Less than three percent of all bird species are active at night and over half of these are owls—they are the nocturnal counterparts of the day-hunting hawks and falcons. Although the largest species is one hundred times the weight of the smallest, all owls are instantly recognizable as such a uniformity which stems from their unique adaptations for their role as nocturnal predators. New species are still being discovered at the threat of one every decade.
Owls occur wherever there is an animal on which they can prey. Most are associated with trees, but others are adapted to living in grasslands, deserts, marshes, or even arctic tundra. The diets, biology, and behavior of many tropical owls are unknown, but about 8o of the 133 species of all owls are thought to be primarily night-hunting and most of the rest can hunt at any time but do so, especially at dusk and dawn.
All owls are easily recognized by their shape: an upright stance, short tail, large head, and dense covering of feathers giving them a neckless, rounded outline. Equally characteristic are the huge frontally placed, often orange or yellow eyes, which stare out from saucer-shaped disks of radiating feathers. Daytime-hunting species have smaller eyes and ill-defined facial disks. Many owls have flexible tufts of feathers above the eyes used in visual communication; these “ear tufts” have no connection with the hearing. All owls have powerful, use-ally feathered legs with sharply curved talons for gripping prey. The short hooked beak is curved downwards and may be hard to see among the feathers.
Birds active only in darkness do not require striking plumage; owls mostly spend the day roosting in quiet places, often pres-seed tight against a tree trunk, and so both sexes are usually similarly patterned with various somber shades of brown to aid con coalmen. If discovered by small birds, owls are mobbed to advertise their presence and to persuade them to move on.
Owls that live in open habitats are paler than those from woodland: desert forms are sandy-colored and the Snowy owl is mainly white to match its arctic surroundings. Some woodland owls have two distinct color-ph ashes– gray in northern coniferous forests, and brown in deciduous woods further south. With few exceptions, juveniles look similar to adults. In most owls, the females are larger than the male, although the dif-ferrous is not usually as marked as in some of the day-hunting birds of prey.
Most owls are “typical” owls of the family Strigidae. The largest genus (Oats) contains 33 screech or stops owls afar-flip though absent from Australia: all medium-sized, unspecialized owls of tropical woodland or scrub.
Nocturnal and feed on insects, but temperate species switch to rodents. The T 2 eagle owls are the equivalents of the large day-hunting buzzards.
They include the sparrow-sized least pygmy owl of tropical South American forests, which shares with the Elf owl of the American West the distinction of being the smallest owl. Both are closely related to the Long-whiskered owlet of the Peruvian Andes, not discovered until 1.976. Most medium-sized owlsinf Indonesia and Australasia are hawk owls (Niño). Of these only the Oriental hawk owl of the Asian mainland has a wide distribution; most of the other 15 species are confined to single islands and their ranges do not overlap. In Australia, where most owl genera are absent (egg Bubo, Outs, Stri,p, and Glaucidium), three Niño species do exist side by side, including the small Booboo owl which also occurs in New Zealand and New GuineaThe`e five Cicada owls are rarely-seemed,um-sized owls of the tropical forests of Africa and the Americas.
Intemperate woodlands are replaced by its Strixspecies: including the extensively studied Tawny owl, whose range extends from Britain ac, boss Europe and northwest Africa to the moue aims of Burma and China. The five Asia species fall into two ecologically distinct groups “long-eared,” found in broad-leaved or conifer° s woodland, and “short-eared” which is fa quant open country.
The genus Athena contains four species and takes its name from the Greek goddess of wisdom Pallas Athena. The Little owl, which sometimes hunts by day, is a familiar sight in open habitats from western European North Africa across to China. In the later800s it was introduced into Britain and New Zealand.
The only New World representative is the Burrowing owl, a long-legged, daytime-hunting, terrestrial species of open treeless grasslands. The genusAegolius is a basically New World family of four geographically-separated, small, nocturnal, forest owls. The most widespread is the Boreal or Tengmalm’s owl. Like several other owls of northern coniferous forests (either the Great grayed or the Hawk owl) its range extends in a belt right across the Old World.
The barn and bay owls from the other family (Tytonidae). They are distinguished from the typical owls by their heart-shaped rather than round faces, middle and inner toes of equal length (the inner is shorterintriguess), serrated middle class,s, and wish-bones fused to the breastbone. The rodent-hunting Barn owl of the open country is one of the most widely distributed of all birds (see box), found on every inhabited continent. The other seven barn owls occur in Africa, islands in the Indian Ocean, and Southeast
Asia, and Australasia. The little-known Common Bay owl is found in Asian forests from India to Java and Borneo; the African Bay owl is known only from single specie-men collected in the Congo in 1951. Despitef their general resemblance, owls are not closely related to the hawks and falcons both groups evolved to exploit similar food supplies and so possess the same basic anatomical features for a predatory way of life.
A study of egg-white proteins sag-gests that the nearest relatives of owls are another nocturnal order—the nightjarsdalliess. Nightjars use their huge mouths to snap up insects in flight, and oil birds find ripe fruit by smell, but owls are unique among nocturnal birds in using vision and/or hearing to locate their food.
The adaptations needed for hunting at night limit the kinds of food that owl’scan exploit efficiently. There are no carrion-feeding owls equivalent to vultures and kites (although some do take carrion occasion-ally), no soaring forms, and owls seldom pursue and capture birds in flight—a com-moon specialization among day-hunting birds of prey.
Owls catch most of their prey on the ground in the open. Woodland owls have short, rounded wings and when huntingsitst quietly on a low perch watching and listening in for small mammals. On hearing a likely noise they rapidly rotate their head until the sound registers equally in both ears; they are then directly facing it. When the source of the sound is pin-pointed, the owl glides silently down towards it; at the last second it swings its feetforwards to hit the prey, often killing it outright. Many owls of open country hunt mainly in flight.
They have long wings which enable them slowly to quarter the ground like the day-hunting harriers, with little expenditure of energy. Long-eared owls spend about 20 percent of the night hunting. Once prey is located, it’s pounced on from a low height in the manner of perch-hunting owls; about one in five attempts is successful. Owls are opportunistic hunters and will often try to catch prey any way they can: insects (and sometimes birds) may be chased in flight, birds are grabbed while roosting, several species (egg little and Burrowing owls) hound across the ground in search of invertebrates, and Tawny owls will plunge into the water to catch frogs.
The specialist fish-in owls swoop down to pluck fish from the theater surface. Roadsides also provide good hunting areas for owls and in developed countries,s many are killed by traffic.
Unlike hawks and falcons, owls carry all but the largest prey in their bill and swallow it whole, head first. Sorting out what is nutritious takes place internally and any indigestible remains such as bones and fur are regurgitated as pellets, which provide a good record of what owls eat. Owls have no crop in which to store food but sometimes cache prey.
Owls feed on a wide variety of animal prey; what they eat depends mainly on their size and the habitat they occupy. Tawny owls living in woodland feed mainly on mice and voles, but in the towns, they feed on birds, especially house sparrows. Small owls are mostly insectivorous; medium-sized ones feed mainly on small rodents or birds; the largest species take mammals (up to the size of hares or even small deer) and medium-sized birds—including other owls and birds of prey!
Owls are well insulated by their dense covering of feathers and get by on about30 percentt less loud than most other birds of equivalent size. They can have a substantial impact on prey populations: in one study, a pair of Tawny owls, consuming a maximum of seven togs (o.7oz) rodents per day, removed 18-46 percent of the Bank voles and 28-70 percent of the Wood mice present in their hunting range in every two months.
Periodic fluctuations in thenumbers of rodents have striking effects on the owls themselves. In years when they are scarce, many owls either do not breed or lay reduced numbers of eggs; Snowy owls and Short-eared owls lay 2-14 eggs, depending on prey availability. Generally, breeding is timed so that food is most plentiful when the young are learning to hunt for themselves and the adults are undergoing their annual molt (which reduces their hunting efficiency).
Extra food is also needed to grow new feathers and to compensate for the reduced insulation properties of their plumage. In tropical regions, breeding is geared to rainfall—small species have young in the nest when the onset of the rainy season produces a flush of insects. Many owls have only a single brood per year, but some open-country species cambered whenever rodents are abundant, and may raise several broods in a year. Most owls can breed in their first year if conditions are suitable.
Many owls adopt another family-planning measure. Single eggs are laid at intervals of two or more days but incubation begins with the first so that the earliest chick to hatch maybe up to three weeks solder than the last. If food is plentiful, all the chicks survive, if not, the youngest die and are eaten by the rest. In this way, brood sizes are adjusted to the food available.
Owls are not great nest-builders: most breed in holes in trees, rocks,s or the ground, but some open-country owls line depressions in the ground,d and Burrowing owls candid their underground nest chambers(although they usually take over prairie dog lairs). Small species appropriate oldwoodpeckerr holes. Large owls are unable to find appropriate old woodpecker holes, and most woodland owls, readily occupy nest boxes provided for them. Large owls are unable to find appropriately sized natural fissures and take over abandoned tree nests of crows or birds of prey.
Incubation tends to be by the female alone, with the smaller male providing althea food from before egg-laying until the young are initially born blind, helpless,s and covered in sparse grayish-white down—no longer need brooding or their prey is torn up for them. This division of labor allows the inert female to accumulate fat reserves and remaininn the nest even when the male finds hunting difficuls, for example, in wet weather.
In many species, the larger female vigorously defends the young against intruders, including humans (some people have even lost eyes to them). Other species have threat displays in which the female tries to make herself look larger and even more fearsome. To further reduce the chances of predation, the young of open-nesting owls grow faster than thosethath are reared in holes and leave the nest before they are fully feathered.
Fledged young beg loudly for food and are often dependent upon their parents for several months before they disperse. Newly independent owls suffer high mortality:
Over half the young Tawny owls die in their first year, many of starvation, but once set-tiled they can expect to live for at least furor five years and some have survived for more than 15. Larger species probably live even longer a captive Eagle owl survived 688 years.
Most owls are territorial and non-migratory, especially those living in the tropics or woodland. Here pairs often spend all their lives in strictly defended territories, switching to alternative prey if one kind becomes unavailable; the populations of such species remain stable over long periods. Northern owls and those of open country which feed mainly on rodents have a narrower range of quarry available totem. They usually defend territories in the breeding season but theft populations tend to fluctuate in parallel with those of their prey. Some are found far outside their normal ranges if food supplies fail.
Large numbers of Snowy owls sometimes appear in these when lemming or hare population shave crashed in the Arctic. The birds are often very tame when in unfamiliar surroundings: North American bird banders catch the impressive-looking Great gray owl by casting out a dead mouse attached to the fishing line (without a hook)–a hungry owl will pounce on it and can be reeled in! Anew owls undertake regular north-south migrations, like the Scoops owl, which
Exploits the summer flush of insects in southern Europe; the Short-eared owl is nomadic, settling wherever prey is temporarily abundant.
Owls that arc territorial throughout the year live in pairs but forage alone so as motto interferes with each other’s hunting. The rest usually live alone outside the breeding season, except some owls of open country which congregate in areas where prey is plentiful; these often roost communally but disperse at dusk to hunt alone. Only the Bur-rowing owl is colonial.
Breeding territories tend to be smaller where more prey is available. In Britain, Tawny owls defend 12-2oha (30-5o acres) in open deciduous woodland where small rodents are abundant but over 4oha (Iooacres) in more sterile conifer plantations. The huge Eagle owl takes bigger, less com-moon prey and needs a correspondingly larger territory: their nests are usually spa-cede 4-5km 2.5-3 mii) apart.
To communicate over such long distances at night, owls have well-developed vocabularies and they are much more vocal than day-hunting birds of prey. The familiar territorial hooting of many owls is equivalent to the song of other birds serving to warn off rival males and to attract a mate. The hoots of male Eagle owls can be heard 4km 2.5 mii) away and like many other owls the pair frequently answer one another in a duet, probably to maintain the pair bond between them.
Hunting owls are generally less vocal and’ the Barn owl does not hoot, perhaps because its light color makes it easily visible. Removing the need for long-distance vocal communication. All owls can produce loud tongue-clicking sounds when they are frightened or angry.
Many owls will answer imitations of their hoots, and territories can be mapped in this way. Local names for owls often reflect their distinctive calls: the names booboo. And saw-whet arc respectively phonetic render-in and description.
Owls sometimes share their habitat with day-hunting birds of prey; on the Galapagos Islands, Short-eared owls hunt both by day and night where the Galapagos hawk is absent, but only at night where hawks are present. Where several owls coexist they are usually of different sizes and feed on different prey in Australia, three hawk owls occur together: the small Booboo owl feedsonn insects and small birds, themedium-sizede: Barking owl takes small marsupial anribirds up to crow-size, and the large Powerful owl catches medium-sized arboreal mar-spiels.
Where owls feed on similar prey in the same habitat, for example, Great-horned and Barred owls in parts of North America. They avoid competition for food by defending territories against rival species. Modern forestry reduces the number of nest sites for owls and in Scandinavia, many nest boxes have been erected for Ural and Tawny owls these species have increased but they prey upon Pygmy owls, which have consequently become rare.
Like most predators, owls are persecuted by man wherever their presence might con-flit with his interests, particularly game preservation. In Europe, Eagle owls have been exterminated in densely populated regions. Ironically, in other areas, these owls are used to lure other birds of prey (which mob them) within gunshot range.
Owls did not suffer as much as some birds of prey when toxic chemicals were introduced as pesticides in agriculture. A greater threat is the destruction of their habitats. The Madagascar grass owl, the Skokie scops owl (discovered in 1965 in the coastal rainforest in Kenya), the races of the Madagascan scoops owls on Seychelles and Comoros, and the Giant scoops owl of the Philippines are all endangered for this reason. The Laughing owl of New Zealand may already be extinct. Its decline followed the introduction from Europe of stoats and weasels which destroyed its nests and com-petted with it for food.
An Owl senses. The eyes and ears of owls are extremely sensitive. The eye (a) differs from the mammalian eye (b) in having the retina closet and equidistantly spaced from the lens. The retina is packed with rods (which only detect black and white) and can function at very low light intensities. Pectin is a structure thought to provide nutrients to the eyeball. The owl’s field of vision (c) is not large but provides good stereoscopic vision over an angle of 70°. This intense tunnel vision is an adaptation to horning in on its prey. The ears of owls (d) are asymmetrically placed to aid in the location of sounds.
Owls are generalized predators their specialization lies not in feeding on a particular type of prey but in catching it in darkness. The modifications which enable owls to do this create their distinctive appearance.
Owls have particularly highly developed hearing and vision, and need oversized skulls to accommodate ear openings and eyes much bigger than those of other birds the largest owls have eyes comparable in size to those of humans. What, therefore, is the advantage of these large frontally placed eyes?
Large eyes can have large pupils to allow more light to fall on the retina (the light-sensitive layer at the back of the eye). A Tawny owl’s eye has 200 times the light-gathering power of a pigeon’s and produces large retinal images to provide the visual acuity necessary to discriminate potential prey. Owls have tubular (rather than spherical) eyes, placed frontally to accommodate the huge lens and cornea.
Unfortunately, tubular eyes have a reduced field of view and are virtually immobile, giving owls a visual field of only 360° compared with a man’s 18o° and a pigeon’s 340°. To overcome this, owls have remarkably flex-bile necks, enabling them to invert their heads as well as to look directly behind! Frontally-placed eyes can also provide binocular vision, in which both eyes view the same area from different aspects. This allows better judgment of distance.
Although owls see much better at night than birds active during the day, the popular belief that their eyes are vastly superior to man’s in the dark but function poorly in bright light is not correct. The Tawny owl has color vision, sees in daylight well as a pigeon, and has eyes only some two to three times more sensitive than men in the dark. Owls can only hunt successfully at night because their visual sensitivity is allied to exceptional hearing. Owls are especially sensitive to sounds withal high-frequency components, such as the rustling of dry leaves some species can even locate and capture small rodents in total darkness just from the noise they make in moving across the woodland floor.
The characteristic facial disks of owls are part of this specialized hearing apparatus. The tightly-packed rows of stiff feathers which make up the rim of this disk reflect high-frequency sounds which are channeled by the mobile facial disks into the ears behind, in the same way, that mammals use their large.
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