At the approach of dawn, most owls seek seclusion in densely foliated trees, vines, or hollows; since they seldom expose themselves by day, many people believe that they cannot see in daylight. But the eyes of an owl are among the most remarkable in the bird world: in darkness the pupils can be dilated almost as wide as the eye itself, giving the retina access to as much light as possible. As light intensity increases, a gradual contraction of the pupils allows just the right amount to reach the retina and form the image, and in very bright light the pupil becomes so small that it appears as a mere dark speck in the yellow iris.
In addition to this unique quality, the retina--that part of the eye which receives the image and transfers it to the central nervous system--has special adaptations for nocturnal vision. It contains rod-shaped receptor cells, effective in gathering light, and cone-shaped receptor cells, which function in bright light and play a major role in color vision. While diurnal birds have a great abundance of cone-shaped cells but comparatively few rods, the owls have many more rods than cones. This gives them superior vision in poor light but at the same time renders them more or less color-blind. They see most images in various shades of gray, black, and white.
The eyes of most birds (songbirds are a good example) are located on each side of the head, giving them a wide field of monocular vision to either side and a limited range of binocular vision directly in front, where there is an overlap of each eye's field of view. The eyes of an owl are situated on the front of a relatively flat face, giving it a wider range of binocular vision than any other bird.
Binocular vision gives the three-dimensional effect necessary for distance determination; it is vital to hawks and owls, which hunt living prey and must be able to judge distances accurately and quickly. Though a desirable trait, it is greatly exaggerated in the owl family and tends to reduce their over-all visual field. An owl sees only what is directly in front of it and must rotate its head to take full account of its surroundings. Because its eyes are set so wide apart and the eyeballs themselves are so rigidly fixed in the sockets, an owl has difficulty focusing at close range and often backs away from near objects in order to see them clearly.
Owls also have the unique ability to drop the top eyelid when blinking, but they raise the lower eyelid when sleeping just as other birds do. Also, like all other birds, they have a transparent nictitating membrane, sometimes referred to as a "third eyelid." It is controlled by involuntary reflex action, and its primary function is to cleanse the eyes and keep them moist.
The ears of an owl, under the feathers of the skull, are long asymmetrical slits located behind the facial disks on either side of the head. They are an essential part of the bird's sensory apparatus, playing as important a role in the detection of prey at night as does vision itself. Experiments conducted by Roger S. Payne of the New York State College of Agriculture seem to have proved that owls are capable of locating prey entirely by sound. Payne placed a captive barn owl in a totally darkened room, the floor of which had been covered with dried leaves. When Payne released live mice in the dark room, the owl caught them without difficulty. Thinking the mouse's body temperature might possibly be aiding in its detection, Payne repeated the experiment, this time dragging a wad of paper through the leaves with a string. The owl struck the paper with high accuracy, leading Payne to conclude that sound alone had guided the bird to its target.
Further tests by Payne indicated that the owl's accuracy depends to some extent on the sound frequency or number of aerial vibrations produced per second. The bird's eardrums were found to be highly directional for sound frequencies over 9,000 cycles per second.
Payne suggests that the owl locates its prey by turning its head until the intensity of all vibrations producing the sound are at a maximum in both ears, at which time it is automatically facing the sound source.
Although Payne's experiments involved only the barn owl, the conclusions of his experiments seem applicable to owls in general, perhaps with some variations between different species.
Owls are not usually credited with a highly developed sense of smell, but information pertaining to this subject is contradictory and unsubstantiated. A Canadian author, Farley Mowat, who apparently kept two horned owls for some length of time, claimed that the scent of skunk on the night air would transform one of his birds into a feathered terror, so great was its anticipation of a feast. Though horned owls occasionally dine on skunk, it is not likely that they normally hunt these animals by scent.
Most owls hunt at night or in the subdued light of late evening or early dawn. When environmental conditions prevent this, however, they adapt quite readily. This trait is best illustrated by certain species of northern owls, including the snowy owl, the short-eared owl, and the hawk owl. Since periods of darkness are short during the summer months, these birds adopt more or less diurnal habits at this time. Summer is also the time of greatest activity in the reproductive cycle. During the dark winter months, they revert to a more completely nocturnal existence again.
Although the Arctic horned owl and other subspecies might well fit into the category of partially diurnal owls, the great horned owl of the temperate zones is pre-eminently a bird of the night. However, it'll occasionally hunt by day when hard pressed for food, especially on dark cloudy days during the winter.
