From tiny hummingbirds to massive whooping cranes, roughly half of the world's more than 10,000 bird species migrate. Longer wings and stronger flying muscles often aid these birds in crisscrossing vast swaths of air. However, a study of nearly all bird species found that many migrators share an unexpected flight aid: lighter-colored feathers.
Researchers report on December 6 that being a tad lighter in color than non-migrating birds may help these long-distance fliers stay calm as they work hard under the hot sun to fly.
Color is known to help birds hide from predators by blending in or attracting mates by standing out. On the other hand, color has subtler effects, such as regulating temperature by absorbing or reflecting light, according to Kaspar Delhey, an ornithologist at the Max Planck Institute for Ornithology in Seewiesen, Germany. Bird eggs laid in colder climates, for example, tend to be darker, which may help keep them warm (SN: 10/28/19).
Migrating birds push their bodies to their physiological limits, resulting in excessive heat. During the day, some species cope by ascending to cooler air. "If overheating is a problem in migratory birds, another way to deal with it would be to evolve lighter colors," Delhey says.
Delhey and his colleagues examined over 20,000 illustrations of 10,618 bird species, ranking plumage lightness and comparing it to how far the birds fly.
The team discovered that lightness increased slightly with migratory distance on average. Long-distance migrators were about 4% lighter than non-migratory, an effect that could not be explained by differences in size, climate, or habitat type for different species.
"There isn't much of a difference," Delhey says, noting that many migrators are darkly colored, possibly for reasons unrelated to flight. However, the pattern was remarkably consistent.
"This pattern is seen in very different groups with very different biologies," Delhey says. "That took us by surprise."
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