People may have a tough time telling one squawking bird from another. Mockingbirds, on the other hand, quickly learn which humans to watch.

"Mockingbirds certainly do not view all humans as equal," Doug Levey, lead author of a study of published this week in Proceedings of the National of Sciences, said in a statement.

How quickly can Northern Mockingbirds (Mimus polyglottos) start recognizing specific people? It only takes two 30-second exposures over a couple of days.

In Levey’s experiment, student volunteers touched birds' nests on the busy UF campus once a day for four days. After the second day, birds began alarm calls and attack flights sooner and sooner, even swooping down Hitchcock-style to graze the heads of intruders. On the fifth day, however, a different human approached the nests, and the birds' response was back to square one.

The researchers conclude that mockingbirds "rapidly learn to assess the level of threat posed by different humans, and to respond accordingly." That, the scientists note, may explain why mockingbirds and some other species have adapted so well to urban environments.

Defense mode can be risky and energy-intensive, so overreacting—or under-reacting—to frequent passersby would be a distinct disadvantage in an urban environment. 

Levey explained that there's no reason to believe that the mockingbirds are in tune with humans in particular. "Mockingbirds and humans haven't been living in close association long enough for that to occur," he said. Rather, the study shows a general attention to detail that might surprise many who have thought crows, ravens and parrots to have the most impressive bird brains.

Image of fluffed-up mockingbird on the University of Florida Campus courtesy of Louis Guillette