Spring is the season for flashy mates, at least for finches. It is only later in the year that the females choose based on genetic diversity, according to new research from two scientists at the University of Arizona. Their 10-year study of a colony of 12,000 finches in Montana has revealed the seasonal dynamics of finch attraction and thereby resolved an evolutionary conundrum.

Previous research had shown that female birds go for the most resplendent mates; in the case of finches, this means the males with the reddest breast. Some scientists argue that such sexual selection is the driving force behind apparently useless displays, such as red breasts or the brilliance of a male peacock's tail. "For such elaborate traits to evolve, you have to have mating patterns where everyone wants the same thing," ecologist Kevin Oh explains.

On the other hand, in a relatively stable population, if all females mated with the flashiest guy, the charm of finches would become inbred. Instead, female finches should seek out males with the most genetic difference from themselves. But this would lead to a variety of hues for male finch breasts. "Even though preference for genetically complementary mates is widely documented, it has always puzzled people that individual differences in mate preference do not prevent the evolution of elaborate ornaments," ecologist Alexander Badyaev notes.

Badyaev has spent the last 10 years photographing, taking DNA samples and measuring a charm of house finches in Montana. By 2004, he had the intimate details of the entire population, including who was mating with whom, who was cheating on whom, and whose genes got passed on to the next generation. He also has pictures of the relative rosiness of any given male finches breast. "We have 10 sequential generations of wild birds completely genotyped," he says. "It's never been done before."

By analyzing that record, the researchers found that early in the mating season, females chose the male finch with the reddest breast. But as the season wore on--and new females entered the charm--they typically chose males with strong genetic differences from themselves. And those tempted to stray typically chose a mate more genetically different than their regular partner, according to the research presented in Proceedings of the Royal Society B.

Although it remains unclear how finches sense genetic difference, the finding does reveal a new strategy for evolving elaborate ornamentation while preventing inbreeding. And that means this behavior might apply to more than just one charm of finches in Montana.