How Birds Perform Amazing Migratory Feats—and the Mysteries That Remain

Migrating birds employ skills far beyond human abilities. They depart on time without the use of a calendar and navigate long distances sans smartphone. They fly for days with few, if any, pit stops. Scientists can explain a lot about these arduous journeys, but the quest to fully understand them continues. 

How do birds prepare their bodies for flight?

Migrating is a major physical challenge. To get ready for the trip, birds can make huge changes to their bodies in a matter of days. Birds rely on fat to fuel flight, in contrast to humans who burn a lot of carbohydrates and sugars during exercise. Some will double their body weight beforehand by gorging on berries, insects, or mollusks. Others pump up the size of their flight muscles, increase their metabolism, and even grow or shrink their digestive organs to pack on or shed weight.

In a study of Gray Catbirds, for example, University of Rhode Island physiologist and ecologist Kristen DeMoranville found the birds’ flight muscles were larger during fall migration than in summer. And while the birds wintered and rested in the tropics, their hearts shrank in size, and they gained fat, perhaps anticipating the next migration season.

Scientists like DeMoranville are working to understand what genes and proteins are responsible for these body-morphing abilities, using findings from human medical research as a starting point. The results could signal which foods are crucial for birds’ journeys, which in turn could help conservationists identify and protect the most important sources for nutrition.

How do birds know when to migrate?

Even captive migratory birds get restless in fall and spring when they’d normally embark. Varied cues help birds decide when to start their journeys, says University of South Carolina ornithologist Nathan Senner, including day length, temperature, rainfall, food availability, and body condition. Birds may also pay attention to others, using social cues to decide when it’s time to leave.

How birds are using this information—and how much they can change their behavior based on what they are sensing—is less clear, says Senner. Researchers are also curious about how juvenile birds work out their migratory schedule when they’ve never made the journey before.

Studies of the European Nightjar show how a bird’s migratory timing can be finely tuned to its fueling needs. Susanne Åkesson, an evolutionary ecologist at Lund University in Sweden, and her colleagues found that nocturnal nightjars hunt more actively on bright, moonlit nights and are also more likely to start migrating after a full moon than a new moon. After gorging for days, as if at a brightly lit buffet, they’re fueled up and ready to take off.

How do migrating birds find their way?

Any animal, be it a bird, shark, or human, needs both a “compass” and a “map” to know where it is and where it is going. Birds can sense which direction they are flying based on the position of the sun, the stars, and Earth’s magnetic field. Artificially changing the magnetic field around a captured migratory bird, for example, can cause it to fly in the wrong direction, says Nikita Chernetsov, an ornithologist at the Zoological Institute Russian Academy of Sciences in St. Petersburg, Russia.

“The map has been much more controversial,” says Richard Holland, a sensory biologist at Bangor University. Here magnetic fields probably play a role, too. Each spot on the Earth’s surface has a unique magnetic signature based on its position relative to the North and South Poles, which birds seem to be able to sense. Research suggests smell may also be important, says Holland: When scientists have suppressed this sense in some species, experiments show that birds can’t find their way as precisely. But scientists don’t know exactly which smells, whether vegetation or sea spray or even air pollution, these birds rely on as a guide. Identifying the biology underlying these navigational abilities—like where magnetic sensors are located in a bird’s body and what genes code for migratory direction—is a major focus of ongoing research.

In addition to these innate abilities, some navigational skills are partly learned. Adult White-crowned Sparrows captured in the western United States and released in the eastern part of the country will find their way back to their usual migratory route, says Holland, who has tracked the birds. Young sparrows, however, will start migrating south, but don’t know to backtrack west, so they end up in the wrong place. In essence, although their internal migrating compass is inherited, how exactly to reach their wintering grounds is learned by doing.

How do birds sleep on long migratory flights?

Some birds take breaks during trips to rest and refuel. Others fly for days, crossing lakes, oceans, mountains, and continents without stopping. Scientists know it is possible for birds to sleep while flying, but don’t know how much sleep different migrating species are actually catching on the wing. Proof birds can catch in-flight z’s comes from tracking Great Frigatebirds, seabirds that can forage over the open ocean for weeks without landing. The frigatebirds would typically rest half their brain at a time, with one eye closed, while gliding upward on spiraling air currents. 

Just because birds can sleep while flying doesn’t mean they do. “We have good reason to think some birds might just not sleep at all during flight,” says Niels Rattenborg, an avian sleep researcher at the Max Planck Institute for Ornithology. For one, there’s no proof birds can sleep while actively flapping their wings. The ocean-going frigatebirds only slept about 45 minutes a night compared to more than a total of 12 hours a day back on land—and none of that was while flapping. What’s more, extreme sleep deprivation doesn’t seem to affect birds as drastically as it does humans. How much time these animals need to sleep is relatively flexible, he says. In fact, related research in migratory Pectoral Sandpipers found males that slept the least during the breeding period performed the best: They fathered more offspring. 

For now, batteries that power gear scientists use to track bird movements and brain activity in flight are too big for most species other than sizable frigatebirds, which wore trackers on their backs and heads. Definitive answers about migratory sleep will require a technological breakthrough.

This story originally ran in the Spring 2022 issue as “Enduring Mysteries.” To receive our print magazine, become a member by making a donation today.