Mobbing

It is not uncommon to see a group of blackbirds or swallows chasing a hawk or eagle, or a group of songbirds fluttering and calling around a perched owl. Such "mobbing" behavior is probably the most frequently observed overt antipredator strategy. Nevertheless, the exact purpose of such noisy group demonstrations remains a matter of some debate.

Mobbing tends to occur most intensely on the breeding grounds. For instance, in April a tape recording of the cries of an Eastern Screech-Owl brought Prothonotary Warblers, Blue-gray Gnatcatchers, and other small songbirds swarming in from their newly established territories in the swamps of South Carolina. A week later the same tape had no discernible effect on warblers moving northward along a ridge in Nashville. Presumably the migrating warblers had less reason to mob, perhaps because they would be leaving the vicinity of the predator anyway. Mobbing may thus function to divert the predator from areas where there are fledglings, or simply to confuse and annoy the predator, in the hope of getting it to move away.

This "move-along" hypothesis, first put forth by E. Curio, a specialist in the biology of predation from Ruhr University in Germany, is supported by the research of ornithologist Douglas Shedd of Randolph-Macon Women's College. Shedd has shown that Black-capped Chickadees will respond to predators in fall and winter, even in January with the temperature 25 degrees below zero. The chickadees, which remain in residence all year long, still find it profitable to mob in winter. Migratory robins, in contrast, sometimes approached a stuffed screech-owl and tape combination outside the breeding season, but never mobbed.

Careful experiments have shown that birds can learn from each other which predators to mob (indeed, one bird in an experiment was taught by another to "mob" a many-colored plastic bottle, although the mobbing was halfhearted). Therefore one function of mobbing may be educational -- to teach young birds the identity of the enemy. Another may be to alert other birds to the presence of the predator, either getting them to join in the mobbing or protecting them, since a predator is unlikely to be able to sneak up on an alert victim. The original mobber may benefit directly by the predator being moved along, or indirectly if the protected birds are its kin.

Much is lacking in our understanding of mobbing. It is not clear why predators don't simply turn on their tormentors and snatch up one or two of the mobbing birds. If they did, presumably mobbing would quickly disappear; that it persists suggests that surprise is an essential element in raptor hunting.

By Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye