On a recent rainy morning in Brooklyn, Tom Stephenson, smiling and serious, dressed in khaki quick dry and a baseball cap with a Cerulean warbler insignia, stepped into Prospect Park, 526 acres of sweeping lawns, forest, lakes, and winding paths. Through layers of car honks, distant jackhammers, and chatty joggers, Stephenson heard a delicate, high pitched trill. Cedar waxwing, he said. Walking beneath a canopy of dogwood trees, he called out, Robin, robin, catbird. The catbird was mimicking other birds, he said, stringing together a jumble of song fragments. It sounded like a distracted robin. A hairy woodpecker! he exclaimed. Another robin, a yellow warbler.