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The mob squad

APRIL HOLLADAY

Globe and Mail Update

February 19, 2008 at 11:18 AM EDT

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.

FURTHER READING

(Answered Feb. 11, 2008)

READERS' ANSWERS

Big birds usually do not have the same aerial maneuverability as smaller, agile birds simply due to the laws of physics involved in size and flight. Hence big birds cannot always reply to nuisance birds with great efficiency unless they are well experienced or in larger numbers (like the Harris hawks who are social birds of prey and hunt in numbers — but they are a rarity or like a murder of crows).

Small birds also tend to fly above a larger bird and peck at the series of muscles that conduct the wing motion while in flight. That peck often disturbs the muscles from functioning in the larger bird's back and such disturbance interrupts flight (and it most probably hurts). You'll often see these birds drop a little in flight before regaining control. In a series of successive pecks this attack can be quite annoying to flight stability and it seems difficult for a larger bird to recover.

Fleeing is more reasonable than expending unnecessary energy thwarting a small pest that is simply more versatile in air. Plus, juvenile/young large birds are often inexperienced in dealing with smaller birds that are viciously determined to protect their nest and/or territory. Older large birds often have learned to use such flight techniques as barrel rolls to flash their talons to warn smaller birds they are a bigger threat and mean business. I've seen many golden eagles and red tailed hawks use this technique to get crows and ravens off their backs.
Adriana C. Croghan, New York, USA

Larger birds, particularly individuals of predatory or quasi-predatory species such as eagles, hawks and falcons, depend upon having distance between themselves and their prey when commencing an attack. They are not close-range 'in-fighters'. By approaching the larger bird and attacking repeatedly while remaining close, the smaller bird negates the larger bird's instinctive offense strategy and often forces it to flee in confusion. And, of course, the larger bird definitely feels threatened.

Someone, World Smaller birds are much more maneuverable than the larger birds they are attacking. The smaller birds can cause appreciable damage to larger birds by pecking them in the head from behind. During the period of nesting and feeding young, smaller birds attack larger birds to drive them from their territories. This often starts a daisy chain reaction.

I once saw a hawk chased by a crow that was chased by a grackle that was chased by a blackbird that was chased by a kingbird that was chased by a sparrow that was chased by a canary. The whole stream of birds moved like an undulating snake in the sky. Once nestlings are safely fledged these same birds will fly right by their former protagonists without any concern.
Russ Schnell, Boulder, Colorado, USA

My best guess, from having seen small birds harassing eagles and turkey buzzards, is small birds can deftly outmaneuver the large bird in midair. Kinda like the difference between a Caddy and a Maserati.
G. A. Buehner, Narrowsburg, New York, USA

Next question for readers to answer: I read lately that a scientist may have discovered the 'Evil' gene that causes Man to kill, murder, start wars, etc. Is this 'discovery' another one of those theories classified as nonsense?

April Holladay lives in Albuquerque, New Mexico. Her column, WonderQuest, appears every second Monday of the month on globetechnology.com. To read April's past columns, please visit her website. If you have a question for April, visit this information page.

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