Every spring and fall, an invisible river of birds flows across the sky. On a good autumn night over the United States, an estimated one billion birds can be airborne at once, most of them traveling after dark when the air is calmer and predators are fewer. They are not fleeing the cold, at least not primarily. They are chasing food, daylight, and a narrow seasonal window to raise the next generation, and they are doing it with a navigation system that puts most human technology to shame.

It Is Mostly About Food, Not the Cold

According to the Cornell Lab of Ornithology’s All About Birds, many species that migrate north in spring are not simply escaping winter, since plenty of birds can survive freezing temperatures as long as food is available. What they cannot survive is a lack of insects, seeds, and budding plants. Northern Hemisphere birds fly north each spring to cash in on a short burst of abundant food and open nesting habitat in places like the Arctic tundra and boreal forest, which are far less crowded than the tropics where many of these species spend the winter.

Daylight Is the Trigger

Longer days at northern latitudes also mean more daylight hours to hunt for food and feed hungry chicks. Cornell notes that changing day length itself acts as the biological cue, triggering a restless, migratory drive in birds known as zugunruhe, a German term describing the fidgety urge to travel that shows up even in birds kept in captivity as the seasons shift.

Distances That Sound Impossible

Some of the resulting journeys are almost hard to believe. Arctic terns and sooty shearwaters log more than 31,000 miles a year crisscrossing the globe. Bar-tailed godwits have been tracked flying nonstop from Alaska to New Zealand in about eight days, without landing once to rest, eat, or drink. Even small songbirds and hummingbirds routinely cross 700 miles of open water over the Gulf of Mexico in a single overnight flight, according to All About Birds.

A Compass Built Into the Eye

How do they not get lost? Part of the answer is genuinely strange. Research described by National Geographic suggests migratory birds have specialized light-sensitive molecules in their eyes that let them essentially see Earth’s magnetic field, appearing as a subtle shading overlaid on their normal vision. In one study, researcher Dominik Heyers and colleagues traced a direct neural pathway from these eye molecules to a brain region called Cluster N, which is highly active during nighttime migratory flight.

A Map to Go With the Compass

A compass alone only tells a bird which way is north. To actually find a specific wintering ground or a home nesting site, birds likely combine that magnetic sense with other cues, including magnetic particles believed to sit in the beak and the position of the stars, which some species appear to calibrate against as chicks. As researchers have put it, an animal capable of migrating thousands of miles needs both a compass and a map, and birds appear to carry a working version of both.