The mob squad

How come big birds don't take a whack at little birds harassing them? Do the big birds feel threatened or just annoyed? Lanney, Sandia Park, New Mexico

Mobbing undoubtedly bothers big birds because it works: the large predator moves on. For example, a carrion crow, looking for eggs or chicks, cruises through the nesting area of black-headed gulls. Screaming gulls dive bomb the crow, defecating. The crow, terribly distracted, can't find eggs or chicks. She moves along, and the gulls stop mobbing her.

Occasionally, the big bird does take a whack at tormentors. In 1946, Hawk Mountain curator Maurice Braun reported seeing a golden eagle capture a red-shouldered hawk harassing him. Here's his account:

"Lying on my back and scanning the zenith with my 7 x 50 binocular, I picked up a small hawk making frequent passes at a much larger, dark bird. The smaller bird persisted in annoying its fellow traveler. I switched to an 18-power glass. The dark bird proved to be an adult golden eagle. It made a sudden thrust forward, executed an Immelmann turn [half looped over, then half rolled to reverse direction] as effortlessly as a fly landing on a ceiling, and then, to my amazement, it seized the smaller hawk, which seemed to put up a momentary, hopeless struggle. Down came the two birds precipitously, the eagle with set wings and clutching its victim. As the eagle plunged to earth, the wings of the smaller bird were fully outstretched, and I glimpsed the ruddy breast of the red-shouldered hawk. The eagle, still clutching its prey, disappeared into the densely wooded flank of the ridge."

In fact, ornithologist Eberhard Curio has documented 35 cases where the big bird attacked, captured or killed little birds mobbing her, the predator. "Taken together, these anecdotes strongly support the hypothesis that mobbingbirds are at deadly risk," he concludes. Curio is a professor of biology at Ruhr-University in Germany.

Why are predator attacks statistically small — though not negligible? The predator normally does not strike the mobbers because maneuvering mid air costs energy, and does not benefit him sufficiently. In this case, the eagle made a kill. He gained a meal. But most little birds are far more maneuverable than the big one, and are in a state of full alert.

Furthermore, as Curio points out, mobbers are careful, which minimizes big-bird whacks. The molesters generally mob or distract the predator to protect their young, and, therefore, cannot afford to take great risks.

Moreover, the little ones don't usually hit the big bird, especially outside of the breeding season, and, therefore, don't threaten it. Ornithologist Millicent Ficken recalls only one such attack in thirty years; she is a professor emeritus in biological sciences at the University of Wisconsin-Milwaukee. A group of Steller's jays mobbed a hawk, confusing him; one struck the hawk on its back. The hawk fell to the ground, but stood up, then flew off.

But "birds of most species mob much more aggressively during the breeding season," emails D. H. Shedd , Thoresen Professor of Biology at Randolph College in Lynchburg, Virginia. During the breeding season many bird species strike a predator, especially an owl (less risky than hitting hawks and the like). "I've often seen chickadees, robins and blue jays strike an owl then." Outside the breeding season, it is less common, because "if the young aren't around to benefit, the risk of approaching a predator that closely is too high."

So, a predator does strike back — even killing — but usually saves energy by escaping the mob.

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