A female bird’s taste in males can change radically from year to year. But fickleness and faddishness are not the reasons. Her preferences, instead, arise from another inscrutable attribute: female intuition. She infers from a list of fashion choices in her potential mates which combination is most likely to add up to the most chicks at the end of the season.

That’s the conclusion of two biologists who reported their work last week in the journal Science. So-called “secondary sexual traits” — fancy plumage, rump color, big beaks — are the product of two forces. One is the competition between males for mates and often for territory. The other is the female’s choice of her mate.

Alexis Chaine, of France’s Laboratory of Evolution and Biological Diversity, and Bruce Lyon, of the University of California at Santa Cruz, studied lark buntings, songbirds of the Great Plains. Over the five years of the study, about 45% of males failed to attract a mate; and even when they did, about 25% of chicks were fathered by other males.

They found that each year a different constellation of traits was associated with “fitness,” allowing the pairs to produce the most offspring. Some traits went out of fashion overnight.

In one example, having a large body and a lot of black rump feathers were marks of reproductive success one year but marks of reproductive failure another year. Food, predators, and weather are constantly in flux in the lark buntings’ habitat. The researchers think the female birds somehow discerned which attributes and behaviors were most favorable for each breeding season, and chose their mates accordingly.