Migratory birds are specially adapted to tackle the remarkable tests of endurance involved in undertaking their long and arduous journeys. And now we know that one of these adaptations might resemble one of our own during the summer months – keeping it cool in lighter colours.
New research has found an unexpected way that migratory birds avoid roasting in the sun: lighter-coloured feathers.
“We found across nearly all species of birds, migratory species tend to be lighter-coloured than non-migratory species,” says Dr Kaspar Delhey, of the Max Planck Institute for Ornithology, Germany.
“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.”
Delhey and his team have studied the effects of climate on bird colouration before; their earlier research showed that in general lighter-coloured birds are found in places with high temperatures and little shade. More recently, they came across new research which showed that some migratory birds fly at higher altitudes during the day compared to night, raising the question as to why the birds would make the trek to climb to such high altitudes.
“Because flying at high altitude is likely costly, these changes required an explanation,” Delhey says. “One possibility was that flying higher, where it is colder, would offset the heat absorbed by the plumage when the sun was shining.”
They realised that another way to reduce the risk of overheating would be to have evolved lighter (more reflective) plumage to absorb less solar radiation in the first place. To test this relationship between plumage lightness and migration they used bird images from the Handbook of the Birds of the World to quantify overall plumage lightness (from 0 = black to 100 = white) for all bird species.
By comparing this data to the species’ migratory behaviour and controlling for other factors known to affect plumage colour – specifically climate, habitat structure, and body size – they found that bird species get increasingly lighter as they migrate farther. Resident birds tended to be darker than short-distance migrants, which in turn were darker than species which travel long distances.
Amazingly this effect remained consistent across different types of birds, with the same pattern holding true for land-dwelling and waterbirds, large and small.
These findings come as another reminder of the significant role of temperature – and climate factors more broadly – in influencing the evolution of animal colouration. With the escalation of global warming, this research also has implications for understanding its impacts and the potential adaptive evolutionary responses they provoke.