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Show 724 The National Geographic Magazine Drawn by Walter A. Weber The Way a Bird Sees The head of this Red-shouldered Hawk is dissected on the left side to show the extraordinary size of the eye. The retina has two depressions, or foveae, for sharpest sight. The line of binocular vision (t) is that for both eyes together; that of monocular vision (c) is that for each eye independently. These are adjusted by the large lens in front, while below lies the dark-colored pecten, behind which is the light-colored end of the optic nerve. The eye is not symmetrical (pages 723 and opposite). light than any other bird, for it nests in the Arctic region during the period of almost perpetual daylight and winters in the Antarctic during a similar period. On the 22,000 miles of its migrations, it enjoys the periods of longest days and shortest nights. Is there a material difference in its eye structure that stimulates such a tremendous urge for migration? So far as I know, no scientist has yet worked this out. Birds' Eyes Adjust Themselves Instantly I have often watched an osprey or a kingfisher, hovering 50 to 75 feet over the lake, suddenly half close its wings and drop headfirst toward a fish it has spied in the water. Despite the rapidly increasing speed of its plunge, it apparently keeps its eyes focused on its prey, for it sometimes swerves when only three or four feet from the water to transfix a moving target! (Page 722.) Human eyes require a fraction of a second to adjust from the distant road to the speedometer of a fast-moving automobile, but with birds adjustment seems to be instantaneous. In the back of a bird's eye a little black vascular comb called the pecten projects from the region of the optic nerve into the vitreous humor behind the lens. One of the supposed functions of this pecten is to throw its shadow on the retina, so that the slightest motion of the object under scrutiny is perceived and telegraphed to the brain. Mammals do not have this structure. The tiniest mite on the bird's nest is immediately picked up if it moves, but even large enemies escape detection if they are quiet. A wounded grouse I once held in my hands at the edge of a wood was obviously frightened at being caught; yet, despite its predicament, it suddenly turned its head to watch the sky. After several moments I made out the tiniest black speck of a hawk flying so far above the earth that it was nearly out of range of human vision. A hawk or an owl, however, that does not move often escapes detection even when close at hand. Bird-eating hawks like the Cooper's and sharp-shinned take advantage of small birds by perching motionless until their prey has become oblivious of their presence. Once while banding canvasbacks on Cayuga Lake, I sat in a blind on a bag of grain from which I had baited the trap, a small enclosure of wire netting in the shallow water. I had spilled inadvertently a trail of corn along the shore. Before I realized what had happened, several black ducks had guzzled their way around and behind my little hideout and were nosing the grain from right under my coat-tails. So long as I kept still, I was just a part of the landscape to them; but the instant I moved they sprang away in such alarm that one boxed my ear with his wing as he passed over my head. Hunters consider the black duck the wariest and most sagacious of all our waterfowl. For this reason friends have remarked rudely that the ducks' failure to distinguish between me and the sack of corn was no compliment! Do Birds Have a Sense of Smell? Some duck hunters aver that the black duck is the only duck that has a sense of smell and that consequently it is as difficult to sneak up on as a deer. This opinion brings up the question of the importance of odors in the bird's world. A few years ago one of our graduate stu- |
