Barn Owl

The barn owl (Tyto alba/仓鸮) is the most widely distributed species of owl, and one of the most widespread of all birds. It is also referred to as the common barn owl, to distinguish it from other species in its family, Tytonidae, which forms one of the two main lineages of living owls, the other being the typical owls (Strigidae). The barn owl is found almost everywhere in the world except polar and desert regions, Asia north of the Himalayas, most of Indonesia and some Pacific islands.[2]

Phylogenetic evidence shows that there are at least three major lineages of barn owl, one in Eurasia and Africa, one in Australasia and one in the New World, and some highly divergent taxa on islands. Some authorities further split the group, recognising up to five species, and further research needs to be done to clarify the position. There is a considerable variation between the sizes and colour of 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 head and back is a mottled shade of grey or brown, the underparts vary from white to brown and are sometimes speckled with dark markings. The face is characteristically heart-shaped and is white in most species. This owl does not hoot, but utters an eerie, drawn-out shriek.

The barn owl is nocturnal over most of its range but in 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. They mate for life unless one of the pair gets killed, when a new pair bond may be formed. Breeding takes place at varying times of year according to locality, with a clutch, averaging about four eggs, 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.

Taxonomy and 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.[3][4] As more species of owl were described, the genus name Strix came to be used solely for the wood owls in the typical owl family Strigidae, and the barn owl became Tyto alba in the barn owl family Tytonidae. The name literally means “white owl”, from the onomatopoeic Ancient Greek tyto (τυτώ) for an owl – compare English “hooter” – and Latin alba, “white”.[2] The bird is known by many common names which refer to its appearance, call, habitat or its eerie, silent flight: white owl, 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, scritch owl, screech owl, straw owl, barnyard owl, and delicate owl.[2][5] “Golden owl” might also refer to the related golden masked owl (T. aurantia). “Hissing owl” and, particularly in the US and in India, “screech owl”, refers to the piercing calls of these birds.[6] The latter name is more correctly applied to a different group of birds, the screech-owls in the genus Megascops.[2]

The ashy-faced owl (T. glaucops) was for some time included in T. alba, and by some authors its populations from the Lesser Antilles still are. 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.[7] 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. sumbaensis, T. d. meeki, T. d. crassirostris and T. d. interposita.[8] However the International Ornithological Committee has doubts about this and states that the split of Tyto delicatula from T. alba “may need to be revisited”.[9] Some island subspecies are occasionally treated as distinct species, a move which should await further research into barn owl phylogeography. According to Bruce in the Handbook of Birds of the World, Volume 5: Barn-owls to Hummingbirds, “a review of the whole group [is] long overdue”.[2] 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. A high amount of genetic variation was also found between the Indonesian T. a. stertens and other members of the alba clade.[10]

The barn owl has a wider distribution than any other species of owl. Many subspecies have been proposed over the years but several are generally considered to be intergrades between more distinct populations. Twenty to thirty are usually recognised, varying mainly in body proportions, size and colour. Island forms are mostly smaller than mainland ones, and those inhabiting forests have darker plumage and shorter wings than those occurring in open grasslands.[11] Barn owls range in colour from the almost beige-and-white nominate subspecies, erlangeri and niveicauda, to the nearly black-and-brown contempta.[2]

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, with a wingspan of some 80 to 95 cm (31 to 37 in). Adult body mass is also variable with male owls from the Galapagos weighing 260 g (9.2 oz) while male Pacific barn owls average 555 g (19.6 oz). 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.[16] 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 eyeslits, the ridge of feathers above the bill somewhat resembling a nose.[17]

The bird’s head and upper body typically vary between pale brown and some shade of grey (especially on the forehead and back) in most subspecies. Some are purer, richer brown instead, and all have fine black-and-white speckles except on the remiges and rectrices (main wing feathers), which are light brown with darker bands. The heart-shaped face is usually bright white, but in some subspecies it is brown. 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. This does not hold true for European males by contrast, where the spotting varies according to subspecies. The bill varies from pale horn to dark buff, corresponding to the general plumage hue, and the iris is blackish brown. The toes, like the bill, vary in colour, ranging from pink to dark pinkish-grey and the talons are black.[2][12]

On average within any one population, males tend to have fewer spots on the underside and are paler in colour than females. The latter are also larger with a strong female T. alba of a large subspecies weighing over 550 g (19.4 oz), while males are typically about 10% lighter. Nestlings are covered in white down, but the heart-shaped facial disk becomes visible soon after hatching.[18]

Contrary to popular belief, the barn owl does not hoot (such calls are made by typical owls, like the tawny owl or other members of the genus Strix). It instead produces the characteristic shree scream, ear-shattering at close range, an eerie, long-drawn-out shriek. Males in courtship give a shrill twitter. Both young and old can hiss like a snake to scare away intruders. 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 sounds or clicking snaps, produced probably by the bill but possibly by the tongue.[17][19]

The barn owl is the most widespread landbird species in the world, occurring in every continent except Antarctica.[20] 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, North, Central and South America.[20] 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 foraging areas nearby become vacant. 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).[21]

In continental Europe the distance travelled 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 the point of origin. 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, and also 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 not beyond their capabilities.[2] In 2008, barn owls were recorded for the first time breeding in New Zealand.[22] The barn owl has been successfully introduced into the Hawaiian island of Kauai in an attempt to control rodents, however it has been found to also feed on native birds.[23]

Behaviour and ecology[edit]

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 and 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. This practice may depend on whether the owl is mobbed by other birds if it emerges in daylight.[2] However, in Britain, some birds continue to hunt by day even when mobbed by such birds as magpies, rooks and black-headed gulls, such diurnal activity possibly occurring when the previous night has been wet making 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.[24]

Barn owls are not particularly territorial but have a home range inside which they forage. For males in Scotland this has a radius of about 1 km (0.6 mi) from the nest site and an average size of about 300 hectares. 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 haysheds and are often small in comparison to nesting sites. As the breeding season approaches, the birds move back to the vicinity of the chosen nest to roost.[25]

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. 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 to 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.[26] The behaviour and ecological preferences may differ slightly even among neighbouring subspecies, as shown in the case of the European T. a. guttata and T. a. alba that probably evolved, respectively, in allopatric glacial refugia in southeastern Europe, and in Iberia or southern France.[12][27]