Science. Migratory birds have lighter feathers

Madrid, 6 (Europe Press)

Migratory birds are specially adapted to find their way over great distances which are great tests of endurance.

Now, researchers have discovered an unexpected way for them to stay calm during those arduous journeys, and that is their bright-colored plumage, as published in the journal Current Biology.

“In nearly all bird species, migratory species tend to be lighter than non-migratory species,” explains Caspar Delhi, of the Max Planck Institute for Ornithology, in Seewiesen (Germany) in a statement. Feathers are chosen in migratory species because it reduces the risk of overheating when exposed to sunlight.”

She adds, “Lighter surfaces absorb less heat than darker surfaces, which anyone in dark clothing can attest to on a sunny day. This will be especially important for long-distance migrants on long journeys during which they are unable to stop in the far shade,” he explains.

Delhi and colleagues studied the effects of weather on bird colors. Their previous studies showed that light-colored birds are generally found where temperatures are high and there is little shade.

They hypothesize that this is due, at least in part, to the birds’ lighter plumage which helps them stay cooler in the sun. At about the same time, researchers came across studies by other people that showed that some birds fly much higher during the day than at night.

“Because flying high may be costly, these changes required an explanation. One possibility was that flying higher, where it is cooler, compensates for the heat absorbed by the feathers when the sun is shining,” Delhi says.

If so, they realized that another way to reduce the risk of overheating was to absorb less solar radiation in the first place. This raised the question of whether migratory species with lighter feathers evolved.

To find out, they quantified the total plumage brightness (0 = black to 100 = white) for all bird species, using pictures of birds from the Birds of the World Handbook. They then compared the coloration data to the species’ migratory behavior, while controlling for other factors known to influence plumage colour.

Overall, the results show that bird species became more visible with increased migration. Thus, resident birds tend to be darker than short-distance migratory birds. Migrants for short distances are darker than bird species that travel longer distances.

Delhey acknowledges that one of the biggest surprises has been the consistency of the effect in different bird species. They observed the same pattern in large and small birds, and the same thing happened with water and land birds.

The results once again confirm the important role that temperature and climatic factors play in the evolution of animal coloration, they assert. The researchers say 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 that light colors are just one of many ways that migratory birds can prevent overheating.

His team will continue to explore the links between migration, climate and other selective factors that shape the evolution of bird plumage colours. They also suggest, in light of the new findings, that future studies should directly investigate how migratory species cope with thermoregulatory challenges.