Migratory birds undertake long and challenging journeys that have selected for a suite of adaptations. Several recent studies revealed that migrating individuals increase their flight altitude dramatically during the day compared to at night. Those studies suggested that the phenomenon is driven by thermoregulation: the ascent to cooler heights during the day may offset heat generated by absorption of sunlight. If thermoregulation is an important selective force on migratory species, migrants should have evolved lighter, more reflective plumage to avoid overheating. A new study, published in the journal Current Biology, shows that migratory bird species are indeed lighter colored.
“We found across nearly all species of birds, migratory species tend to be lighter colored than non-migratory species,” said Dr. Kaspar Delhey, a researcher in the Max Planck Institute for Ornithology.
“We think that lighter plumage coloration is selected in migratory species because it reduces the risk of overheating when exposed to sunshine.”
“Lighter surfaces absorb less heat than darker ones, as anybody wearing dark clothes on a sunny day can attest!”
“This would be particularly important for long-distance migrants that undertake extensive flights during which they cannot stop to rest in the shade.”
Dr. Delhey and colleagues had been studying the effects of climate on bird coloration.
Their earlier studies showed that, in general, lighter colored birds are found where temperatures are high and there is little shade. Presumably that’s at least in part because the birds’ lighter plumage helps to keep them cooler in the hot Sun.
Around that same time, the researchers came across studies by others showing that some birds fly at much higher altitudes during the day compared to at night.
“Because flying at high altitude is likely costly, these changes required an explanation,” Dr. Delhey said.
“One possibility was that flying higher, where it is colder, would offset the heat absorbed by the plumage when the Sun was shining.”
To find out, the scientists quantified overall plumage lightness (from 0 = black to 100 = white) for all bird species, using bird images from the Handbook of the Birds of the World.
They then compared the data on coloration with the species’ migratory behavior, while controlling for other factors known to effect plumage color.
Overall, the findings show that bird species get increasingly lighter as they migrate more: resident birds tend to be darker than short-distance migrants; short-distance migrants are darker than bird species that travel farther.
“One of the biggest surprises was how consistent the effect was across different types of birds. We saw the same pattern in birds large and small. The same held true in waterbirds and land-dwelling birds, too,” Dr. Delhey said.
Kaspar Delhey et al. Migratory birds are lighter coloured. Current Biology 31 (23): PR1511-R1512; doi: 10.1016/j.cub.2021.10.048
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