Looks matter: if swallows aren't spiffy, mates' fidelity is iffy.

If a male barn swallow's plumage is more attractive than that of other males, his mate is less likely to have furtive flings with other wooers, new research suggests. Even plumage changes during the breeding season, after birds have paired up for the year, have a significant effect.

"The bad news for male swallows is that the mating game is never over," says Rebecca Jo Safran, an evolutionary biologist who was at Cornell University when she and her colleagues there conducted their investigation.

The researchers studied the barn swallow, Hirundo rustica erythrogaster, which scientists regard as socially monogamous. In such species, females bond with one male at the beginning of each breeding season, but genetic studies of offspring indicate that females often copulate with other males on the sly.

Breast plumage varies among male barn swallows from pale reddish brown to dark chestnut, says Safran, now at Princeton University. Previous studies have shown that males with darker breast feathers are more attractive to females, she notes.

In the new experiment, conducted during the 2002 breeding season in rural areas near Cornell, the scientists captured some male barn swallows and darkened the reddish tone of their breast plumage with a felt-tip marker. That created a "supersexy male," says Safran, "one dressed in a Gucci suit rather than one from J.C. Penney."

At the same time, the researchers took blood samples from all the birds, removed the eggs from their nests, and checked the paternity of the eggs.

For those pre-touch-up clutches of eggs, about 70 percent of the offspring had been fathered by the resident male. But in subsequent clutches tended by males that had received a breast makeover, the percentage fathered by the resident male jumped to 95 percent. Paternity percentage for males painted with a colorless marker or simply captured and released either remained at about 70 percent or dropped in subsequent clutches, say Safran and her colleagues in the Sept. 30 Science.

The new research "is really a very nice study" that opens a suite of interesting questions, says Bob Montgomerie of Queen's University in Kingston, Ontario, who has studied the color of barn swallows. For instance, did females decide that the color-enhanced males were worthier partners? Or did the more-colorful males scare off would-be philanderers?

Other studies will be required to answer such questions, says behavioral ecologist David F. Westneat of the University of Kentucky in Lexington. The current research, however, is the first to suggest that female barn swallows may be constantly assessing their mates' fitness and that males need to continuously advertise their prowess, lest they lose out to other suitors.