Christine Stracey remembers the first time a mockingbird dive-bombed her head. Since 2005, the graduate student at the University of Florida, Gainesville, has been invading the birds' nests in her neighborhood, counting eggs and banding chicks for a research project. Over time, she noticed that the birds were getting aggressive with her: They would squawk and swoop toward her as soon as they saw her coming. The mockingbirds seemed to have it out for Stracey in particular; they ignored passersby and even gardeners working right beneath their nests. "By the end of the summer, I was absolutely convinced that the birds knew me and did not like me!" she says.
The idea that birds can recognize individual humans isn't new. Any parrot owner will tell you that Polly knows the difference between the owner and a stranger. And scientists have shown that crows can identify people by sight: In one unpublished experiment, the birds scolded researchers wearing caveman masks who had caught and banded them months earlier, but they ignored "neutral" figures wearing Dick Cheney masks. Still, no one had actually published work on the ability of birds to recognize people in the wild, and mockingbirds aren't considered to be as intelligent as parrots or crows.
So Stracey persuaded her adviser, ecologist Doug Levey, to design an experiment to see if mockingbirds could really recognize individual people. First, Levey, Stracey, and colleagues located a brooding mockingbird on campus. Then they asked one volunteer to stand near the nest for 30 seconds and touch it for half of that time on four consecutive days while the mother mockingbird was present. With each visit, the bird grew more agitated. At first, the mother bird waited until the person came close and then flew to a nearby bush to shout out alarms calls, a behavior called flushing that birds do to distract predators in the wild. But by day four, mom was up and out of her nest when the volunteer was almost 14 meters away--and she or her mate dive-bombed the volunteer's head.
On the 5th day, the team asked a completely new person to approach the nest. This time, the mother bird reset her behavior, responding as she had to the other volunteer on the first day. The team ran the experiment on 23 other nests, with the same results, showing that the mockingbirds were able to spot their intruder out of the hundreds of people who passed within meters of their nest each day, the researchers report online today in the Proceedings of the National Academy of Sciences.
The mockingbirds' acute perception may explain why these birds flourish in urban settings, while other birds like warblers and tanagers do not, says Levey. For an animal to live in a city, it must cope with a barrage of potential threats. The birds can't just squawk at anything, he explains, because bringing attention to the nest can attract other predators.
This paper is "a beauty," says John Fitzpatrick, an ornithologist at Cornell University. "It's amazing what a bird brain can do." He says the findings expose the subtle interplay between the natural and human world. Most people assume that birds are minding their own business, he says, when in reality they are quite in tune with the people passing by.