Sex and the Single Bird

This occasional feature, written by researchers around the world, highlights new discoveries in the sciences and what they mean for your outlook.

THE BIG IDEA: Every summer, male songbirds called lark buntings shed their dull winter plumage and fly north from Mexico and Texas to build ground nests on the short-grass prairie of the Pawnee National Grassland, northeast of Denver. By the time female birds arrive two weeks later, the males are brightly adorned, often with glossy black feathers and white wing patches that flash as they zoom skyward and float back to the ground, singing all the while. These fancy traits and aerial displays may ward off other males, and they also send a come-hither signal to females. Scientists have always assumed that female birds, looking for good fathers, consistently choose males that exhibit the same exaggerated sexual traits. But my research shows that their preferences change from year to year. Sometimes females even select partners that look completely different from the previous season's mates.

HOW WE DISCOVERED IT: For five summers, I teamed up with Bruce Lyon, a biologist at the University of California at Santa Cruz. We lured male birds to baited feeders, tagged them with colored bands, took blood samples and documented their traits -- from body and wing-patch color to body and beak size. Then we released the birds and used binoculars to follow their breeding success. When the chicks hatched, we ran paternity tests and tracked the number of offspring fathered by each male.

WHAT WE FOUND: For male lark buntings, reproductive success depends on whatever traits are in vogue among females that season. By staying flexible and seeking out partners with the physical qualities most needed at the moment, females ensure that more chicks successfully leave the nest. If the prairie is overrun by ground snakes, for example, mother birds might choose the most protective males -- a quality that might be signaled by wing-patch size. If grasshoppers are scarce the next year, maybe they will look for partners with big beaks, which might make them good providers.

WHY IT MATTERS: Female mating preferences are nothing new -- female guppies look for males with bright orange and black spots, for example -- but scientists generally believed that those preferences were fixed, as if coded into the species' genes. Now we've shown that female birds can change their minds from year to year, which means that they help drive genetic variation. So what might explain the many bright colors and elaborate songs of birds of paradise? The woman's prerogative.

-- Alexis Chaine is a researcher at the National Center for Scientific Research in Moulis, France

In cooperation withScience magazineand theAmerican Association for the Advancement of Science. To see videos of lark buntings, visit the Web sitewww.sciencemag.org/wpoutlook.