Color Me Sexy

Forget about personality. Females judge males by their looks.

In the market for “love?” Better take a clue from the North American barn swallow. How you look can make all the difference.

New research shows that the female North American barn swallow, even after pairing with a male, still comparison shops for sexual partners. She is especially interested in the chestnut color of the males’ breast and belly feathers, according to a team of researchers including one from Arizona State University.

The scientists found that if the male barn swallow’s brown breast is not as dark as other males in the population, the female is more likely to leave him and copulate with another male. The finding has evolutionary implications, says Kevin McGraw, an assistant professor in the School of Life Sciences at ASU. It shows how ornamental traits like the deep color of a male barn swallow (or the antlers of a deer); while costly to males, give them an edge over their rival suitors.

McGraw was part of a research team that included scientists at Cornell University in Ithaca, N.Y. “We found that female barn swallows were more likely to be faithful to their partners when we experimentally altered their mates’ appearance to make his feathers more colorful,” McGraw explains. “This is the first time a study has ever demonstrated that female birds can make rapid decisions, based on changing qualities of their mate, about whether or not they should cheat on him,” he adds.

“The bad news for male swallows is that the mating game is never over,” says Rebecca Safran, who led the research team as a post-doctoral student at Cornell. “It is dynamic and continual. This is something that most humans can relate to. Just think of how much time and money we spend on our looks and status long after we have established stable relationships.”

In addition to McGraw and Safran (now at Princeton University), other team members included Irby Lovette and Colby Neuman, both of the Cornell Laboratory of Ornithology.

“Even after he’s secured a mate, our results show that a male barn swallow still must maintain an attractive appearance. That’s akin to staying clean shaven, or keeping his ‘suit’ clean to please his partner and continue to mate with her,” McGraw explains.

Like many songbirds, cheating is common among barn swallows (Hirundo rustica erythrogaster). About half of all male barn swallows typically care for at least one young chick that was actually fathered by another bird. The ASU-Cornell researchers used the birds’ cheating ways to test factors that might keep a female faithful to her mate.

Male barn swallows have a wash of reddish-brown color from their throats to their bellies. This color varies among birds from pale to deep chestnut. Previous studies by Safran demonstrated that male color may indicate a male’s quality. Color can be an indicator of health, status, or the ability to raise young. As a result, this color signal may be involved in mate selection decisions.

To assess mate selection decisions, the researchers removed the first set of eggs laid by 30 pairs of barn swallows. The females were forced to mate again. However, before females chose their mates for a second nest, the researchers captured the males and randomly assigned them one of three treatments.

They painted the throat, breast or belly feathers of some males with a brown marker. The idea was to enhance their feathers to match the darkest (most attractive) males in the population. A second group of males were left alone. Members of the third group of birds were painted with a clear marker to ensure the coloring process did not bias results.

The pairs were then allowed to breed again. The scientists then conducted comparative DNA tests on the offspring from the first and second breeding bouts.

All 30 females remained socially paired with their original mate, but they were sexually active with other males. Males with enhanced color fathered a substantially larger percentage of offspring in their second nests compared to their first nests. Males whose color was unchanged fathered the same number or fewer chicks than they had in their first nests.

“Our goal now is to understand how certain males keep better plumage than others,” says McGraw. The ASU scientist studies the molecules that color a bird’s plumage and the evolutionary importance of color for birds.

Lots of factors can have an effect on the color of a bird’s feathers.

“Ultraviolet radiation from the sun is one factor,” McGraw adds. “Soiling is another. Even feather-degrading bacteria are known to affect the color of bird feathers once they are grown. Perhaps the best males are those who spend more time preening and protecting their plumage.”—Skip Derra