Wednesday, February 16, 2011

Owl

Owls are a widely distributed group of birds of prey broadly characterized by large heads, flat faces, forward directed eyes, hooked beaks (the size of which is obscured by facial feathers), strong legs, sharp clawas, and soft feathers. The owl order, Strigiformes, comprises three families. One of these, the Protostrigidae, is extinct. These owls lived in North America during the Eocene Epoch, about 45 million years ago. The remaining two families are the barn owls and the typical owls.

The barn owl family, Tytonidae, contains two genera, Tyto and Phodilus, with 10 or 11 living species and 11 known fossil species. The earliest record of the barn owls is form France and dates back to the Miocene Epoch, about 19 million years ago. Barn owls range from 27 to 53 cm (11 to 21 in) in length and have relatively small eyes, a heart shaped face, a comblike margin on the middle claw, and a light coloration and sparse markings on the underparts of the body. The typical owls, family Stringidae, comprises 25 - 31 genera, three of them extinct, with 124 - 137 living species and 25 extinct species. The earliest unknown typical owls range from 13 to 71 cm (5.1 to 28 in) in length and have large eyes, a rounded or gogglelike face, feathered toes in many species, two tufts of head feathers resembling ears or horns in many species, and usually dark coloration and heavy markings on the underparts of the body.

Most owls are nocturnal, but several, including the pygmy owls, genus Glaucidium, are creapuscular, or twilight active, hunting mainly at dawn and dusk. A few owls, such as the burrowing owl, Speotyto cunicularia, and the short eared owl. The smallest owls are also active during the day. The smallest owls are probably the pygmy owls, some of which are only 13 cm (5.1 in) long, have a 32 cm (12.6 in) wingspan, and weight only 50 g (1.76 oz). The largest owls are the eagle owls, Bubo bubo and B.lacteus, which may reach 71 cm (28 in) long, have wingspan of just over 2 m (6.6 ft), and weight about 4 kg (almost 9 lb).


Most owls see well in poor light, but all see well in bright daylight - something that even the most nocturnal northern species have to do during the arctic summer. The Owl's eyes are almost immovable, and the owl must turn its head to look elsewhere; however; some species can rotate their heads horizontally 270o, or about 3/4 of a circle, and can turn their head completely upside down (180o). The eyes are more forwardly directed in owls than in other birds, and the visual fields of the two eyes therefore overlap, creating a binocular span of 60o to 70o, or about 1/6 of a circle. The overlapping visual fields of the two eyes are a perquisite for instantaneous stereoscopic vision, or dept perception.

The owl's ear openings are located behind and to the side of the eyes. They are covered by sparse, sound-transparent feathers that form the flat face. Most owls have an ear opening about the size of the eye, whereas a few species have extremely large ear openings that occupy the whole height of the head. Nine genera of owls have a remarkable anatomical asymmetry in the ear openings - the ear opening differ in position and shape so that one is not the mirror image of the other. Owls can determined the direction of the sound source on a horizontal level by comparing the intensity of a sound heard in the two ears; generally, the ear nearest the sound source will receive the highest intensity. Owls also need to know the vertical direction of a sound to be able to strike prey concealed by vegetation or snow, and this ability is provided by the asymmetrical ears. The soft feathers contribute to a noiseless flight, enabling owls to surprise their prey.

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