Owl - Anatomy

Anatomy

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 sport 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—as are those of other birds—so they must turn their entire head to change views. Owls can rotate their heads and necks as much as 270 degrees in either direction. 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.

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. As noted above, 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.

Owl eggs usually have a white color and an 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 fact 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 to raise their young.