Birds and people are "sight animals." For both, the eyes are the dominant sense organs, vastly more important than their inferior sense of smell. The reasons for our sensory similarity to birds can be found in human evolutionary history. At one point the ancestors of Homo sapiens were small, tree-dwelling primates. When leaping from limb to limb and snatching of insect prey with the hands, sharp, binocular vision was very handy; those of our forebears that tried instead to smell the location of a branch on which to land were unlikely to survive to reproduce. And since in the breezy treetops odors quickly dissipate, they do not provide good cues for detecting food, enemies, or mates. Birds, flying higher and faster than primates leap, naturally also evolved sight as their major device for orienting to the world.

Most birds have binocular vision. It is especially well developed in predators that must precisely estimate ever-changing distances to moving prey. Their eyes tend to be rotated toward the front of the head, so that the visual fields of each eye overlap to some degree. This trend is most pronounced in owls, whose eyes are almost as completely overlapping in field as ours. Small birds that are likely to be prey for raptors tend to have their eyes set on the sides of the head, permitting them to watch for danger in all directions. At the opposite extreme from the owls are the woodcocks, mud probers with eyes set high and back on the head, out of the way of vegetation and splattering mud and in a position to look out for predators. In fact, the woodcock has better binocular vision to the rear than to the front!

Shorebirds, waterfowl, pigeons, and other birds that have minimal binocular vision seem to depend on differences in apparent motion between close and distant objects for much of their depth perception. When a bird's eye is moving, closer objects appear to move at a faster rate than do distant objects -- a phenomenon familiar from the way roadside telephone poles seen from the window of a moving car appear to pass more rapidly than the distant landscape. Presumably to enhance this distance-measuring method, shorebirds, and waterfowl often bob their heads up and down, and pigeons move theirs back and forth while walking. Even birds with relatively good binocular vision may use apparent motion to aid them in estimating distance; perched New Guinea kingfishers often "post" up and down on their legs before diving after prey. To see how this works, move your head with one eye closed and note the relative motion of close and distant objects.

The term "hawk-eyed" accurately describes many birds. For example, both raptors that must see prey at great distances and seed eaters that must pick tiny objects off the ground have eyes designed for high "visual acuity" -- the capacity to make fine discriminations. There is, in fact, evidence that hawks can distinguish their prey at something like two or three times the distance that a human being can detect the same creature. Interestingly, even with such visual acuity, Cooper's Hawks are known to hunt quail by their calls.

One way that birds have attained such a high degree of acuity is by having relatively large eyes. A human eye weighs less than I percent of the weight of the head, whereas a starling's eye accounts for some 15 percent of its head weight. But more than size alone appears to account for the astonishing performance of the eyes of hawks. Evolution has arranged the structure of their eyes so that each eye functions very much like a telescope. The eye has a somewhat flattened lens placed rather far from the retina, giving it a long "focal length," which produces a large image. A large pupil and highly curved cornea admit plenty of light to keep the image on the retina bright.

Visual acuity in birds is also enhanced by the structure of the retina itself, which has tightly packed receptors and possesses other adaptations for producing a fine-grained image. Most of those receptors are the type called "cones." "Rods," the receptors of the vertebrate retina that are specialized to function in dim light, are relatively rare. Thus daytime acuity is, in part, achieved at the expense of night vision -- a small price to pay for birds that are inactive at night anyway. In those relatively few species that are nocturnal, such as owls, rods predominate.

Considering the frequent evolution of gaudy colored plumage, it is not surprising that birds active in the daytime have color vision (nocturnal birds are thought to be color blind), and that color perception is often obvious in bird behavior. One can watch a hummingbird moving from red flower to red flower; bowerbirds show color preferences when decorating their bowers. Just how refined that color vision may be has proven difficult to determine. However, the diversity of visual pigments found in birds' eyes, and the presence of an array of brightly colored oil droplets inside the cones, suggest that avian color perception may surpass our own. There is also evidence that some birds' eyes are sensitive to ultraviolet light. In hummingbirds the adaptive significance of this is clear, since some flowers from which they drink nectar have patterns visible in the ultraviolet end of the light spectrum. Why pigeons have the ability to see ultraviolet remains a mystery. Equally surprising is the recently discovered ability of pigeons to detect the plane of polarized light. This probably serves them well in homing.

SEE: Raptor Hunting; The Color of Birds; How Owls Hunt in the Dark; The Avian Sense of Smell.
