Meeting Abstract
Birds display a bewildering variety of colors that have fascinated biologists since Darwin and Wallace, who began a long-running debate about the causes of sexual differences in plumage color (dichromatism). There are, however, many monochromatic species of birds, with both sexes dull or both brightly colored, but, to date, the causes of this variation have received little study. Here we show using plumage reflectance data from a worldwide sample of 977 species of birds that most variation in bird plumage occurs along an axis of sexual similarity, rather than dichromatism. Dichromatism is associated primarily with indices of sexual selection, such as social mating system and testes size, while the extent and direction of similar plumage in both sexes is associated with ecological and behavioral variables, such as habitat type, migratory behavior and mode of development. Both natural and sexual selection have influenced the evolution of bird coloration, but in many respects they have acted on two different axes: sexual selection on an axis of sexual differences and natural selection on an axis of sexual similarity. Since most evolutionary transitions have been to monochromatism, we suggest that natural selection on both sexes has been the most common source of selection on plumage color.
