Migratory birds may have evolved to be lighter in color than their nonmigratory counterparts, as paler plumage absorbs less heat than darker feathers, according to a study published yesterday (December 6) in Current Biology

Color has been co-opted by birds to aid in everything from camouflage to mate selection, but it’s not only vivid hues that have an evolutionary benefit. Prior research has shown, for instance, that birds nesting in cold climates often lay eggs that are darker, and therefore retain heat better, and many species use drab coloring or black and white contrast to blend in.  

Whether color has any bearing on migration—a behavior roughly half of all bird species engage in to some extent—has rarely been studied. Two recent papers documented how great snipes (Gallinago media) and great reed warblers (Acrocephalus arundinaceus) fly at much higher altitudes during the day than at night, leading the researchers to suggest that it may be a way to keep cool. If thermoregulation is a strong selective force on migratory species, might they also evolve lighter plumage to avoid overheating?

To study whether plumage color has any relation to migration, researchers analyzed roughly 20,000 scientific illustrations of more than 10,000 species of bird—almost every species described by science. The team, led by Kaspar Delhey, an ornithologist at the Max Planck Institute for Ornithology in Germany, ranked how light each species’ plumage is and compared the ranking to the distance it flies on its migration. 

The researchers found that, on average, lightness increased with migratory distance, a trend that was largely consistent regardless of size, climate, or habitat. Birds with the longest migrations were roughly 4 percent lighter than nonmigrators. “It’s not a big difference,” Delhey tells Science News, adding that many species didn’t align with the trend and there could be other variables contributing to the pattern. Still, he says, the group was surprised by how “very different groups with very different biologies show this pattern.”

In the paper, the authors report that their findings align with that of another study, published in Science Advances in 2015, that identified a similar association between plumage lightness and migration. At the time, the University of Wisconsin-Milwaukee group suggested that such a trend might be explained if bright color conferred some benefit in territorial competitions for mates. But Delhey and his collaborators note that their findings better support existing observations of thermoregulation in other migrating species, account for sexual dimorphism between males and females, and include data on far more species (the 2015 study included only 977). As a follow-up, they suggest that future research could look at how plumage color interacts with other adaptive forces such as body and bill size to craft an “optimal migratory phenotype.”