The roles of ecological factors and sexual selection in the evolution of white wing patches in ducks

Most studies of the evolution of sexual ornamentation have dealt with plumage attributes. White plumage patches are widespread in birds, the disproportionate role of wearing costs makes their evolutionary trajectories unique, and their visual assessment is less biased than that of other color categories. Still, comparative studies of white patches are very rare. We examined the evolution of white wing patches in ducks (Anatinae), assessing both sex-specific trait expression and dichromatism. Habitat openness, nest site (cavity or open), or the length of the incubation period did not predict white patch expression. Patch size on the wing coverts of females increased with parental care contribution by males. Covert patch size relative to the wing surface was positively related to body size in males, suggesting a role in sexual competition. White wing patch expression was unrelated to the frequency of social mate change or testis size, a measure of general sperm competition intensity. However, covert patch size in both sexes showed strong negative correlation with the phallus length of males, an indicator of the prevalence of forced copulations. Further studies are needed to clarify the role of flight feather patches and the factors that limit the exaggeration of white wing patches in this and other groups.