Birds rapidly learn to identify people who have previously threatened their nests and sounded alarms and even attacked those people, while ignoring others nearby, researchers report in Tuesday's edition of Proceedings of the National Academy of Sciences.
"This shows a bird is much more perceptive of its environment than people had previously suspected," said Douglas J. Levey, a professor in the zoology department of the University of Florida.
"We are a part of their environment and we are a concern to them," Levey said in a telephone interview.
The researchers are studying mockingbirds as part of an effort to better understand how species adapt to urbanization.
With more and more areas being converted into towns and cities, animals that adapt well seem to be those that are especially perceptive about their environment, he said.
"We do not think mockingbirds evolved a specific ability to respond to humans, rather we think that mockingbirds are naturally perceptive about their environment, especially threats to their nests."
A graduate student involved in research on bird nesting noticed that when she would make repeat visits to peoples' yards the birds would alarm and attack her, while they would ignore people gardening or doing other things nearby, Levey said.
Indeed, it seemed they could even recognize her car, and she had to start parking around the corner.
So research team members decided to run their own tests in which people would approach mockingbird nests around the university campus, touch the nest, and then move on.
The study involved 10 people who varied in age, sex and amount of hair and facial hair, and dressed differently on different days, Levey said. The individuals approached a total of 24 mockingbird nests. They would approach the nests from different directions and at various times of day.
For four days the same student would approach and touch a nest, and then leave. The birds began reacting to them in advance starting on day three - fleeing the nest, sounding alarms and dive bombing the researchers.
"You may be walking by a bird and think it's just minding its own business. But if there is a nest nearby, you are its business," Levey said.
The researchers were surprised that the response was as rapid and dramatic as it was, Levey said.
It might have been expected from crows, ravens and parrots - birds known to be highly intelligent - but not from songbirds living in a natural setting, he said.
When, on the fifth day, a different student would approach the mockingbird nest, the birds did not respond in advance.
And even on the days when they were attacking a person they perceived as a threat, the birds ignored dozens of other passers-by.
Past studies have sought to determine if birds could choose between two individuals, or pictures of individuals, to get a food reward, Levey said.
This research was different in that the bird needed to pick out one person they had seen before, not always dressed the same or coming from the same direction, while streams of other people were walking by.
And the birds succeeded after having seen the person just twice.