Researchers have found an interesting principle in birds: the further they fly during migration, the lighter their plumage is on average. It stands to reason that this principle has to do with protection against overheating: The reflective plumage absorbs less light energy when flying in the blazing sun. The rule characterizes the most diverse groups of birds, show the statistical evaluations. The researchers explain that some species are exceptions.
Black, gray, motley or completely white – as is well known, there is a wide range of plumage colors and patterns in the bird world. The appearance of the animals can have to do with different aspects: Some colors and patterns serve as camouflage from predators, while others are more dedicated to intra-species communication. With some bird species, the males even sacrifice the protective function of a simple plumage in favor of an impressive blaze of colors for finding a partner. Basically: The aspect that was most important for survival and reproductive success in the development of a bird species was the color of the plumage.
Light-reflecting feathered?
At the beginning of the study by the researchers led by Kaspar Delhey from the Max Planck Institute for Ornithology in Seewiesen, there was initially the assumption that a thermoregulatory effect also played a role in the development of bird colors. In previous studies, they had already found that lighter colored bird species tend to be found in habitats with high temperatures and little shade. The background to the connection was obvious: The rather light plumage helps to keep the birds cooler in the sunlight. Everyone knows the principle who has already experienced the cooling effect of light-colored clothing in the sunshine.
As the scientists report, the migratory behavior of the research topic moved into their focus through further indications: Studies have shown that some bird species fly much higher during long-haul flights during the day than at night. “Since flying at high altitude is likely to be comparatively expensive, there has to be an explanation for it,” says Delhey. One possible cause would be that the birds seek out the higher layers of the air because it is cooler there. In doing so, they could get rid of the heat that the plumage absorbs when flying in the blazing sun. If this is actually the reason, it also seems plausible that some species prevent them from absorbing too much sunlight in the first place – through their plumage color, the researchers explain.
The further, the brighter
In order to find out to what extent migratory species have actually developed lighter feathers, the scientists quantified the overall brightness of the plumage for all bird species using images from the “Handbook of the Birds of the World” – from 0 = black to 100 = white. They then compared the data on the coloration with the migration behavior of the species, taking into account other factors that could influence the plumage color.
The evaluations and statistical analyzes then showed: “We have found in almost all bird species that migratory birds tend to be lighter in color than resident birds,” says Delhey. There was also a gradual effect: the further birds fly, the lighter their plumage is on average. Resident birds tend to be darker than short-range migrants and these animals are less bright than species that migrate further, according to the data analyzes. The pattern is apparently also widespread: the researchers found the rule in large and small birds, as well as in waterfowl and land-dwelling birds.
Rule with exceptions
“It was a great surprise to see how uniform the effect is in the various bird groups,” says senior author Bart Kempenaers from the Max Planck Institute for Ornithology in Seewiesen. The results therefore suggest that many bird species reduce the risk of overheating during their migrations by using a light plumage color. “This is obviously particularly important for long-distance migrants who cannot rest in the shade on their long flights of more than 2000 kilometers,” says Kempenaers.
In some species, however, the plumage color does not follow the rule, emphasize the researchers. But as they explain, this is not a contradiction in terms: Delhey points out that many factors influence the coloring of birds, and light colors are just one of many ways that migratory birds can prevent overheating. In addition to flying higher, they can only be on the move at night, and other methods seem possible to give off excess heat. That means: In species with alternative strategies, the plumage could easily be darker and thus take on a signaling or camouflage function.
Delhey and his colleagues want to stay true to the research topic: They plan to investigate the relationships between migration, climate and other selective factors that influence the evolution of plumage colors in birds more closely.
Source: Cell Press, Max Planck Institute for Ornithology in Seewiesen, specialist article: Current Biolog, doi: 10.1016 / j.cub.2021.10.048