Barn Owl

The city in Russia is spelled Barnaul.
Barn Owl
Light adult, representative of T. a. alba or T. a. delicatula
Light adult, representative of T. a. alba or T. a. delicatula
Conservation status
Status iucn3.1 LC.svg
Least Concern (IUCN 3.1)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Aves
Subclass: Neornithes
Infraclass: Neognathae
Superorder: Neoaves
Order: Strigiformes
Family: Tytonidae
Subfamily: Tytoninae
Genus: Tyto
Species: T. alba
Binomial name
Tyto alba
(Scopoli, 1769)
Global range in green
Global range in green
Synonyms

Lechusa stirtoni Miller, 1956
Strix alba Scopoli, 1769
Strix pratincola
Tyto delicatula (Gould, 1837)

The Barn Owl (Tyto alba) is the most widely distributed species of owl, and one of the most widespread of all birds. It is also referred to as Common Barn Owl, to distinguish it from other species in the barn-owl family Tytonidae. These form one of the two living main lineages groups of owls, the other being the typical owls (Strigidae). T. alba is found almost anywhere in the world outside polar and desert regions, as well as all of Asia north of the Alpide belt, New Zealand, and most of Indonesia and the Pacific islands.[1]

It is known by many other vernacular names, some of them rather ambiguous. They often refer to the appearance, habitat or the eerie, silent flight: White Owl, Silver Owl, Demon Owl, Ghost Owl, Death Owl, Night Owl, Rat Owl, Monkey-faced Owl, Church Owl, Cave Owl or Stone Owl. Golden Owl might also refer to the related Golden Masked-owl (T. aurantia). Hissing Owl and – particularly in the USA – "screech owl" refer to the piercing calls of these birds, but the latter term usually refers to typical owls of the genus Megascops.

The Ashy-faced Owl (T. glaucops) was for some time included in T. alba, and by some authors its Lesser Antilles populations insularis and nigrescens still are. The Barn Owls from the Indopacific region are sometimes separated as Eastern Barn-owl, Australian Barn-owl or Delicate Barn-owl (T. delicatula). While this may be warranted, it is not clear between which races to draw the line between the two species. Also, some island subspecies are occasionally treated as distinct species. While all this may be warranted, such a move is generally eschewed pending further information on Barn Owl phylogeography.[1]

Contents

Description

The Barn Owl is a pale, long-winged, long-legged owl with a short squarish tail. Depending on subspecies, it measures c.25-45 cm in overall length, with a wingspan of about 75-100 cm. Tail shape is a way of distinguishing the Barn Owl from true owls when seen in flight, as are the wavering motions and the dangling feathered legs. The light face with its peculiar shape and the black eyes give the flying bird an odd and startling appearance, like a flat mask with oversized oblique black eyeslits, the ridge of feathers above the bill somewhat resembling a nose.[2]

Its head and upperparts are a mixture of buff and grey (especially on the forehead and back) feathers in most subspecies. Some are purer richer brown instead, and all have fine black-and-white speckles except on the remiges and rectrices, which are light brown with darker bands. The heart-shaped face is usually bright white, but in some subspecies it is browner. The underparts (including the tarsometatarsus feathers) vary from white to reddish buff among the subspecies, and are either mostly unpatterned or bear a varying amount of tiny blackish-brown speckles. The bill varies from pale horn to dark buff, corresponding to the general plumage hue. The iris is blackish brown. The toes, as the bill, vary in color; their color ranges from pinkish to dark pinkish-grey. The talons are black.[1]

Contrary to popular belief, it does not hoot (such calls are made by typical owls, like the Tawny Owl or other Strix). It instead produces the characteristic shree scream, ear-shattering at close range. It can hiss like a snake, and when captured or cornered, it throws itself on its back and flails with sharp-taloned feet, making for an effective defence.[2]

Subspecies

Adult, probably of T. a. alba.
Aduilt of T. a. guttata in flight, Sandesneben (Germany).
Adult, probably of T. a. pratincola.
Adult, probably of T. a. pratincola.
Adult T. a. pratincola of the population introduced to Hawaiʻi.

Across its vast range, the Barn Owl has formed many subspecies, but several are considered to be intergrades between more distinct populations today. Still, some 20-30 seem to be worthy of recognition as long as the species is not split up. They vary mainly in size and color, sometimes according to Bergmann's and Gloger's Rules, sometimes more unpredictably. This species ranges in color from the almost beige-and-white nominate subspecies, erlangeri and niveicauda to the nearly black-and-brown contempta:[1]

Upperparts grey and light buff. Underparts white, with little if any black spots.
Large. Similar to alba but darker above, and with conspicuous speckling overall.
Large. Upperparts pale orange-buff and brownish-grey, underparts whitish with few speckles. Face white.
Upperparts grey and orange-buff. Underparts whitish to light buff with little speckling. Face white. Resembles pale Old World guttata.
More grey on upperparts than alba. Underparts buff to rufous with some dark speckles. Face whitish. Females are on average redder below than males.
Similar to alba; slightly darker above, more speckles below. Tail with 4 bark brown bars.
Large. Upperparts grey and orange-buff. Underparts whitish to light buff with much speckling. Face white. Resembles pale Old World guttata, but usually more speckles below.
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.
Upperparts golden-brown and grey with very bold pattern. Underparts light buff with extensive speckles. Face white.
Smallish. Upperparts dark brownish grey with bold pattern, including lighter brown bands on remiges and rectrices. Underparts golden brown with extensive speckles. Face buff.
Upperparts very grey. Underparts light buff with extensive speckles. Face white.
Similar to dark pratincola; less grey above, coarser speckles below.
Smallish. Almost uniformly dark reddish brown above. Reddish buff below, with some speckling. Face reddish buff.
Similar to alba; smaller and noticeably short-winged.
Large, particularly the bill. Similar to javanica; tail whitish with black bars.
Almost black with some dark grey above, the white part of the spotting showing prominently. Reddish brown below
Small. Similar to guttata, but breast region light buff.
Similar to alba; breast region always pure unspotted white.
Small. Similar to schmitzi but breast darker, approaching guttata. Face light buff.
Large. Similar to javanica; tail whitish with grey bars, underparts silvery-white with arrowhead-shaped speckles (larger than in javanica).
Similar to guttata, but less reddish. Face buff.
Similar to ernesti; upperparts lighter and yellower.
Similar to alba, but noticeably speckled below.
Similar to delicatula; darker, with stronger bill and feet.
Similar to delicatula; darker, with orange hue.
Similar to tuidara but larger.
Similar to pratincola; smaller and paler on average.
Large. Similar to furcata; paler in general. Resembles Old World alba.

Ecology and status

This is a bird of open country such as farmland or grassland with some interspersed woodland, usually below 2,000 m ASL but occasionally as high as 3,000 m ASL in the Tropics[3]. This owl prefers to hunt along the edges of woods. It has an effortless wavering flight as it quarters pastures or similar hunting grounds. Like most owls, the Barn Owl flies silently; tiny serrations on the leading edges of its flight feathers help to break up the flow of air over its wings, thereby reducing turbulence and the noise that accompanies it[4].

A Barn Owl skull, showing the rodent-killer beak

It hunts by flying low and slowly over an area of open ground, hovering over spots that conceal potential prey. The Barn Owl feeds primarily on small vertebrates, particularly rodents. Studies have shown that an individual Barn Owl may eat one or more rodents per night; a nesting pair and their young can eat more than 1,000 rodents per year[5]. Locally superabundant rodent species in the weight class of several grams per individual usually make up the single largest proportion of prey, no matter whether they are Muridae[6] or Cricetidae[7]. Such animals probably make up at least three-quarters of the biomass eaten by each and every T. alba.[8]

The diet is supplemented with local small vertebrate and large invertebrate life. A Barn Owl will eat anything it can subdue and that is more than a beakful, from small invertebrates weighing less than 0.05 grams to birds weighing as much as the owl itself, like the Spotted Nothura (Nothura maculosa). Contrary to what is sometimes assumed, the Barn Owl does not eat domestic animals on any sort of regular basis; it might snatch a young chicken or guinea pig once or twice in its life, but that is all. Regionally, different foods outside of rodents are utilized as per availability. On bird-rich islands, a Barn Owl might contain some 15-20% birds in its diet, while in grassland it will gorge itself on Orthoptera such as Copiphorinae katydids, Jerusalem crickets (Stenopelmatidae) or true crickets (Gryllidae). Bats and even toads and squamates may as well make up a minor but conspicuous part of the prey.[8]

An Uhlenloch ("owl-hole") in northern Germany lets Barn Owls access the attic for nesting.

The Barn Owl has acute hearing, with ears placed asymmetrically for improved detection of sound position and distance, and it does not require sight to hunt. Hunting nocturnally or crepuscularly, it can target and dive down, penetrating it talons through snow, grass or brush to seize rodents with deadly accuracy. Compared to other owls of similar size, the Barn Owl has a much higher metabolic rate, requiring relatively more food. Pound for pound, Barn Owls consume more rodents – often regarded as pests by humans – than possibly any other creature. This makes the Barn Owl one of the most economically valuable wildlife animals to farmers. Farmers often find these owls more effective than poison in keeping down rodent pests, and they can encourage Barn Owl habitation by providing nest sites.[9]

Nestlings are covered in off-white down all over, but already have the typical barn-owl face.

The Barn Owl builds a nest of sorts, but unlike in typical owls it is just a scrape in any unsorted debris as has assembled in a hollow with narrow entrance. Typical nest sites include tree stumps and cliff crevices, but these owls will readily nest in attics, vacant and ruined buildings, and even wells, chimneys, hunting blinds and similar locations. The usual clutch consists of roughly half a dozen eggs, but may be as small as 2; very large clutches of more than a dozen eggs might be of 2-3 females.[1]

Predators of the Barn Owl include large American opossums (Didelphis), the Common Raccoon (Procyon lotor), and similar carnivorous mammals, as well as large raptors such as hawks, eagles, and other owls. Among the latter, the Great Horned Owl (Bubo virginianus) and the Eurasian Eagle-owl (B. bubo) are noted predators of Barn Owls. Some fall also victim to large snakes, but the biggest threat are humans and their pets, in particular house or feral cats.

Barn Owls are relatively common throughout most of their range. However, locally severe declines from organochlorine (e.g. DDT) poisoning in the mid-20th century and rodenticides in the late 20th century have affected some populations[1]. While the Barn Owl is prolific and will often make good any population decrease soon enough, they are rare in Britain today. The most recent survey their population in the UK at around 4,400 breeding pairs. In the USA, Barn Owls are listed as endangered species in seven Midwestern states.

Common names like "Demon Owl", "Death Owl" or "Ghost Owl" show that for long, the rural population considered Barn Owls to be birds of evil omen in many places. Consequently, they were often persecuted by farmers, unawares of the benefit these birds bring. As late as 1975, hunting by fearful locals was limiting the population of T. a. gracilirostris on Fuerteventura. In our time, rodenticide poisoning is the main threat for the Canary Barn-owl, which in the Chinijo Archipelago is on the verge of disappearance while on Fuerteventura only a few dozen pairs remain overall. There, the abandonment of much agricultural land and the subsequent decline of rodent pests seem to have decreased the owl's numbers even further. Only on Lanzarote does a somewhat larger number of these birds still seem to exist, but altogether this particular subspecies is precariously rare: Probably less than 300 and perhaps less than 200 birds still exist, and it is classified as insuficientemente conocida ("data deficient") by the Spanish Ministry of Environment. Similarly, the birds on the western Canary Islands which are usually assigned to the nominate subspecies (though this seems suspect on grounds of biogeography) have declined much, and here wanton destruction seems still to be significant. On Tenerife they seem to be not uncommon, while on the other islands, the situation looks about as bleak as on Fuerteventura. Due to the assignment to the nominate subspecies, which is common in mainland Spain, the western Canary Islands population is not classified as threatened.[10]

Footnotes

  1. ↑ 1.0 1.1 1.2 1.3 1.4 1.5 Bruce (1999)
  2. ↑ 2.0 2.1 Bruce (1999), Svensson et al. (1999): pp.212-213
  3. ↑ Cisneros-Heredia (2006)
  4. ↑ Ehrlich et al. (1994): pp.250-254
  5. ↑ Ingles (1995), PGC (2008)
  6. ↑ E.g. Black Rat (Rattus rattus) or House Mouse (Mus musculus): Cisneros-Heredia (2006), Motta-Junior (2006)
  7. ↑ E.g. Delicate Vesper Mouse (Calomys tener), Hairy-tailed Bolo Mouse (Bolomys lasiurus) or Black-footed Pygmy Rice Rat (Oligoryzomys nigripes): Motta-Junior (2006)
  8. ↑ 8.0 8.1 Ehrlich et al. (1994): pp.250-254, Motta-Junior (2006)
  9. ↑ UF (1999), Day (2001)
  10. ↑ Álamo Tavío (1975), Palacios (2004), MES (2006)

References

Further reading

External links