| Some years ago Dr. Frank M. Chapman of The American Museum of Natural History, New York City, hid some dead animals and tested the ability of the turkey vulture to locate decomposing animal matter by smell alone. As soon as the carcasses produced odors through advanced decay, numerous vultures were attracted to the spot of concealment. From this, one may conclude that smell is well developed in vultures; but there is no way of knowing whether or not a red-tail could detect similar odors, because hawks are not attracted to such putrid material as a source of food. It was later suggested that flies swarming around the carcass attracted the vultures, which associated their presence with a dead animal.
Mammals are well known for their highly developed sense of smell and use it to detect the approach of an enemy, and to locate food. Predatory animals usually have an even better olfactory sense, which enables them to follow their prey by the scent on the ground. But such a sense would be of little use in the air, where the scent would be only momentary and of no use at all in tracking. As ample compensation for their poor sense of smell, predatory birds, therefore, have evolved the superior visual acuity for which they are well known, and locate prey and detect enemies chiefly by sight.
The eyesight of an eagle is legendary, but the red-tailed hawk enjoys equally sharp vision. The eyes of a red-tail are very large and occupy as much, or more, space in the head as the brain, each eye being almost as large in diameter as a full grown man's.
The eyes of birds are classified as either flat, globose, or tubular. Flat eyes, characteristic of pigeons, have relatively slight convexities on their corneal surfaces, while the distance through the eye from cornea to retina is shorter than from the dorsal to ventral walls. In other words, flat eyes are broader than they are deep.
Globose and tubular eyes of the birds of prey have much greater corneal convexities. In globose eyes, either distance through them is about the same. In the tubular eyes of the red-tailed hawk, the distance between cornea and retina tends to be longer and the sides of the eye are noticeably concave, giving a tubular effect. This affords better vision at longer distances, an adaptation with obvious advantages for such a bird. An interesting feature of the red-tail's eye is the nictitating membrane, or "second eyelid." Although laymen frequently liken this membrane to a windshield-saying that it protects the hawk's eye as he flies through the rain, and so on-actually, its primary function is to cleanse and moisten the eye.
 The red-tail has both monocular and binocular vision. The binocular field of view covers about 50 degrees, considerably less than the flat-eyed birds, which may require the broader view to detect more numerous enemies. The hawk's eye, however, can adjust rapidly from distant viewing to close vision, as it must do in the pursuit of prey.
The ability of the red-tail and other hawks to spot distant objects in the sky, visible to the human eye only through binoculars, is remarkable. A captive red-tail makes an excellent "spotter" bird during the autumn hawk migration and will point out countless high-flying birds that otherwise would escape human detection. The captive merely cocks its head, to look up with one eye. The passing bird is usually watched for several seconds, and the spotter often cranes its neck, possibly to better judge the distance. It may also be true that such monocular vision in hawks functions like an adjustable telescope, with a zooming-in effect made possible by muscular action on the lens.
Although the hearing of birds is inferior to that in most mammals, it is nonetheless well-developed and plays an important role in their lives. Hunting, the chief occupation of the red-tailed hawk, is accomplished primarily by vision. However, the distress cries of most creatures often attract a nearby red-tail, especially the high-pitched squealing of rabbits and other rodents. Since injured or handicapped animals are more readily captured, it is possible that bearing plays a more important function in relation to hunting than is generally realized.
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