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Show Sights and Sounds of the Winged World 723 to your canary, he may seem to be listening as he watches your lips move, but he does not hear a word you say. When I talk to Jim Crow in a low voice, it is little wonder he turns a deaf ear to my crooning. If I speak loudly, even at the same pitch, I doubtless set up overtones that become audible to him, and he pays attention. What a lot of noise he is spared! His world is somewhat different from mine in this respect, although he doubtless hears more of the world's uproar than does the canary or the starling. In our laboratory at Cornell, Ernest Edwards, a graduate student, made tests of the hearing of horned owls and determined that, in spite of the comparatively low frequency of the call (150), the owl's range of hearing extends downward only to about 70 cycles a second. This explains why the ruffed grouse can drum with impunity even at night in woods inhabited by horned owls, for the vibration frequency of the grouse drum is only 40 a second and therefore below the hearing range of the owl. Most species of birds utter distinctive call notes-chips, chirps, or squeaks quite apart from their songs. Is it too farfetched to suppose that their ears are especially attuned to these notes, which sound much alike to man, and that they are thus able to exchange with their own kind a considerable range of feeling and experience? On the other hand, man's wide range of hearing enables him to detect many sounds inaudible to birds. When I place some unfamiliar object before my crow, he usually scrutinizes it carefully, turning his head first on one side and then on the other, as if listening to see if it could make a noise. He is not listening any more than the robin cocking his head to one side as he seeks worms on the lawn. Birds, having their eyes on the sides of their heads, do not see near objects straight ahead so easily as those at the side. They turn their heads to center the vision of one eye on the object. How Owls See at Night On the retinas of such birds as the robin and the crow, there are two spots of keenest vision, one for monocular vision of things close up and one for binocular vision of more distant objects (pages 724, 725). Owls, whose eyes are directed forward, see more as we do and turn their heads in the direction they wish to see most clearly. When the light fails and we begin to see poorly, the owl starts to waken, and the iris in its eye opens up to let in a maximum of light. In one of my photographs the distended iris proves that the picture was taken by flashlight in the dark. In bright sunlight the pupil might be the size of a pinhead. In still another way, the owls have an advantage over man: Their retinas, like those of cats, are more sensitive to the blue end of the spectrum, which dominates at night. The retina contains two types of light-sensitive cells, the rods and the cones. The cone cells, which contain a drop of oil, are apparently more sensitive to the red end of the spectrum, while the rods are more sensitive to the blue. In the retina of the owl and other night-roaming creatures, the rods predominate. Is it not possible that the periods of activity of the many kinds of birds are determined even more by the proportion of rods and cones in their retinas than by the size of their eyes or the activity of the pigment cells? Cardinals Sing Early and Late Certain species start their May morning concert much earlier than others. For example, a pair of cardinals are the first dawn visitors to the feeding station near my window and the last to leave at night. The chickadees, however (Plate IV), and the woodpeckers (Plates II, III) often do not show up for an hour after dawn, and they retire long before the cardinals. When winter shadows lengthen on Cayuga Lake, the canvasbacks and bluebills cease diving near shore and swim farther out to sleep. While sleeping, they paddle occasionally with one foot to keep traveling in circles and not drift ashore. They obviously do not like the dark. On the other hand, the black ducks that have been dozing out the day in the middle of the lake now become active. Like the woodcock (Plate IX), they go in search of food just when the diving ducks cease their activities. Differences in responses to sunlight control all birds' selection of habitat and time their daily activities. To a barn owl, the world viewed by the light of the moon and stars must be a cold, colorless place, but to the swallows (Plate X), which dart about in full sunlight, it takes on a riot of color. The ovenbird and mourning warbler choose the dense shade of the woodlands for their abode; the orioles (Plate VI), cuckoos (Plate V), and kingbirds (Plate XV) like the light shade of gardens; the sandpiper (Plate XIII) and the terns (Plate I) prefer the bright sun of the open shores. And each species may be unhappy in the preferred habitat of the others. The Arctic tern probably enjoys more day- |