Sexual Competition Makes Some Female Birds as Big and Flashy as Males

Despite what you learned in science class, it turns out female birds aren’t always drab in comparison to males. Sure, the peacock displays gorgeous plumage, while the peahen’s earthy tones are less likely to dazzle the human observer. But consider the superb starling of East Africa: males and females are both highly ornamented and have similar wing size.

The reason for the females’ finery is likely competition to be the egg layer; superb starlings live in large family groups, where several adults tend to the chicks of a single mother. So in a group where not all individuals breed, the flashiest female is likeliest to get lucky, Dustin Rubenstein of Columbia University and Irby Lovette of Cornell Laboratory of Ornithology report in Nature.

“If competition for reproductive opportunities is intense in both sexes in species that live in family groups,” says Rubenstein, an assistant professor of ecology, evolution and environmental biology, “it stands to reason that the traits that are typically only elaborate in males might also be elaborate in females in cooperatively breeding species.”

To find out, the researchers looked at the 45 species of African starlings. Some of the species employ the family group approach to tending nests and some raise their chicks solo. In addition, in some species males and females look the same, but in others, they look quite different.

Rubenstein and Lovette found that in more than 80 percent of cooperative breeders—the ones that live in family groups—females resembled males; the same was true for only about 30 percent of non-cooperative breeders.

“This goes beyond starlings,” says Rubenstein. “Any species that lives with relatives might be expected to show similar patterns. This type of complex social behavior is not only common in birds, but also many mammals—including humans—and insects.”

[Superb startling social groups can contain as many as 30 individuals, and the helper relationships among group members are extremely complex. Males and females are both highly ornamented. Photo: Dustin Rubenstein]