Migratory birds may have evolved lighter plumage than nonmigratory counterparts because lighter plumage absorbs less heat than darker feathers. Birds have used color to aid in everything from camouflage to mate selection, but it’s not just bright colors that have an evolutionary advantage. Prior research has shown, for example, that birds nesting in cold climates frequently lays darker eggs, which retain heat better, and that many species use drab coloring or black and white contrast to blend in.
Migratory birds are specially adapted to navigate extreme distances that represent remarkable endurance tests. Now, researchers reporting in the journal Current Biology on December 6th have discovered an unexpected way for migratory birds to stay cool during such long journeys: lighter-colored feathers.
“We discovered that migratory species tend to be lighter colored than non-migratory species across nearly all bird species,” said Kaspar Delhey of the Max Planck Institute for Ornithology in Seewiesen, Germany. “We believe that lighter plumage coloration is chosen in migratory species to reduce the risk of overheating when exposed to sunlight. As anyone who has worn dark clothing on a sunny day can attest, lighter surfaces absorb less heat than darker ones. This is especially important for long-distance migrants who must fly for long periods of time without being able to stop and rest in the shade.”
We discovered that migratory species tend to be lighter colored than non-migratory species across nearly all bird species. We believe that lighter plumage coloration is chosen in migratory species to reduce the risk of overheating when exposed to sunlight. This is especially important for long-distance migrants who must fly for long periods of time without being able to stop and rest in the shade.
Kaspar Delhey
Researchers examined nearly 20,000 scientific illustrations of more than 10,000 bird species—nearly every species described by science—to see if plumage color has any relationship to migration. The team, led by Kaspar Delhey, an ornithologist at Germany’s Max Planck Institute for Ornithology, ranked the lightness of each species’ plumage and compared it to the distance it flies on its migration.
Delhey and colleagues were researching the effects of climate on bird coloration. Previous research had shown that lighter-colored birds tend to congregate in areas where temperatures are high and there is little shade. Presumably, this is due, at least in part, to the birds’ lighter plumage, which helps to keep them cool in the hot sun. Around the same time, the researchers came across research showing that some birds fly at much higher altitudes during the day than at night.
“Because flying at high altitude is likely expensive,” Delhey explains, “these changes required an explanation. One theory was that flying higher, where it was colder, would offset the heat absorbed by the plumage when the sun shone.”
If this is the case, they realized that another way to reduce the risk of overheating is to absorb less solar radiation in the first place. It sparked the thought, “Have migratory species evolved lighter feathers?” To find out, they used bird images from the Handbook of the Birds of the World to quantify overall plumage lightness (from 0 = black to 100 = white) for all bird species. The researchers then compared the coloration data to the species’ migratory behavior, while controlling for other factors known to influence plumage color.
Overall, the findings show that as bird species migrate more, they become lighter. As a result, resident birds are darker than short-distance migrants. Short-distance migrants are darker than long-distance migrants. One of the most surprising findings, according to Delhey, was how consistent the effect was across different types of birds. They noticed the same pattern in both large and small birds. The same was true for both waterbirds and landbirds.
The findings serve as yet another reminder of the importance of temperature and other climate factors in shaping the evolution of animal coloration. According to the researchers, they also have clear implications for understanding the effects of global warming and potential adaptive evolutionary responses.
Delhey points out that many factors influence bird coloration, and light colors are just one of many ways migratory birds avoid overheating. His team will continue to investigate the links between migration, climate, and other selective factors that influence the evolution of bird plumage colors. In light of the new findings, they also propose that future research should directly test how migratory species cope with thermoregulatory challenges.
The researchers discovered that lightness increased with migratory distance on average, a trend that held true regardless of size, climate, or habitat. Birds with the longest migrations weighed about 4% less than nonmigratory. “It’s not a big difference,” Delhey tells Science News, adding that many species did not follow the trend and that other factors could be at work. Nonetheless, he claims that the group was taken aback by how “very different groups with very different biologies show this pattern.”
