The largest owls are two similarly sized eagle owls the Eurasian eagle-owl ( Bubo bubo) and Blakiston's fish owl ( Bubo blakistoni). Around the same diminutive length, although slightly heavier, are the lesser known long-whiskered owlet ( Xenoglaux loweryi) and Tamaulipas pygmy owl ( Glaucidium sanchezi). The smallest owl-weighing as little as 31 g ( 1 + 3⁄ 32 oz) and measuring some 13.5 cm ( 5 + 1⁄ 4 in)-is the elf owl ( Micrathene whitneyi).
Other anastomoses between the carotid and vertebral arteries support this effect. They also have adaptations to their circulatory systems, permitting rotation without cutting off blood to the brain: the foramina in their vertebrae through which the vertebral arteries pass are about 10 times the diameter of the artery, instead of about the same size as the artery as in humans the vertebral arteries enter the cervical vertebrae higher than in other birds, giving the vessels some slack, and the carotid arteries unite in a very large anastomosis or junction, the largest of any bird's, preventing blood supply from being cut off while they rotate their necks. Owls have 14 neck vertebrae compared to seven in humans, which makes their necks more flexible. Owls can rotate their heads and necks as much as 270°. Their far vision, particularly in low light, is exceptionally good. Caught prey can be felt by owls with the use of filoplumes-hairlike feathers on the beak and feet that act as "feelers". As owls are farsighted, they are unable to clearly see anything within a few centimetres of their eyes. Although owls have binocular vision, their large eyes are fixed in their sockets-as are those of most other birds-so they must turn their entire heads to change views. Most birds of prey have eyes on the sides of their heads, but the stereoscopic nature of the owl's forward-facing eyes permits the greater sense of depth perception necessary for low-light hunting. The feathers making up this disc can be adjusted to sharply focus sounds from varying distances onto the owls' asymmetrically placed ear cavities. Owls possess large, forward-facing eyes and ear-holes, a hawk-like beak, a flat face, and usually a conspicuous circle of feathers, a facial disc, around each eye.
Owls are divided into two families: the true (or typical) owl family, Strigidae, and the barn-owl family, Tytonidae.Ī group of owls is called a "parliament". They are found in all regions of the Earth except the polar ice caps and some remote islands. Owls hunt mostly small mammals, insects, and other birds, although a few species specialize in hunting fish. Exceptions include the diurnal northern hawk-owl and the gregarious burrowing owl. Owls are birds from the order Strigiformes ( / ˈ s t r ɪ dʒ ə f ɔːr m iː z/), which includes over 200 species of mostly solitary and nocturnal birds of prey typified by an upright stance, a large, broad head, binocular vision, binaural hearing, sharp talons, and feathers adapted for silent flight. Left Strigidae: Tawny owl ( Strix aluco), Eurasian eagle-owl ( Bubo bubo), Little owl ( Athene noctua), Northern saw-whet owl ( Aegolius acadicus) Right Tytonidae: Barn owl ( Tyto alba), Lesser sooty owl ( Tyto multipunctata), Tasmanian masked owl ( Tyto novaehollandiae castanops), Sri Lanka bay owl ( Phodilus assimilis).
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