In many bird species, the males are far brighter and flashier than females, and we typically think that’s because males compete while females choose. Now, a team studying this so-called sexual dichromatism of songbirds have discovered a link between female coloration and migration. The findings, published in Proceedings of the Royal Society B this week, suggest that sexual selection isn’t the strongest driver of color differences between the sexes.
Previous studies revealed a geographical pattern among a colorful group of songbirds called wood-warblers in North America: At northern latitudes, males tend to be the far more colorful ones, while the plumage of the two sexes are similarly bright at lower latitudes in the tropics. Of the 108 or so species of wood-warblers, 48 of them breed in the north and head south for the winter. Because molting occurs on their wintering grounds, all migratory wood-warblers fly northwards in their (sexier) breeding plumage.
There are multiple hypotheses about the evolution of sexual dichromatism among wood-warblers. Two of the more female-centric hypotheses focus on the loss of female coloration with the evolution of migration. According to one of them, drab colors help protect the female from visual predators during migration; the other suggests a relaxation of selection for female social signaling (with color and ornaments) at higher latitudes.
To test these hypotheses, a Trinity University trio led by Troy Murphy compared sexual dichromatism to three variables: the presence of migration, migration distance, and breeding latitude. They also studied wood-warbler ancestry.
Migratory species, they found, showed a higher degree of dichromatism than species who stay put; most non-migratory warblers displayed little contrast between the males and females. Additionally, migration distance was a good predictor of dichromatism: Long-distance migrants tended to have more distinct males and females. For instance, female bay-breasted warblers—who migrate about 7,000 kilometers (4,350 miles) between their northern breeding grounds and their Caribbean wintering grounds—are gray and white, while the males boast yellows and browns, Science reports. But slate-throated redstarts of both sexes flaunt bright colors around their year-round residences in Mexico and Central America.
The findings support the hypotheses that the advent of migration influenced the costs and benefits associated with female ornamentation. In migratory warblers, two mechanisms—relaxed social selection and predatory costs along longer migratory routes—simultaneously accounted for the loss of colorful female plumage.
If the loss of female ornamentation is a driver of sexual dichromatism, then social or natural selection may be a stronger contributor to variation in dichromatism than sexual selection, they write. That also means that dichromatism evolved repeatedly due to the loss of female coloration over evolutionary time. As for males, the benefits they gain with bright colors likely still outweigh the threat of being conspicuous to predators.