From teeny hummingbirds to giant whooping cranes, roughly half of the world’s more than 10,000 bird species migrate. Longer wings and beefed-up flying muscles often help these birds crisscross vast expanses of air. But a study of nearly all bird species suggests many migrators share another unexpected flight aid: lighter-colored feathers.
Being a tad more lightly colored than non-migrating birds may help these long-distance fliers stay cool as they work hard under the hot sun to fly, researchers report December 6 in Current Biology.
It’s known that color can help birds hide from predators by blending in, or attract mates by standing out. But color has subtler effects too, including regulating temperature by absorbing or reflecting light, says Kaspar Delhey, an ornithologist at the Max Planck Institute for Ornithology in Seewiesen, Germany. For example, bird eggs laid in colder climates tend to be darker, which may help keep them warm (SN: 10/28/19).
Migrating birds push their bodies to the physiological limit, which creates excess heat. Some species cope by ascending to cooler air during daytime. “If overheating is a problem in migratory birds, another way of dealing with that would be to evolve lighter colors” that absorb less heat, Delhey says.
Delhey and his colleagues analyzed over 20,000 illustrations of 10,618 bird species, ranking plumage lightness for each species and comparing that with how far the birds fly. On average, lightness slightly increased with migratory distance, the team found. The longest-distance migrators were about 4 percent lighter than non-migrators, an effect that wasn’t explained by size, climate or habitat type for different species.
“It’s not a big difference,” Delhey says, noting that many migrators are darkly colored, perhaps for reasons unrelated to flight. But the trend was remarkably consistent.
“Very different groups with very different biologies show this pattern,” Delhey says. “That surprised us.”