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. Most birds of prey sport eyes on the sides of their heads, but the stereoscopic nature of the owlâ€™s forward-facing eyes permits a greater sense of depth perception necessary for low-light hunting. Although owls have binocular vision, their large eyes are fixed in their sockets, as with other birds, and they must turn their entire head to change views. Owls can rotate their heads and necks as much as 270 degrees in either direction. Owls are farsighted and are unable to see anything clearly within a few centimeters of their eyes. Caught prey can be felt by owls with the use of filoplumes, which are small hair-like feathers on the beak and feet that act as â€œfeelersâ€. Their far vision, particularly in low light, is exceptionally good.
The smallest owl is the Elf Owl (Micrathene whitneyi), at as little as 31 g (1.1 oz) and 13.5 cm (5.3 inches). Some of the pygmy owls are scarcely larger. The largest owls are two of the eagle owls; the Eurasian Eagle Owl (Bubo bubo) and Blakistonâ€™s Fish Owl (Bubo blakistoni)â€”which may reach a size of 60 â€“ 71 cm (28.4 in) long, have a wingspan of almost 2 m (6.6 ft), and an average weight of nearly 4.5 kg (10 lb).
Different species of owls make different sounds; the 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 to funnel the sound of prey to their ears. In many species, these 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.
Owl eggs usually have a white colour and almost spherical shape, and range in number from a few to a dozen, depending on species. Eggs are laid at intervals of 1 to 3 days and do not hatch at the same time. This accounts for the wide variation in the size of sibling nestlings. Owls do not construct nests, but rather look for a sheltered nesting site or an abandoned nest in trees, underground burrows, or in buildings, barns and caves.
Most owls are nocturnal, actively hunting for prey only in the 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 also active during the day; examples are the Burrowing Owl (Speotyto cunicularia) and the Short-eared Owl (Asio flammeus).
The serrations on the leading edge of an owlâ€™s flight feathers reduce noise.Much of the owlsâ€™ hunting strategy depends on stealth and surprise. Owls have at least two adaptations that aid them in achieving stealth. First, the dull coloration of owlsâ€™ feathers can render them almost invisible under certain conditions. Secondly, serrated edges on the leading edge owlsâ€™ remiges muffle an owlâ€™s wing beats, allowing its flight to be practically silent. Some fish-eating owls, for which silence is of no evolutionary advantage, lack this adaptation.
An owlâ€™s sharp beak and powerful talons allow it to kill its prey before swallowing it whole (unless it is too big). 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 â€œowl pelletsâ€, which are plentiful and easy to interpret, are often sold by companies to schools for dissection by students as a lesson in biology and ecology