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THE PRINCIPLE OF RAPID PEERING, IN BIRDS

JOSEPH GRINNELL

Observation shows that the commoner birds may be divided into two categories with regard to their method of obtaining food. The two categories comprise, first, those kinds of birds which perch and wait passively for the appearance, within reach, of moving objects of food value; and, second, those birds which themselves are almost continually in motion and thereby seek out items of food which are stationary. It is not to be inferred that all birds will fall definitely into one or the other of the classes here described. Indeed, some species will come to mind which occupy a borderland position; and furthermore, there are other types of behavior altogether, as illustrated by the vultures and by the owls.

As an instance of the first category, let us recall the behavior of a Hermit Thrush (Hylocichla guttata). The bird moves along the shaded ground for a distance, then strikes a statuesque pose during which it surveys with intent eye the ground round about. If nothing attracts its attention, the bird shifts its field of vision ahead, by another advance of a few feet, and again assumes a rigid pose. Occasionally the field of possibilities is improved by the bird's flicking over with its bill a leaf or two. But always the statuesque pose is quickly resumed. If, during this pose, some object on the ground happens to move, it catches the bird's eye, by reason of this movement, and is captured.

Another example of this first category is afforded by the Black Phoebe (Sayornis nigricans), which selects an open perch three or four feet above the ground where it sits until some insect flies by. Then the bird, having quickly sighted the insect by reason of the latter's motion, sallies forth and, with remarkable accuracy of aim, snaps it up.

In these two birds, and in scores of others which will come to mind, such as the Great Blue Heron and the White-rumped Shrike, the relation is that of a motionless predator on the lookout for moving prey.

The second category is well illustrated by the Brewer Blackbird (Euphagus cyanocephalus) when it forages on a lawn. The bird walks rapidly and continuously, turning this way and that, with head moving quickly to and fro in unison with its footfalls. The motion of the head involves changes in the position of the eyes; and the bird is able, with wonderful acuteness, to detect among the grass blades motionless insect larvae. A gullet-full of these larvae, to be conveyed to the waiting young blackbirds, is obtained in an amazingly brief period.

It is this second type of behavior that I wish to discuss in an endeavor to arrive, if possible, at an acceptable explanation. The more I reflect upon the observed actions of birds, and of animals generally, the more I am confirmed in the conviction that there is no such thing as wasted effort. In other words, the "excess-of-vigor" idea, or more poetically the "joy-of-living" concept, does not properly explain the lively, sprightly, cheerful, joyous aspect of many birds-although these humanistic terms are often applied to them, even in our more serious ornithological literature. Really, every movement, every phase of activity, has its explicit meaning in the bird's program of maintaining existence. Hence an adequate explanation of these movements, involving stern utility, should be looked for with earnestness by every serious naturalist.

Let us now cite the commonly observed actions of the Ruby-crowned Kinglet (Regulus calendula). People sometimes term this bird vivacious or nervous, because of its rapid or seemingly fidgety movements. Elliott Coues designated it as "the picture of restless, puny energy, making much ado about nothing." As a person watches the Kinglet passing through and over the peripheral foliage of a trec, incessant motion is, to be sure, its striking feature. This motion is seen to involve both the whole body, as operated by wings and clinging feet alternately, and the head, a member to be looked upon as the vehicle not only of the beak, the instrument of capture, but of the eyes, the chief sense organs concerned with the search for food. Even when poised out in front of a spray of foliage, on rapidly beating wings in almost the manner of a hummingbird, the Kinglet is continually moving his head this way and that. By virtue of this incessant motion on the part of the eye, he sees in relief, as he could not otherwise, the outlines of the motionless, fixed scale insect, the aphid, or the leaf caterpillar, and thereupon apprehends such insect, to devour it if it prove of food value.

Thus, the Brewer Blackbird, the Ruby-crowned Kinglet, most of the Warblers, and numerous other birds in the same behavior category, seek after stationary objects, and locate them by means of rapid movements on their own part. Our own human experience will help here in making the explanation clear.

Of easy verification is the greater ease with which a person catches sight of a distant moving object as compared with the same or a similar object at rest. Any hunter will recall the experience of having nearly or quite passed an object (such as a deer or a rabbit), in plain sight all the while, without noticing it—until the moment it moved; then as it started up and traveled across the background in which all other objects were stationary, the eye was quickly attracted to it and held by it.

The result of the reverse sitation is, it seems to me, similar. That is to say, when one is looking for an object, such as a golf ball in the grass or an animal which he knows to be stationary like everything else in the background of bushes or trees, then he, the searcher himself, moves back and forth, at the same time intently observing the objects at varying distance from him as the seemingly differential movements separate them from one another so that perspective is emphasized. Indeed it is instinctive for a person to crane his neck, as the saying goes, to move the head and the upper part of the body this way and that, when looking for an object in the near distance. Objects all of the same color, shape, and size, or nearly so, which are at different distances will be separated out of the background, when viewed by an eye in side to side motion, as they never could be separated if viewed by an eye in a fixed position. The principle of the parallax, in astronomy, obviously enters here, in that an apparent displacement of the object observed is brought about by the actual displacement of the observer.

Furthermore, rate of movement bears direct ratio to ease of detection; the more rapidly an object moves across the landscape, the quicker is it picked up by the eye-within certain practical limits. The slow movements of a snake or a land salamander are much more likely to be overlooked than the sudden dash of a rabbit or a mouse. And conversely, the faster a searcher moves, again within certain limits, the quicker will he "pick up" the object sought.

To refer to the birds again, think of an individual of an insectivorous species, one closely fitted by structure into an ecologic niche so that its food is restricted in kind and location but must be found in adequate quantity to meet the needs of a high-pressure organism. Such a bird must cover much territory every day in order to get this requisite food supply. Remember that the maintained bodily temperature of passerine birds

varies from 107 to 111 degrees F., and that their rate of living must be so much faster than our own as to make very difficult, on our part, an understanding of the sensations and reactions of such birds to their surroundings. Their experiences in an hour may total our own experiences of a day.

To get food is relatively easy for the Kinglets and Warblers when insect life is plentiful, in spring and summer; the remittal of the exigencies of food-getting then allows for the carrying on of other essential functions, such as reproduction and molting. But sooner or later there comes a time of lessening food supply, more or less of an annual famine period. And this is the critical time in the existence of the individual and also of the species. It is during these recurrent periods of most extreme food shortage that the whole equipment of the bird, including such intangible things as phases of behavior, is put to the vital test. The merest twist of the head may then be a matter of life and death. There is then no chance for parleying with stern nature. Recall the enormous expectation of casualty in, say, Kinglets— approximately 300 per cent annually, figured on the basis of the minimum population at the advent of the breeding season. In other words, 300 must perish each year for the 100 that survive until May.

All these considerations lead me to look for, in the forage behavior of different birds, specializations which adapt each bird closely to its own food source. One class of food items is characterized by motility and is chosen by a category of birds with particular structures and behavior coördinated with what might be called a phlegmatic temperament. Another class of food items is characterized by fixity-lack of motion. To obtain this class of food the bird called nervous or vivacious applies the principle of rapid peering. This faculty of rapid peering enables it to find successfully small stationary objects in foliage and among branches or grass blades, as if these objects were moving and the bird itself were quiet.

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