Owls are a group of birds that belong to the order Strigiformes, constituting 200 extant bird of prey species. Most are solitary and nocturnal, with some exceptions (e g the Northern Hawk Owl). Owls hunt mostly small mammals, insects and other birds, although a few species specialize in hunting fish. They are found in all regions of the Earth except Antarctica, most of Greenland, and some remote islands. Owls are characterized by their small beaks and wide faces, and are divided into two families: the typical owls, Strigidae; and the barn-owls, Tytonidae.
Owls have 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. The feathers making up this disc can be adjusted in order to sharply focus sounds that come from varying distances onto the owls' asymmetrically placed ear cavities. 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. Although owls have binocular vision, their large eyes are fixed in their sockets, so they must turn their entire head to change views. As owls are farsighted, they are unable to see clearly anything within a few centimeters of their eyes. Caught prey can be felt by owls with the use of filoplumes, like feathers on the beak and feet that act as "feelers". Their far vision, particularly in low light, is exceptionally good.
Owls can rotate their heads and necks as much as 270 degrees. Owls have fourteen neck vertebrae as compared to 7 in humans which makes their necks more flexible. 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 the neck is rotated. Other anastomoses between the carotid and vertebral arteries support this effect.
The smallest owl, weighing as little as 31 grams (1 oz) and measuring some 13.5 centimetres (5 in), is the Elf Owl (Micrathene whitneyi). Around the same diminutive length, although slightly heavier, are the lesser known Long-whiskered Owlet (Xenoglaux loweryi) and Tamaulipas Pygmy Owl (Glaucidium sanchezi). The largest owl by length is the Great Grey Owl (Strix nebulosa), which measures around 70 cm (28 in) on average and can attain a length of 84 cm (33 in). However, the heaviest (and largest winged) owls are two similarly-sized eagle owls; the Eurasian Eagle-Owl (Bubo bubo) and Blakiston's Fish Owl (B. blakistoni). These two species, which are on average about 2.53 cm (1.00 in) shorter in length than the Great Grey, can both attain a wingspan of 2 m (6.6 ft) and a weight of 4.5 kg (10 lb) in the largest females.
Different species of owls make different sounds; this wide range of calls aids owls in finding mates or announcing their presence to potential competitors, and also aids ornithologists and birders in locating these birds and recognizing species. The facial disc helps owls to funnel the sound of prey to their ears. In many species, these discs are placed asymmetrically, for better directional location. The plumage of owls is generally cryptic, but many species have facial and head markings, including face masks, ear tufts and brightly coloured irises. These markings are generally more common in species inhabiting open habitats, and are thought to be used in signaling with other owls in low light conditions.
Most owls are nocturnal, actively hunting their prey only in darkness. Several types of owl, however, are crepuscular, active during the twilight hours of dawn and dusk; one example is the Pygmy owl (Glaucidium). A few owls are active during the day also; examples are the Burrowing Owl (Speotyto cunicularia) and the Short-eared Owl (Asio flammeus). The serrations on the leading edge of an owl's flight feathers reduce noise. Owl eyes each have nictitating membranes that can move independently of each other, as seen on this Spotted Eagle-Owl in Johannesburg, South Africa
Much of the owls' hunting strategy depends on stealth and surprise. Owls have at least two adaptations that aid them in achieving stealth. First, the dull coloration of their' feathers can render them almost invisible under certain conditions. Secondly, serrated edges on the leading edge of owls' remiges muffle an owl's wing beats, allowing an owl's flight to be practically silent. Some fish-eating owls, for which silence has no evolutionary advantage, lack this adaptation. An owl's sharp beak and powerful talons allow it to kill its prey before swallowing it whole. Scientists studying the diets of owls are helped by their habit of regurgitating the indigestible parts of their prey (such as bones, scales and fur) in the form of pellets. These "owl pellets" are plentiful and easy to interpret, and are often sold by companies to schools for dissection by students as a lesson in biology and ecology.