TAKOMA PARK, Md.—A plump robin wearing a tiny metal backpack with an antenna hops around a suburban yard in Takoma Park, then plucks a cicada from the ground for a snack. Ecologist Emily Williams watches through binoculars from behind a bush. On this clear spring day, she’s snooping on his dating life. “Now I’m watching to see whether he’s found a mate,” she said, scrutinizing his interactions with another robin in a nearby tree. Once the bird moves on at season’s end, she’ll rely on the backpack to beam frequent location data to the Argos satellite, then back to Williams’ laptop, to track it. The goal is to unravel why some American robins migrate long distances, but others do not. With more precise information about nesting success and conditions in breeding and wintering grounds, “we should be able to tell the relative roles of genetics versus the environment in shaping …