the most economically valuable wildlife animals for agriculture. Farmers often find these owls more effective than poison in keeping down rodent pests, and they can encourage barn owl habitation by providing nesting sites.[34]

Breeding

Egg, Collection Museum Wiesbaden, Germany

Barn owls living in tropical regions can breed at any time of year, but some seasonality in nesting is still evident. Where there are distinct wet and dry seasons, egg-laying usually takes place during the dry season, with increased rodent prey becoming available to the birds as the vegetation dies off. In arid regions, such as parts of Australia, breeding may be irregular and may happen in wet periods, with the resultant temporary increase in the populations of small mammals. In temperate climates, nesting seasons become more distinct; and there are some seasons of the year when no egg-laying takes place. In Europe and North America, most nesting takes place between March and June, when temperatures are increasing. The actual dates of egg-laying vary by year and by location, being correlated with the amount of prey-rich foraging habitat around the nest site.[44] An increase in rodent populations will usually stimulate the local barn owls to begin nesting; and, consequently, two broods are often raised in a good year, even in the cooler parts of the owl's range. [6]

Females are ready to breed at ten to eleven months of age. Barn owls are usually monogamous, sticking to one partner for life unless one of a pair dies. During the non-breeding season they may roost separately; but as the breeding season approaches, they return to their established nesting site, showing considerable site fidelity. In colder climates, in harsh weather, and where winter food supplies may be scarce, they may roost in farm buildings and in barns between hay bales; but they then run the risk that their selected nesting hole may be taken over by some other species. Single males may establish feeding territories, patrolling the hunting areas, occasionally stopping to hover, and perching on lofty eminences where they screech to attract a mate. Where a female has lost her mate but maintained her breeding site, she usually seems to attract a new spouse.[45]

Clutch of eggs

Once a pair-bond has been formed, the male will make short flights at dusk around the nesting and roosting sites and then longer circuits to establish a home range. When he is later joined by the female, there is much chasing, turning, and twisting in flight, and frequent screeches, the male's being high-pitched and tremulous and the female's lower and harsher. In later stages of courtship, the male emerges at dusk, climbs high into the sky, and then swoops back to the vicinity of the female at speed. He then sets off to forage. The female meanwhile sits in an eminent position and preens, returning to the nest a minute or two before the male arrives with food for her. Such feeding behaviour of the female by the male is common, helps build the pair-bond, and increases the female's fitness before egg-laying commences.[45]

Barn owls are cavity nesters. They choose holes in trees, fissures in cliff faces, the large nests of other birds such as the hamerkop (Scopus umbretta), and, particularly in Europe and North America, old buildings such as farm sheds and church towers. Buildings are preferred to trees in wetter climates in the British Isles and provide better protection for fledglings from inclement weather. Tree nests tend to be in open habitats rather than in the middle of woodland, and nest holes tend to be higher in North America than in Europe, because of possible predation by raccoons (Procyon lotor). No nesting material is used as such but, as the female sits incubating the eggs, she draws in the dry furry material of which her regurgitated pellets are composed, so that by the time the chicks are hatched, they are surrounded by a carpet of shredded pellets. Oftentimes other birds such as jackdaws (Corvus monedula) nest in the same hollow tree or building and seem to live harmoniously with the owls.[46]

Brood prior to fledging, beginning to shed their nestling down

Before commencing laying, the female spends much time near the nest and is entirely provisioned by the male. Meanwhile, the male roosts nearby and may cache any prey that is surplus to their requirements. When the female has reached peak weight, the male provides a ritual presentation of food and copulation occurs at the nest. The female lays eggs on alternate days and the clutch size averages about five eggs (the range being two to nine). The eggs are chalky white, somewhat elliptical, and about the size of bantam eggs. Incubation begins as soon as the first egg is laid. While the female is sitting on the nest, the male is constantly bringing more provisions, and they may pile up beside the female. The incubation period is about thirty days, hatching takes place over a prolonged period, and the youngest chick may be several weeks younger than its oldest sibling. In years with a plentiful supply of food, there may be a hatching success rate of about 75%. The male continues to copulate with the female when he brings food, which makes the newly hatched chicks vulnerable to injury.[45]

The chicks are at first covered with greyish-white down and develop rapidly. Within a week they can hold their heads up and shuffle around in the nest. The female tears up the food brought by the male and distributes it to the chicks. Initially, the chicks make a "chittering" sound but this soon changes into a food-demanding "snore". By two weeks old they are already half their adult weight and look naked, as the amount of down is insufficient to cover their growing bodies. By three weeks old, quills are starting to push through the skin and the chicks stand, making snoring noises with wings raised and tail stumps waggling, begging for food items which are now given whole. Atypically among birds, barn owl chicks can "negotiate" and allow weaker ones to eat first, possibly in exchange for grooming.[47] The male is the main provider of food until all the chicks are at least four weeks old, at which time the female begins to leave the nest and starts to roost elsewhere. By the sixth week the chicks are as big as the adults, but have slimmed down somewhat by the ninth week when they are fully fledged and start leaving the nest briefly themselves. They are still dependent on the parent birds until about thirteen weeks and receive training from the female in finding, and eventually catching, prey.[45]

Moulting

Feathers become abraded over time and all birds need to replace them at intervals. Barn owls are particularly dependent on their ability to fly quietly and manoeuvre efficiently. In temperate areas the owls undergo a prolonged moult that lasts through three phases over a period of two years. The female starts to moult while incubating the eggs and brooding the chicks, a time when the male feeds her, so she does not need to fly much. The first primary feather to be shed is a central one, number 6, and it has regrown completely by the time the female resumes hunting. Feathers 4, 5, 7, and 8 are dropped at a similar time the following year and feathers 1, 2, 3, 9 and 10 in the bird's third year of adulthood. The secondary and tail feathers are lost and replaced over a similar timescale, again starting while incubation is taking place. In the case of the tail, the two outermost tail feathers are first shed, followed by the two central ones, the other tail feathers being shed the following year.[48]

The male owl moults rather later in the year than the female, at a time when there is an abundance of food, the female has recommenced hunting, and the demands of the chicks are lessening. Unmated males without family responsibilities often start losing feathers earlier in the year. Their moult follows a pattern similarly prolonged as that of the female. The first sign that the male is moulting is often when a tail feather has been dropped at the roost.[48] A consequence of moulting is the loss of thermal insulation. This is of little importance in the tropics, and barn owls there usually moult a complete complement of flight feathers annually. The hot-climate moult may still take place over a long period but is usually concentrated at a particular time of year outside the breeding season.[49]

Predators and parasites

Three barn owl chicks threatening an intruder

Predators of the barn owl include large American opossums (Didelphis), the common raccoon, and similar carnivorous mammals, as well as eagles, larger hawks, and other owls. Among the latter, the great horned owl (Bubo virginianus), in the Americas, and the Eurasian eagle-owl (B. bubo) are noted predators of barn owls. Despite some sources claiming that there is little evidence of predation by great horned owls, one study from Washington found that 10.9% of the local great horned owl's diet was made up of barn owls.[50][51][52] In Africa, the principal predators of barn owls are Verreaux's eagle-owls and Cape eagle-owls.[53][54] In Europe, although less dangerous than the eagle-owls, the chief diurnal predators are the northern goshawk (Accipiter gentilis) and the common buzzard (Buteo buteo). About 12 other large diurnal raptors and owls have also been reported as predators of barn owls, ranging from the similar-sized Cooper's hawk and scarcely larger tawny owl to huge bald and golden eagles.[55] The goshawk and the eagle-owls are on the increase because of the greater protection these birds now receive.[28]

When disturbed at its roosting site, an angry barn owl lowers its head and sways it from side to side, or the head may be lowered and stretched forward and the wings outstretched and drooped while the bird emits hisses and makes snapping noises with its beak. Another defensive attitude involves lying flat on the ground or crouching with wings spread out.[16]

Barn owls are hosts to a wide range of parasites. Fleas are present at nesting sites, and externally the birds are attacked by feather lice and feather mites which chew the barbules of the feathers and which are transferred from bird to bird by direct contact. Blood-sucking flies, such as Ornithomyia avicularia, are often present, moving about among the plumage. Internal parasites include the fluke Strigea strigis, the tapeworm Paruternia candelabraria, several species of parasitic round worm, and spiny-headed worms in the genus Centrorhynchus. These gut parasites are acquired when the birds feed on infected prey.[56] There is some indication that female birds with more and larger spots have a greater resistance to external parasites. This is correlated with smaller bursa of Fabricius, glands associated with antibody production, and a lower fecundity of the blood-sucking fly Carnus hemapterus, which attacks nestlings.[57]

Lifespan

Captive bird landing
Landing on a handler's gloved hand. Captive birds often live longer than wild ones.

Unusually for a medium-sized carnivorous animal, the barn owl exhibits r-selection, producing a large number of offspring with a high growth rate, which have a low probability of surviving to adulthood.[58] While wild barn owls are thus on average short-lived, the potential lifespan, or longevity, of the species is much higher. Captive individuals may reach 20 years of age or more; a captive barn owl, in England, lived to be over 25 years old. Occasionally, a wild bird reaches an advanced age. The American record age for a wild barn owl is 11.5 years, while a Dutch bird was noted to have reached the age of 17 years, 10 months. Taking into account such extremely long-lived individuals, the average lifespan of the barn owl is about four years, and statistically 2/3 to 3/4 of all adults survive from one year to the next. This mortality is not evenly distributed throughout the bird's life, and only one-young-in-three manages to live to its first breeding attempt.[6]

The most significant cause of death in temperate areas is likely to be starvation, particularly during the autumn and winter, when first-year birds are still perfecting their hunting skills. In northern and upland areas, there is some correlation between mortality in older birds and adverse weather, deep-lying snow, and prolonged low temperatures. Collision with road vehicles is another cause of death, and may result when birds forage on mown verges. Some of these birds are in poor condition and may have been less able to evade oncoming vehicles than fit individuals. In some locations, road mortality rates can be particularly high, with collision rates being influenced by higher commercial traffic, roadside verges that are grass rather than shrubs, and where small mammals are abundant.[59] Historically, many deaths were caused by the use of pesticides, and this may still be the case in some parts of the world. Collisions with power-lines kill some birds; and being shot accounts for others, especially in Mediterranean regions.[60]

Status and conservation

Barn owl on Lithuanian 5 litų silver coin (2002)

Barn owls are relatively common throughout most of their range and not considered globally threatened. If considered as a single global species, the barn owl is the second most widely distributed of all raptors, behind only the peregrine falcon. It is wider-ranging than the also somewhat cosmopolitan osprey. Furthermore, the barn owl is likely the most numerous of all raptors, with the International Union for Conservation of Nature (IUCN) estimating, for all barn owl individuals, a population possibly as large as nearly 10 million individuals (throughout the Americas, the American barn owl species may comprise nearly 2 million).[1][61] Severe local declines due to organochlorine (e.g., DDT) poisoning in the mid 20th century and rodenticides in the late 20th century have affected some populations, particularly in Europe and North America. Intensification of agricultural practices often means that the rough grassland that provides the best foraging habitat is lost.[62] While barn owls are prolific breeders and able to recover from short-term population decreases, they are not as common in some areas as they used to be. A 1995–1997 survey put their British population at between 3,000 and 5,000 breeding pairs, out of an average of about 150,000 pairs in the whole of Europe. In the US, barn owls are listed as endangered species in seven Midwestern states (Ohio, Michigan, Indiana, Illinois, Wisconsin, Iowa, and Missouri), and in the European Community they are considered a Species of European Concern.[4][6]

In Canada, barn owls are no longer common and are most likely to be found in coastal British Columbia south of Vancouver,[63] having become extremely rare in a previous habitat, southern Ontario. In spite of a Recovery Strategy, particularly in 2007–2010 in Ontario,[64] only a handful of wild, breeding barn owls existed in the province in 2018.[65] This is primarily because of disappearing grasslands where the bird hunted in the past, but according to a study, also because of "harsh winters, predation, road mortality and use of rodenticides".[66] The species is listed as endangered overall in Canada, due to loss of habitat and a lack of nesting sites.[67]

Captive barn owl near Lake Erie, in Ontario; the species has become extremely rare in this province

In the Canary Islands, a somewhat larger number of these birds still seem to exist on the island of Lanzarote, but altogether this particular subspecies (T. a. gracilirostris, the Canary barn owl) is precariously rare: perhaps fewer than two hundred individuals still remain. Similarly, the birds on the western Canary Islands, which are usually assigned to this subspecies, have severely declined; and wanton destruction of the birds seems to be significant. On Tenerife they seem relatively numerous; but on the other islands the situation looks about as bleak as on Fuerteventura. Due to the assignment to this subspecies of birds common in mainland Spain, the western Canary Islands population is not classified as threatened.[68][69][70]

Cultural aspects

Common names such as "demon owl", "death owl", "ghost owl", or "lich owl" (from lich, an old term for a corpse) show that rural populations in many places considered barn owls to be birds of evil omen. For example, the Tzeltal people in Mexico regard them as "disease givers".[71] These owls don't "hoot", instead emitting raspy screeches and hissing noises, and their white face and underbelly feathers, visible as they fly overhead, make them look "ghostly". Consequently, they were often killed by farmers who were unaware of the benefits these birds bring.[72] Negative perceptions can also be attributed to the false belief that they could eat large animals, such as chickens and cats.[73] In South Africa, barn owls are often associated with witchcraft and are persecuted. In some South African cultures, these owls are used in 'muthi' (traditional medicine) and are believed to give special powers when consumed.[74][75]

Nest boxes

An Eulenloch ('owl-hole') in northern Germany lets barn owls access the attic for nesting

Nest boxes are used primarily when populations suffer declines.[76] Although such declines have many causes, among them are the lack of available natural nesting sites. Early successes among conservationists have led to the widespread provision of nest boxes, which has become the most used form of population management. The barn owl accepts the provided nest boxes and sometimes prefers them to natural sites.[77]

Conservationists encourage farmers and landowners to install nest boxes by pointing out that the resultant increased barn owl population would provide natural rodent control.[78] The nest boxes are placed under the eaves of buildings and in other locations. The upper bound of the number of barn owl pairs depends on the abundance of food at nesting sites.[79]

Surveillance

A nest box can also be regarded as an animal surveillance device, as they give direct access to the breeding location. While the diet of the barn owl has been studied, other behaviours–such as breeding–are not well known. Surveilling animals can lead to the discovery of new scientific and industrial fields. For example, biologists and engineers can work on barn owl surveillance techniques and devices, while social scientists document what cause humans to want to observe an animal.[80][81]

In Switzerland, a research group aims to install RFID tag readers on the entrances to the nest boxes, thus allowing tracking of barn owl movements from nest box to nest box. Information about the behaviour of the owls prior to breeding could be obtained using such surveillance.[82]

In the United Kingdom, the "Barn Owl Nest Box Scheme" is promoted by the World Owl Trust[83] and has many participants in local areas, such as Somerset, where a webcam was set up inside a nest box in which 7 young were reared in 2014.[84] Another barn owl nest box live-streaming webcam–located in California, United States–has proved popular online.[85] In May 2012, it was revealed that farmers in Israel and Jordan had, over a period of ten years, replaced pesticides with barn owls in a joint conservation venture called "Project Barn Owl".[86]

Other research tools include using GPS trackers fitted onto the barn owl, to allow precise location tracking of the owl. Tracks obtained enabled the identification of three types of movement: hunting, straight-lined flights, and roosting. By superimposing the track onto a map, the bird's foraging habits could be studied.[87]

Alternative rodent control technique

In some conservation projects, the use of rodenticides for pest control was replaced by the installation of nest boxes for barn owls, which has been shown to be a less costly method of rodent control.[88]

In Malaysia, large areas of rainforest were felled to make way for oil palm plantations; and with few tree cavities for breeding, the barn owl population, with its ability to control rodent pests, diminished. In a trial, the provision of two hundred nest boxes saw almost one hundred percent occupancy; and as the programme expanded, the plantations supported one of the densest barn owl populations in the world.[62] Similarly, providing nesting boxes has increased the number of barn owls in rice-growing areas of Malaysia, where the rodents do much damage to the crop. Although barn owl numbers have increased in both these instances, it is unclear as to how effective this biological control of the rats is, as compared to the baiting and trapping that occurred previously.[89]

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Bibliography

Further reading

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