Migratory birds often travel incredibly long distances to find warmer weather, more resources, and nesting locations. One unusual way they’ve adapted to make these long treks easier is through lighter-colored feathers, a recent study finds.
Research shows that in nearly all bird species, migratory birds tend to be lighter colored than non-migratory species.
Kaspar Delhey of the Max Planck Institute for Ornithology, Seewiesen, Germany, says he and his colleagues have been studying the evolution of bird colors for several years and recently became interested in the effects of climate change on color.
“One of the outcomes of these studies was finding that birds tend to be lighter coloured in warm regions of the world with little shade (like deserts). We have speculated that birds are lighter in these environmental conditions because light plumage colours reflect more solar radiation, absorb less heat, and hence keep birds cooler in the sun,” Delhey tells Treehugger.
Earlier this year, the researchers read two studies that found two long-distance migratory birds—the great reed warbler and the great snipe—greatly increased their altitude between night and day on those trips. The authors suggested that perhaps the birds were flying higher where the air is colder during the daytime to lower the risk of overheating.
“When we read this, we wondered whether there was a connection between these patterns and our results linking temperature and plumage colours: if migratory birds are selected to keep cool in the sunshine we would expect them to also be lighter coloured,” Delhey says. “This led us to test whether indeed migratory birds would be lighter coloured across all species of birds.”
The results were published in the journal Current Biology
Calculating Lightness and Migration
For their study, researchers measured the lightness of feather colors for every bird species with a scale from 0 (black) to 100 (white). They used images from the “Handbook of the Birds of the World” to assign the numbers. Then they compared that lightness data with each species’ migratory behavior, while controlling for factors such as climate, habitat structure, and body size that could also affect plumage color.
They found that migratory birds tended to be lighter colored than birds that don’t migrate.
“We suspect that lighter plumage contributes to keep migratory birds cooler by absorbing less solar radiation when these birds are exposed to constant sunshine during their long, and often non-stop, flights,” Delhey says.
“It should be kept in mind as well that while this effect was found across very different groups of birds, it does not apply to every single species, as there are also many dark migratory species. Thus, evolving lighter plumage colours is but one of possible ways to avoid overheating while migrating.”
Other adaptations include flying higher, migrating only at night when sunshine is not an issue, or evolving in other ways that would shed extra heat. For example, some birds become smaller.
Researchers also found that feather colors often got lighter the more a bird migrated. Plumage became progressively lighter from non-migratory species to short-distance migratory species (those that migrate less than 2,000 kilometers on average) to long-distance migratory birds (those that travel more than 2,000 kilometers).
“Our results reinforce the importance of climatic factors, not only on the evolution of bird colours, but also on other aspects of bird biology such as their migratory strategies. If, as our results and that of other studies suggest, thermoregulation is an important concern for migratory birds, this would have clear implications in the context of ongoing global warming,” Delhey says.
“The big question then becomes if future increases in temperature will disrupt the ability of birds to migrate long-distances non-stop without overheating.”
