Harmony and Conflict in the Living World - Alexander F. Skutch [31]
At the sight of a flying hawk, the gregarious California Ground Squirrel utters a loud cheesk, which is repeated by neighboring squirrels as each slips into its hole. A series of different notes warns the community of the presence of a snake; and yet another call advises neighbors that a person, a dog, or a coyote is approaching (Bourlière 1954).
Many kinds of animals find safety in numbers. At the approach of a Peregrine Falcon, European Starlings, flying in a flock, bunch
Page 58
more compactly and make sudden, closely coordinated turns. A falcon dashing into such a dense flock at tremendous speed would injure itself by colliding with some of the starlings, hence will try to seize only an isolated individual (Tinbergen 1951). Similarly, a flock of Cedar Waxwings, repeatedly menaced by a Cooper's Hawk, contracted into a dense mass and veered aside in unison each time the raptor tried to seize one of them, always failing (Meyerriecks 1957). The November 1988 issue of National Geographic includes an underwater photograph by David Doubilet of a school of Crevalle Jackfish swimming in close contact while they chased a great barracuda that they had thwarted when it attacked them. On the ground, in the air, and under the water, gregarious animals employ similar methods to baffle their enemies.
The caterpillars of several kinds of butterflies live in clusters, sometimes dozens or hundreds of them forming a conspicuous, compact sheet on the bark of a tree. Since at least some kinds of these gregarious caterpillars are palatable to birds, one might suppose that to make themselves so visible would be disastrous. But it has been demonstrated in the case of the Small Tortoiseshell and the Peacock that certain birds, such as redstarts, hesitate to attack the caterpillars while clustering, although they devour one of the same kind if they find it alone. Why the birds avoid clustered harmless caterpillars I do not know.
In other ways, too, the massing together of small creatures may give them a measure of safety. Midges flitting back and forth in a dense aggregation or crowded tiny animals darting at random in the water may be harder to catch than if they were more thinly dispersed. A predator closely pursuing one of them seems to be thrown off the track by another suddenly crossing its path. Allee, who noticed that goldfish ate fewer rather than more Daphnia when these small crustaceans were very crowded in the water, called this the "confusion effect."
From ancient times, it has been known that an owl drowsing on an exposed perch in the daytime is often surrounded by a crowd of small birds of the most varied kinds, all flitting closely around the sleepy raptor and calling in a medley of voices. The Greeks, as we
Page 59
learn from a remark in the Olympian discourse of Dio Chrysostom, supposed that the birds were admiring the owl; but the modern explanation is different. Similar behavior, known as "mobbing," is elicited by a perching hawk, a snake, a cat, and indeed any animal dangerous to small birds or their nests. Attracted by the hubbub, I have sometimes found nothing more formidable than a very large, moribund moth at the center of the crowd of excited birds. The birds scarcely ever touch the creature that they mob; but I have known Riverside Wrens and Rufous-fronted Thornbirds to peck snakes a dozen times their own length, always being careful to keep away from the serpent's head. Mobbing serves to warn every small animal in the vicinity that a potential enemy is present. It is