From Scientific American:
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.
By analyzing genetic records collected over 10 years, 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.
More here.