A study of Lark Buntings shows that female songbirds look for different traits in prospective mates, depending on the conditions present in a particular year.
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.Apparently this is the first time changes in preference have been documented in female songbirds.
A link provides some good pictures and videos of lark bunting mating displays.