One researcher found that Gray Catbirds’ flight muscles are larger during migration, and while the birds wintered in the tropics, their hearts became reduced in size, and they gained fat, perhaps in anticipation for the spring migration season (photo by Paul Konrad).
To learn more about extraordinary activities related to preparing for and undertaking migration, Audubon recently tracked down some information to answer some of the most frequently asked questions about migrating birds. Obviously, birds have super-skills far beyond human abilities: They depart on time without the use of a calendar or watch, and they navigate long distances without a smartphone or map. They fly for days with few, if any, rest stops. Their bodies undergo remarkable changes so they are in top shape to undergo long migration flights.
How do birds prepare their bodies for flight? Migrating is a major physical challenge for birds. To get ready, some birds can make huge changes in their bodies in a matter of days. Birds rely on fat to fuel migration flights, in contrast to humans who burn a lot of carbohydrates and sugars during exercise. Some birds double their body weight before migration by gorging on insects, berries, earthworms, or mollusks. Others increase the size of their flight muscles, charge up their metabolism, and even grow or shrink digestive organs to increase or lose weight.
For example, during a study of Gray Catbirds, University of Rhode Island physiologist and ecologist Kristen DeMoranville found catbirds’ flight muscles were larger during migration. And while the birds wintered in the tropics, their hearts were reduced in size; but they gained fat, perhaps in anticipation for the spring migration season.
Biologists like DeMoranville are working to understand what genes and proteins are responsible for such body-morphing abilities, using information from human medical research as a starting point. The results could signal which foods are crucial for birds’ migratory flights, which in turn could help conservationists identify and protect the most important sources for nutrition for various birds.
How do birds know when to migrate? Varied cues help birds decide when to start their migrations, explained University of South Carolina ornithologist Nathan Senner, including day length, temperature, rainfall, food availability, and body condition. Birds may also pay attention to social cues to decide when it’s time to leave wintering areas.
How birds are using this information, and how much they can change their behavior based on what they are sensing is less clear, added Senner. Researchers are also curious about how juvenile birds work out their migratory schedule when they’ve never made the journey before.
Studies of European Nightjars show how birds’ migratory timing can be finely tuned to its feeding requirements. Susanne Akesson, an evolutionary ecologist at Lund University in Sweden and her colleagues found that nocturnal nightjars hunt more actively on bright moonlit nights, and they are also more likely to start migrating after a full moon than after a new moon. After gorging on flying insects for “days,” they become adequately “fueled” and ready to begin migrating.
How do migrating birds find their way? Any bird, 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. Research shows that by artificially changing the magnetic field around a captured migratory bird can cause it to fly in the wrong direction, informed Nikita Chernetsov, an ornithologist at the Zoological Institute of Russian Academy of Sciences in St. Petersburg, Russia.
“The map has been much more controversial,” explained Richard Holland, a sensory biologist at Bangor University, but magnetic fields probably play a role. Each location 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 and utilize.
Research also suggests smell may be important, noted Holland: When scientists have suppressed smell in some species, experiments showed that birds can’t find their way as precisely. But scientists don’t know exactly which smells some birds rely on as a guide, whether vegetation or sea spray or even air pollution are olfactory factors. Identifying the biology underlying these navigational abilities – like where magnetic sensors are located in a bird’s body and what genes are coded for migratory direction – is a major focus of ongoing research.
In addition to these innate abilities, some bird navigational skills are partly learned. Adult White-crowned Sparrows captured in the western United States and released in the eastern part of the country can find their way back to their usual migratory route, Holland revealed. His data show that young sparrows he tracked show they will start migrating, but they don’t know they need to backtrack west to end up in the right place. In essence, although their internal migrating compass is inherited, they learn how to reach their wintering areas through experience.
How do birds sleep during long migratory flights? Some birds take breaks during migrations to rest and refuel. Others fly for days, crossing lakes, oceans, mountains, and continents without stopping. Biologists know it is possible for birds to sleep while flying, but they don’t know how much sleep different species are actually getting on the wing during migrations. Proof birds can catch in-flight Z’s comes from tracking Great Frigatebirds, large seabirds that are known to search for foods 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.
Just because birds can sleep while flying doesn’t mean they do sleep though. “We have good reason to think some birds might not sleep at all during flight,” says Niels Rattenborg, an avian sleep researcher at the Max Planck Institute for Ornithology. First, there’s no proof that 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 the flight sleep was done while flapping their wings.
What’s more, extreme sleep deprivation doesn’t seem to affect birds as drastically as it does humans. How much time birds need to sleep is relatively flexible, Niels explained. In fact, related research with migrating Pectoral Sandpipers found males that slept least during the nesting season performed best, in that they fathered more offspring.
For now, batteries that power transmitters that biologists use to track bird movements and brain activity in flight are just too big for most species aside from the big frigatebirds that wore transmitters on their backs and heads. Definitive answers about migratory sleep will require a technological breakthrough to miniaturize batteries and associated hardware, although that’s probably just a matter of time.
