Shades of Fidelity

Barn Swallow DNA study finds that when it comes to faithfulness, redder is better

By Rebecca J. Safran

Barn Swallow by Marie Read

Birds are among the most elaborately decorated creatures on the planet. They possess incredibly colorful feathers ranging from the dazzling trains of male peacocks to the electrifying breasts of sunbirds, and all sorts of feathered ornaments and elongated streamers that are characteristic of many birds of paradise and swallows. Beyond being beautiful to look at, these traits provide critical information about a male's ability to compete for and attract a mate.

For example, classic research in Europe has shown that female Barn Swallows apparently prefer to mate with males that have the longest tail streamers--the outermost, elongated tail feathers. These males are healthier and more successful at raising young than males with shorter streamers.

Here in Ithaca, New York, however, female Barn Swallows are partial to the reddish color on their mates' breast and belly. In a recent study that I published with collaborators Colby Neuman, Kevin McGraw, and Irby Lovette in Science (September 30, 2005), we found that if we improved a male's appearance using colored markers, he earned greater points with his mate--and more offspring as a result. The study shows that even after Barn Swallows have paired up for the season, females still assess their mates and adjust their mating decisions if their mates undergo changes in appearance.

Rebecca Safran removes a Barn Swallow from a mist net at a study site in Tompkins County, New York.

Photo by Kevin Stearns

In 1998, when I initiated my study of Barn Swallows in Tompkins County, New York, I assumed that females were judging males based on their tail streamers, as do Barn Swallows in Europe. However, when I analyzed the data from that first field season, I was puzzled. Tail streamer length did not appear to be an indicator of a male's breeding success in my study area, a result I corroborated five years and hundreds of measurements later.

In the course of my research, I also noticed that male Barn Swallows in my study area were a lot more colorful than those in Europe. A trip to the American Museum of Natural History's specimen collection confirmed that male Barn Swallows in North America were darker and had shorter tail streamers. Were Barn Swallows in North America using color to assess one another? I had to find out.

Three Barn Swallows showing plumage variation. The bird at right has the most extensive reddish coloration.

Photo by Marie Read

In collaboration with Kevin McGraw, a former graduate student in the Department of Neurobiology and Behavior at Cornell and now an assistant professor at Arizona State University, we altered the appearance of a group of males using nontoxic permanent ink markers. We painstakingly chose a color to match the darkest male in our population and an ink that didn't mat down or otherwise alter the feather structure. To make sure that painting a male per se did not affect the results of our experiment, we painted some males with a clear marker pen. We left a third group of males unpainted for comparison.

Next, we wanted to study how females responded to the changes in their mate's plumage. Although Barn Swallows choose a single partner with whom to raise their young, they sometimes sneak matings with their neighbors. If a male became less desirable, would his mate be more inclined to cheat?

To find out, Cornell undergraduate Colby Neuman, Evolutionary Biology Program director Irby Lovette, and I determined the paternity in each brood. Nest predation is common during first breeding attempts, after which the pair lays another clutch of eggs. We simulated nest loss by removing the first clutch of eggs and determined how many offspring a given male sired by conducting DNA tests on the eggs in the Lab's Evolutionary Biology facility.

On the same day that we removed the eggs, we recaptured all of the males in the experiment and gave their feathers one of the three treatments. Despite the changes to their feathers, the males renested with the same partners as before. However, males with darkened feathers sired a greater proportion of young in their second brood. In contrast, the males in the other groups, whose feathers did not change in appearance, sired the same number of young in their second nests, or fewer; several of these males lost paternity to neighboring males. The females with mates whose color we enhanced apparently found them more desirable as mates and fathers of their young.

These results are intriguing because they show that females can rapidly assess their mates and update these assessments in response to changes in appearance. Long after Barn Swallows have paired up for the season, males have to keep on impressing their mates. This may come as no surprise if you think of the billions of dollars that members of our own species spend on improving and maintaining our looks long after we have committed to a mate. Although ornithologists have long noted that male birds expend energy on brilliant and extravagant plumes to win a mate, this study of Barn Swallows has revealed why males keep up their beauty long after initial mating decisions have been made.

Rebecca Safran conducted her study of Barn Swallows as part of her doctoral dissertation in Cornell's Department of Ecology and Evolutionary Biology and the Cornell Lab of Ornithology's Evolutionary Biology Program. She is now a postdoctoral researcher at Princeton University.

For permission to reprint all or part of this article, please contact Laura Erickson, editor, Cornell Lab of Ornithology, 159 Sapsucker Woods Rd., Ithaca, NY, 14850. Phone: (607) 254-1114. email: lle24@cornell.edu