Sex differences in avian plumage evolution: stronger effects of natural selection and social competition on females

Sexual dichromatism in birds may evolve via sexual selection for increased elaboration in male plumage (Darwin hypothesis) or, alternatively, by natural selection for increased crypsis in female plumage (Wallace hypothesis). Both these effects may be counteracted by social competition promoting ornamentation in females. However, the relative roles of sexual, natural, and social selection in shaping patterns of plumage dichromatism have proved difficult to disentangle. To test these hypotheses in conjunction, we estimated the strength of sexual selection on males, the level of predation risk on females and the intensity of social selection across 1283 species of suboscine passerines. We then used phylogenetic analyses to evaluate the drivers of sex differences in plumage elaboration and brightness. The results confirm that sexual selection increases plumage elaboration in males but not females, driving overall dichromatism. In contrast, predation risk more strongly affected female plumage, limiting brightness in species with exposed nests and female-only incubation. Similarly, social competition for year-round territories predicted greater plumage elaboration in both sexes, particularly in females. Our findings show that sexual dichromatism arises not only through sexual selection on male ornamentation, but also via natural and social selection pressures, which have greatest effect on female plumage evolution.