Every spring and fall, a spectacle unfolds in the night sky as millions of birds attempt long, perilous journeys between their summer breeding and wintering grounds.
Most of the thousands of bird species that engage in this annual migration travel at night, when wind currents are smoother and the moon and stars guide their way.
The birds typically follow established flyways, generally north-south routes that offer the best opportunities for rest and refueling along the way. Multiple bird species share these flight paths as they contend with rough weather, dehydration, starvation, and the threat of predation. (Read more about the legendary treks of migratory birds.)
Arctic terns, for instance, undertake pole-to-pole roundtrips spanning more than 60,000 miles—a record, believed to be the world’s longest migration of any animal. Other migrations involve birds flying east-west or up and down mountains. Even flightless birds migrate, such as the Adélie penguin, which makes a nearly 8,000-mile trek through frigid Antarctica.
Because migration is such an integral part of the avian life cycle, it was likely almost as prevalent thousands of years ago as it is today, says Martin Wikelski, director of the Max Planck Institute for Ornithology and a National Geographic Explorer.
Why some birds migrate and others don’t is the focus of a complex and active field of research. Finding food generally is believed to be the main driver. Additional motivations could include to escape from inclement weather and to reduce exposure to predators or parasites, especially during breeding season.
New technological advances, such as sophisticated GPS tags and radar-detection systems, are giving scientists unprecedented opportunities to observe bird migration.
As part of his ICARUS project, for instance, Wikelski has outfitted some birds with Fitbit-like devices that track their movements and the environmental conditions they encounter.
These miniature solar-powered satellite transmitters could one day reveal animal migrations and behavior at a global scale from space.
“There’s just so much to learn,” Wikelski says. “I’ve been tracking birds for over two decades, and the ease with which birds seamlessly migrate between worlds is absolutely astounding.”
Which birds migrate?
Roughly half of the world’s nearly 10,000 known bird species migrate, including several songbirds and seabirds, waterfowl and waders, as well as some raptors. The Northern Hemisphere has the most diverse array of migratory birds.
Among the most well known are Arctic-breeding bar-tailed godwits, champions of endurance. In 2020, scientists recorded a godwit undertaking the longest-known nonstop migratory flight between Alaska and New Zealand, traveling more than 7,500 miles across the Pacific Ocean for 11 days straight. (Learn why birds matter, and are worth protecting.)
There are also feathered migrants that fly far and fast. The great snipe, for instance, covers distances exceeding 4,200 miles and reaches speeds of up to 60 miles per hour when traveling nonstop between Europe and sub-Saharan Africa, making it the fastest flying migratory bird.
Even tiny birds embark on gargantuan journeys. Calliope hummingbirds—North America’s smallest bird—make 5,600-mile roundtrips between the high-elevation meadows and open forests of the northern Rockies and the pine-oak forests of Mexico.
Most species of migratory birds may be partial migrants, meaning that some populations or individuals within the species migrate while others stay put. A fraction of American robins, for example, remain near their breeding grounds across seasons while others travel south and then return north.
Yellow-eyed juncos breeding at high elevations along southeastern Arizona’s mountains are most likely to migrate up to a mile downslope during severe snowy winters, compared to those at lower elevations facing fewer food constraints. Even tropical birds, especially insectivores, undertake short-distance elevational trips.
How do they know where to go?
In addition to following celestial cues, such as the position of the sun, stars, and the moon, adult birds use a magnetic compass to navigate. Even when there are no landmarks, this internal “GPS system” can prevent them from getting lost.
Such navigational acumen can enable individual birds to move through regions not typically traveled. In experiments, when solo-flying common cuckoos were transported nearly 1,500 miles away from their breeding grounds prior to migration, they often steered back to their normal migratory routes.
But what about inexperienced birds migrating for the first time? In one experiment, geographically displaced young common cuckoos navigated back to roughly the same flight path used by those birds that weren't displaced from their home. (Read about amazing animal navigators.)
Whether this navigational capacity is inherited and innate or learned is an ongoing debate. “I think it’s a combination of innate tendency, but you learn from others on the way,” says Wikelski, who has been tracking common cuckoos since 2012.
One way to learn might be tuning into nocturnal flight calls from other migrating birds. Distinct from a bird species’ regular vocalizations, these acoustic signals could especially guide the inexperienced, sometimes even those of other species, Wikelski says.
How do they know it’s time to go?
For some birds, changes in environmental conditions, such as the length of the day, may trigger migration by stimulating hormones, telling the birds it’s time to fly.
Birds’ internal biological clocks can also detect when a season shifts, using cues such as changes in light and possibly air temperature.
Once the birds are in migration mode, a feeding frenzy ensues. This allows the birds to accumulate fat to power their journeys, says Lucy Hawkes, a migration scientist at the U.K.’s University of Exeter who currently tracks Arctic terns.
“Somehow, [the birds] know that they have to migrate soon and get massive,” Hawkes says.
Local and regional weather conditions, such as rain, wind, and air temperatures can also influence decisions about when migratory birds take to the skies.
Migrating in a changing world
Overall, migration schedules seem to be shifting, as a result of climate change. “It looks like bird migrations are commencing a little earlier in the spring,” says Kyle Horton, an aeroecologist at the University of Colorado who uses radar technology to map realtime and historical bird migrations in the United States.
Black-throated blue warblers, for example, are migrating almost five days earlier now, on average, than they did in the 1960s. Canada-bound American robins are arriving 12 days earlier in the spring than they did in 1994. Migrating whooping cranes are showing up nearly 22 days earlier at their stopover site in Nebraska in the spring and leaving almost 21 days later in the fall than they did in the 1940s. (Learn how climate change has affected the annual migration of the yellow warbler.)
Such early starts to migration may benefit birds if plant and insect productivity at the breeding grounds mirror the trend. However, not all migratory birds may be able to adapt to a warming world, and if they did, the full costs of doing so remain unclear.
As scientists continue to unravel the mysteries of bird migration, the phenomenon remains one of nature’s great wonders.
“They’re flying all night, feeding all day, and doing it again,” Horton says. “That’s sort of remarkable."