THE EVOLUTION OF PLUMAGE DICHROMATISM IN THE NEW-WORLD BLACKBIRDS - SOCIAL SELECTION ON FEMALE BRIGHTNESS

Plumage dichromatism in which males are more brightly colored than females is generally thought to result from sexual selection for bright coloration in males. As predicted by the sexual-selection hypothesis, plumage dichromatism in New World blackbirds (Passeriformes: Icterinae) was found to be higher in polygynous species in five of six independent clades within which mating system varies. However, both within these clades and in the subfamily as a whole, a phylogenetic analysis revealed that most changes in dichromatism result from changes in female plumage brightness. Female plumage coloration was found to be generally more evolutionarily labile than male plumage coloration, and most of the plumage changes that clearly result in a change in dichromatism were attributable to changes in female brightness. The combined findings that dichromatism is higher in polygynous than in monogamous species and that change in dichromatism results from change in female plumage color suggest that dichromatism evolution in icterines results from some form of social selection on females rather than sexual selection on males.