Humans have noted the phenomenon of avian migration for several thousand years and produced some misguided explanations for a bird's return home come spring—Aristotle hypothesized that redstarts mysteriously transmogrified into robins for the winter, then back again—but only in the past decades have we begun to grasp just how birds manage such a feat.
Most species are born with migration routes genetically encoded in their brains. The timing of their return, too, is governed by an internal clock that tracks the changing ratio of daylight to darkness as winter progresses, and longer days trigger their instinct to head north.
"They really have to accomplish two separate but interlocking tasks," says Scott Weidensaul, naturalist and author of Living on the Wind: Across the Hemisphere With Migratory Birds. "They have to orient, and they have to navigate." Think of orientation as the compass and navigation as the map. One of the most important cues for orientation is the night sky. Like sea explorers, birds note the movement of stars as they swing around the stationary Polaris to the north.
But the night sky can be riddled with star-obscuring clouds, and some birds migrate by daylight. Their other key fallback is the earth's magnetic field. Numerous species contain deposits of magnetite at the base of their bill, and scientists long assumed birds used these like magnets that pull their noses north. But recent studies have shown instead that a visual element is more vital to their sense of the magnetic field through quantum entanglement, a state in which a pair of electrons can be separated by vast distances yet affect each other's behavior. Essentially, photons of light enter a bird's eye and galvanize these intertwined electrons into chemical reactions that, when multiplied across the eye, could create a map-like image of the magnetic field.
Other tools include scent—seabirds can smell their way back to a single burrow among millions in pitch black—and hearing. Birds migrating up the Great Plains, for example, can orient "in part by keeping the sounds of the Atlantic Ocean in one ear and the wind in the the high passes of the Rockies in the other, with the rumble of seismic activity in the mountains of central Mexico directly behind," says Weidensaul.
Along the way birds follow coastlines, mountains, and river valleys that funnel them northward. Once closer to home, memory and recognition of smaller features like trees or buildings help them find their exact nesting areas.
The real miracle is the seamless fusion of these tools over vast distances. "Birds really do have a remarkable ability to home back to a particular place," Weidensaul says. "They're coming back to the same back yard, the same tree. They have a whole suite of cues they can use to get there. And it may be that, depending on global conditions, they learn to rely on one more than the others."