In Montana, as in most of the Lower 48, American Robins are present year-round. So I had long assumed that the cheerful songbird hopping around my yard in the fall had nested nearby that summer. Imagine my surprise, then, to learn it may have spent those months a thousand miles or more to the north. Where these common birds go, and what drives their decision to migrate long or short distances—or simply stay put—are mysteries that scientists are just starting to solve. “They’re a truly cosmopolitan bird,” says Emily Williams. “They may be the most widespread songbird in North America, and yet there’s so much we don’t know about them.” Williams, a former biologist at Alaska’s Denali National Park, is taking part in the first-ever study to track the movements of individual robins across seasons. Scientists have a solid grasp on the behavior and physiology of the birds during the breeding season but a poor handle on their experiences the rest of the year. Four...