When the little guys win one

Science News, March 14, 1998 by Susan Milius

Underdogs of all species, take heart.

Ornithologists in Australia have documented that small birds win benefits by taking on a predator 20 times their size.

A variety of bird species will mob a predator such as a hawk, harassing it by raising a ruckus and repeatedly performing displays, swooping, or striking at the victim. There may be safety in numbers, but many of these so-called mobs number only two or three birds, says Chris R. Pavey, formerly of the University of Queensland in Brisbane.

Most studies of the benefits of mobbing have focused on birds that pick on somebody near their own size, he says. He's more interested in the likes of the grey butcherbird, all 75 grams of it, which mobs a species called the powerful owl. These hawkish-looking predators can weigh up to 1,700 g.

To a big owl, small birds pose no more danger than flying hors d'oeuvres. Yet pestering an owl at its daytime roost pays off for the smaller birds, say Pavey and Queensland colleague Anita K. Smyth. In 20 percent of mob scenes, the owl left, the researchers report in the February Animal Behaviour.

This harassment may explain why owls so often roost in the rain forest, Pavey speculates. Only one of the seven mobbing species spends much time there, and owls snoozing in the thick vegetation suffer less than a third of the disturbances they endure elsewhere. Rain forest covers only 12 percent of the study area, yet the owls roost there about half the time.

By checking pellets regurgitated by owls, Pavey and Smyth estimated that nonmobbing birds were nine times more likely to be eaten as mobbing species. Although not convinced by those data, Keith L. Bildstein of Hawk Mountain Sanctuary in Kempton, Pa., is nevertheless impressed by the Australian mobsters' power. "It actually shapes the behavior of the predator," he says.