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Migratory birds find their way home thanks to lighter-colored feathers

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Macro view of lying violet feather with other color feather on background

By Mark Waghorn via SWNS

Migratory birds find their way home thanks to lighter-colored feathers, according to new research.

Pale plumes help them keep cool during round trips of up to 10,000 miles.

The marathon treks are among the most spectacular sights in nature. Flocks can number in their millions - resembling a tornado.

Their remarkable powers of endurance have baffled scientists for decades.

via GIPHY

"We found across nearly all species of birds, migratory species tend to be lighter colored than non-migratory species,"said lead author Dr. Kaspar Delhey, of the Max Planck Institute for Ornithology, Seewiesen, Germany.

"We think lighter plumage coloration is selected in migratory species because it reduces the risk of overheating when exposed to sunshine."

One of the reasons the Arctic remains colder than the equator is because more heat from the sun is reflected off the ice - back into space.

"Lighter surfaces absorb less heat than darker ones, as anybody wearing dark clothes on a sunny day can attest," Delhey said.

"This would be particularly important for long-distance migrants that undertake extensive flights during which they cannot stop to rest in the shade."

For instance, nightingales and willow warblers which spend winters in Africa and summers in Europe are light brown and light green, respectively.

Sanderlings, long-distance migratory shorebirds, breed in the Arctic and visit the UK in the winter - benefiting from their grey and white hue.

White-rumped sandpipers make one of the longest migrations of any North American bird.

They sometimes fly from Arctic Canada to winter in southern South America - sometimes covering 2,500 miles without a rest.

The most famous long-distance migrants, such as swallows, breed in Europe and spend the winter in Africa.

At least 4,000 species are regular migrants - about 40 percent of the world's total.

In far northern regions, such as Canada or Scandinavia, most birds move south to escape winter.

In temperate areas like the UK about half the species migrate including cuckoos, swifts and other insect-eaters that can't find enough food.

On the other hand, blackbirds in your garden in January are often winter visitors from Eastern Europe.

Delhey's team had earlier been studying the effects of climate on bird coloration.

They found in general lighter-colored varieties occur where temperatures are high and there's little shade.

The birds' lighter plumage helps to keep them cooler in the hot sun. Other analyses also showed some birds fly at much higher altitudes during the day than at night.

"Because flying at high altitude is likely costly, these changes required an explanation," Delhey said.

"One possibility was flying higher - where it's colder - would offset the heat absorbed by the plumage when the sun was shining."

Another way to reduce the risk of overheating would be to absorb less solar radiation in the first place.

It suggested migratory species evolved lighter feathers, explained Delhey.

The researchers quantified overall plumage lightness for all species from zero to 100 - or black to white - using images from the Handbook of the Birds of the World.

They compared the coloration data with migratory behavior - taking into account while other factors known to affect plumage.

Overall, species got increasingly lighter as they migrated more. So resident birds tend to be darker than short-distance migrants.

And the latter are darker than birds that travel farther. One of the biggest surprises was how consistent the effect was across different types of birds, said Delhey.

The same pattern emerged in birds large and small - holding true in waterbirds and land-dwelling birds, too.

The discovery is another reminder of the important role of temperature and climate factors more broadly in shaping the evolution of animal coloration.

They also shed fresh light on the impacts of global warming and potential adaptive evolutionary responses, said the researchers.

Many factors influence bird colors and light shades are one of many ways migrants avoid overheating, said Delhey.

His team will continue exploring the connections between migration, climate and other selective factors that shape the evolution of plumage in birds.

Future studies should directly test how migratory species cope with thermoregulatory challenges, they said.

In tropical regions, such as the Amazon rainforest, fewer species migrate, since the weather and food supply there are more reliable all year round.

The study is in Current Biology.

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