When Birds of a Feather Mob Together, It's Usually Bad News for Predators
Ornithologist Millicent Ficken was recording bird calls in Arizona's Chiricahua Mountains when she heard the screams of a Cooper's hawk on a nearby ponderosa pine. "After the hawk's last scream, a Steller's jay sounded the alarm with its harsh wah call," recalls Ficken. Seconds later, a half-dozen jays were flocking about the hawk, darting in close to its back and head, and joining in a raucous chorus of wahs. Then one of the jays, despite being half the size of its target, struck the hawk on its back and sent the raptor plummeting to the ground. The hawk stood there briefly and then flew off.
Zoologists call this massing together of birds to attack a common enemy "mobbing." In this case, the jays were mobbing a hawk that specializes in eating birds their size. A widespread phenomenon among birds, mobbing may involve a mix of bird species or, as with the jays, just one species attacking a predator. Rarely, however, do the birds actually strike the object of their wrath; the jay's attack was the only instance that Ficken, a professor emerita at the University of Wisconsin-Milwaukee, has witnessed in more than 20 years of research.
In many cases, mobbing is a noisy affair in which each species uses its own special calls to bring out its allies. By surveying the mobbing calls of more than 50 species of birds, Ficken has provided additional insights into this behavior. "I discovered that there is no one stereotypic pattern to the calls as had been previously believed, but an array of different vocalizations among the various species of birds," she says.
Earlier research had suggested that mobbing calls enabled other birds to locate the bird sounding the alarm. Such calls would start abruptly and cover a wide range of frequencies, so they could be readily picked up by other birds. "The black-capped chickadee's mobbing call is a classic example of this type," says Ficken. But she discovered that less than half the species she studied conformed to this classic pattern.
Among other species, she found different kinds of mobbing calls. Some flock associates--birds that often hang out together--have mobbing calls unique to their groups. "Brown creepers, golden-crowned kinglets and Mexican chickadees, for instance, are frequent flock associates," says Ficken, "and their high-pitched, buzzy mobbing calls are strikingly similar." Bridled titmice and ruby-crowned kinglets give mobbing calls that are nearly identical. "The similarity in their calls may help to coordinate the group mobbing responses," she adds.
Her research has also revealed similarities within some groups of birds with a common ancestry such as corvids (crows and jays), nuthatches and vireos. But underscoring the complexities of the avian world, she found that the mobbing calls within other related groups of birds such as flycatchers and chickadees were very different. Why the variation? "There are many possible reasons, such as differences in ancestry and flock associations," says Ficken. "But we really don't know. It's a poorly understood area."
Despite the differences in calls, however, the main function of mobbing appears to be to get the predator to move out of the vicinity. Stanley Temple, a wildlife ecologist at the University of Wisconsin-Madison, sees evidence for this theory in his observations as a falconer. On one occasion, one of his red-tailed hawks caught a crow and brought it to the ground. The screaming of the captured crow sounded the alarm for the crows in a huge nearby roost and "they descended like a black tornado on the hawk," Temple recalls, and the captured crow got away.
Ficken's observations also bear out the "move along" hypothesis. The Steller's jays she watched in Arizona certainly sent the Cooper's hawk on its way in a hurry. With its initial mobbing call, the first jay also put other jays on the alert, averting any surprise attacks by the hawk--another likely reason for mobbing.
One thing about mobbing is clear: It helps little birds even the odds against big predators. Ficken recently observed this in Arizona, where she saw five hummingbirds mob and chase away a northern pygmy-owl more than three times their size. Says Ficken: "Even small prey species can defeat their larger enemies by mass action."
Wisconsin writer Mariette Nowak once discovered a bobcat with the assistance of a mob of wrens and warblers.