MOCKINGBIRDS MAY all look alike to you, but the feeling is not mutual. University of Florida researcher Doug Levey recently devised a test in which a person approached a mockingbird nest once a day for four days in a row, briefly touching the nest. Each day, the parent birds reacted more quickly to the intruder. But on the fifth day, a different person approached the nest—and the birds reacted as slowly as they had to the first person on the first day. The study, published in the Proceedings of the National Academy of Sciences, surmises that this ability to tell threatening people from mere passersby may help explain why mockingbirds thrive even in close contact with humans. —Hannah Schardt