In the spring of 1949, in a coastal town in Lower Saxony, ornithologist Gustav Kramer trapped a few migratory songbirds in cages and put them outside. Every night, he noticed, the birds oriented themselves in the same direction, wings partially spread and quivering, their beaks tilted skyward. They hopped on their perches, agitated and excited, fluttering to the side of the cage facing their migration route. And because songbirds use the North Star to navigate, they did it only after sunset, when they could see the night sky. Kramer called the condition zugunruhe, a German compound of hug (to move) and unruhe (unrest). Migratory restlessness.

It happens in people, too. There’s a time to leave, when you feel a stirring. For most, it’s in the fall, when, as Joni Mitchell sang in “Ur...