Study Reveals Bird-Migration Mystery

By outfitting blackbirds with heart-rate monitors, scientists debunked a long-held assumption about the benefits of spending the winter in warm climates.

For migrating birds, fall brings difficulty and danger. To reach warm winter climes, many birds must fly hundreds or thousands of miles, expend immense amounts of energy and successfully dodge storms, skyscrapers and other potential threats.

Still, scientists have long assumed that a basic trade-off made migration worth the gamble: Once birds arrived at their wintering grounds, they wouldn’t need to work so hard to stay warm, saving substantial amounts of energy. “But nobody ever tested this,” said Nils Linek, a behavioral ecologist at the Max Planck Institute of Animal Behavior in Germany.

Now, Dr. Linek and his colleagues have done so. Their findings, based on a partially migratory population of German blackbirds, challenge the conventional wisdom. Even in the depths of winter, blackbirds basking in balmy southern Europe or northern Africa did not spend any less energy than those riding out the cold in Germany, the scientists found.

“It’s sort of shocking that there isn’t this net benefit,” said Scott Yanco, an animal ecologist at the University of Michigan and an author of the paper. (Dr. Yanco conducted the research while at Yale University.)

The study, which was published in Nature Ecology & Evolution on Wednesday, also revealed that the migrants began preparing for their fall journeys several weeks in advance, saving up energy for the flight by slowing their metabolism at night.

Together, the results suggest that migration is “way more complex than all the theories predicted,” Dr. Linek said. “There were a lot of surprises.”