ITHACA, N.Y. -- Even after they have paired with a male, thefemale North American barn swallow still comparison-shops for sexualpartners. And forget personality; the females judge males by theirlooks -- the reddish color of the males' breast and belly feathers.
Ifthe male's red breast is not as dark as other males in the population,the female is more likely to leave him and then secretly copulate withanother male, according to a Cornell University study featured on thecover of the journal Science (Sept. 30, 2005).
"The bad news formale swallows is the mating game is never over," said lead authorRebecca Safran, who conducted the study while a Cornell postdoctoralresearcher in ecology and evolutionary biology, and in the CornellLaboratory of Ornithology. "It is dynamic and continual. This issomething that most humans can relate to -- think of how much time andmoney we spend on our looks and status long after we have establishedstable relationships."
Barn swallow (Hirundo rusticaerythrogaster) males have a wash of reddish-chestnut color from theirthroats to their bellies, and this color varies among birds from verypale red-brown to a dark rusty-red. Like many songbirds, half of allmale barn swallows typically care for at least one young chick that wasactually fathered by another bird. The researchers used this widespreadphenomenon of cheating to test the factors that may keep a female barnswallow faithful to her mate. Sometimes males even rear an entire nestof illegitimate young.
After all pairs had laid their first setof eggs, Safran removed the eggs so that the females would mate again.Before the females chose their mates for their second nest, Safrancaptured the males and randomly assigned them to one of threetreatments. She either painted their throats, breast and belly featherswith a red marker to enhance their feathers to match the darkest -- andmost attractive -- males in the population, or left them alone orpainted them with a clear marker to ensure that results were not biasedby the coloring process. Then she let the pairs breed again. Sheconducted comparative DNA tests on the offspring from the first andsecond nests.
In the research, all 30 females remained sociallypaired with their original male mate, but they were sexually activewith other males. The males with enhanced color fathered asubstantially larger percentage of offspring in their second nests.Males whose color was unchanged fathered the same number or fewerchicks than they had in their first nests. "The study shows that thefemales are paying close attention to these signals and that theyrespond quickly to changes in their mate's appearance," said Safran.
Thereddish breast and belly feathers indicate a male's quality, such ashis health, status or ability to raise young, Safran speculates.
Theactual cue that female barn swallows use to assess potential matesdiffer according to regional tastes. For example, classic studies haveshown that in the very closely related European barn swallow (H.rustica rustica), males with long tail feathers attract more mates.Although many previous studies have investigated mating patterns inbirds and other animals, this is the first study of its kind tometiculously rule out biases such as age, size and initial variation insignals of male quality, like coloration, and to demonstrate thatmate-selection decisions are continual and dynamic. The results of thestudy have implications for the evolution and upkeep of showyornamental traits -- such as a peacock's tail or a deer's antlers --that are costly for males to maintain but give them an edge over rivalmales. "If females are assessing mates on a day-to-day basis, itexplains why males continue to maintain costly ornaments even when theymight appear to have served their purpose," said co-author IrbyLovette, assistant professor and director of the Cornell Lab ofOrnithology's Evolutionary Biology program.
"Our goal is now tounderstand how certain males keep a better plumage than others," saidKevin McGraw, Cornell Ph.D. '03, one of the co-authors who is now anassistant professor at Arizona State University in Tempe. "Factors likeultraviolet radiation from the sun, soiling and even feather degradingbacteria are known to affect the color of bird feathers once they aregrown, and perhaps the best males are those who spend more timepreening and protecting their plumage."
The paper's otherco-author is Colby Neuman, Cornell B.S. '05. In early September, Safranbegan a new position as a postdoctoral researcher at PrincetonUniversity. Supporters of the study included: the National ScienceFoundation, the American Association of University Women, the AmericanOrnithologists' Union and the Animal Behavior Society.
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