Male ducks may look for clues to a female’s health in the white of her wings

In the bird world, the ladies traditionally size up and select their mates. But a recent study out of Norway (Animal Behavior, February 3, 2006) suggests that sometimes the guys get to be choosy too. Ornamental white bands on the wings of cryptic brown female Common Eiders could indicate breeding and nurturing quality.

Female Common Eider

Charles Darwin first defined “sexual selection” in The Origin of Species as the “struggle between individuals of one sex, usually males, for the possession of the other sex.” He elaborated on his theory in The Descent of Man, and Selection in Relation to Sex, outlining two types of selection—male-male contests, in which males compete for the right to mate with the opposite sex, and mate choice, in which one sex, usually females, select mates based on an appealing characteristic or behavior.

Male Common Eider

Both examples exist among birds. Male Ring-necked Pheasants possess spurs on their legs for fighting. And the male peacock with the longest tail and brightest plumage usually gets the girl. Though beauty may not be on the female’s mind, coloration and ornamentation can demonstrate how much extra energy a male can afford to spend on his appearance.

Since Darwin’s day, sexual selection has played an important role in evolutionary studies. Research, however, has focused mostly on male ornaments. Though females sometimes display similar decorations, they are often assumed to be a side effect—similar to the males’ markings, but paler and less showy. But the new study suggests that white bands in female eiders could indicate health. After all, mate choice can work both ways.

Common Eiders, the largest ducks in the Northern Hemisphere, live in arctic and subarctic coastal marine habitats. They breed in colonies and stick with one mate during each mating season. But the female birds take a hit during breeding, often starving—losing 40 percent of their body mass and facing a weakened immune system. Those that cannot cope often abandon their young. Since these females are entirely responsible for incubating eggs and caring for the little ones, males want to find the best mom.

Lucky for them, they can get a clue before they commit. After each breeding season, females produce white wing bands. The Norwegian researchers, Sveinn Are Hanssen, Ivar Folstad and Kjell Einar Erikstad, found birds with whiter bands lose less mass and maintain immune levels during breeding. Therefore, they are better able to care for the young. So while the females look for the bright pinks, greens, and blacks in their mate, the males may look only for a defined white badge.

“My new view is that I look at sexual selection from a female perspective, where the female takes a more active part,” Hanssen said.

For those who cannot immediately accept the challenge to traditional views on sexual selection, the authors suggest the ornament may also serve a function in female-female interactions, as status signals. Sometimes, when one female abandons a brood of young, another takes  over. Status could play a role. Also, sometimes the eiders care for their ducklings cooperatively in groups. The white could help females locate good potential brooding buddies. Either way, the study suggests past science has paid less-than-enough attention to ornamentation found only in female species.