A study has found that the further bird species migrate, the paler their feathers tend to be.
The research looked at nearly every species – nearly 10,000 – in the world. And the theory behind paler plumage in migrants is probably because it helps birds flying in sunlight to keep cool, says Kaspar Delhey at the Max Planck Institute for Ornithology in Seewiesen, Germany.
"The difference is small, but this is common when it comes to explaining variation in colouration across an entire class of vertebrates," says Delhey. "Plumage colouration is extremely diverse and many factors can drive the evolution of colours, of which migration is but one."
Pale plumage may help species such as Snow Bunting migrate (Lukasz Pulawski).
Several recent studies suggest that overheating is more of a problem for birds flying long distances than previously thought. Another study found that some migrating birds fly much higher during the day than they do at night despite having to expend more energy to stay aloft in thinner air. This suggests that the heat of the sun makes them much more likely to overheat, and made Delhey and his colleagues wonder whether migrating birds have lighter plumage to help them cope.
The researchers used photos from the Handbook of the Birds of the World to rate the lightness of birds' plumage from 0 to 100, treating males and females as separate species because coloration is so often different between the sexes. They found that short-distance migrants were significantly lighter than resident birds, and long-distance migrants were significantly lighter than short-distance migrants, even without controlling for factors such as body size or the environmental conditions where birds live
One implication of these findings is that migrating birds are going to find it even harder to stay cool as the planet heats up. Rising temperatures may force darker species to fly only during the night while migrating, says Delhey, or to fly even higher during the day.
"These responses will probably entail costs, which may reduce individual survival, with potential negative consequences at the population level," he says.
Reference
Delhey K, Dale J, Valcu M & Kempanaers N. 2021. Migratory birds are lighter coloured. Current Biology. DOI: 10.1016/j.cub.2021.10.048
