Sex Study: Flashy Feathers Cause Female Swallows to Cheat

Brian Handwerk
for National Geographic News
September 29, 2005

For male barn swallows on the make, success is all about keeping their feathers in trim.

The birds attract mates by the color intensity of their feathers—a signal of their desirability. A new study suggests that if males' appearances change for the better, females are less likely to cheat.

"[Appearances] are also important for establishing sneaky pair bonds behind your mate's back," said biologist Rebecca Safran, who led the Cornell University study. "Upkeep and maintenance are just as important as the first impression."

Between two successive breeding bouts, Safran and her colleagues turned birds from duds to studs by improving their plumage color with nontoxic inks.

Birds whose feathery mojo was enhanced sired more offspring than those whose plumage remained the same. Improved males also sired more than they had in their first breeding season with the same female.

"It's interesting that birds should be doing such constant evaluation," said Olivia Judson, an evolutionary biologist at Imperial College in London. "Now a male has to not only get territory, build a nest, and seduce somebody—he's also got to keep her.

Choosy But Not Promiscuous

Safran's team used DNA tests to determine how many young birds in a nest were actually fathered by their mother's mate, both before and after his plumage was manipulated.

Researchers studied established pairs, allowing them to eliminate other possible factors, like age or territory, from the mate-choosing process.

"We can say with great certainty that feather color affects female preference for particular males and that changes in the signal have really important consequences in terms of whose offspring you're caring for," Safran said.

Female barn swallows (Hirundo rustica) made their sexual assessments repeatedly and in a short period of time—on the fly, as it were.

Judson cautions that the term "promiscuous" doesn't adequately describe the barn swallow's sexual behavior.

Continued on Next Page >>


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