Kimball, R.T. and J.D. Ligon. 1999. Evolution of avian plumage dichromatism from a proximate perspective. American Naturalist 154: 182-193.

Several studies have indicated that sexual plumage dichromatism is a result of four proximate mechanisms: estrogen, testosterone, luteinizing hormone, and nonhormonal factors. In estrogen-dependent dichromatism, dull plumage coloration develops in the presence of estrogen, while bright coloration develops in its absence. In testosterone-dependent and luteinizing hormone-dependent plumage dichromatism, bright plumage develops in the presence of these hormones, while dull plumage develops in their absence. Placing the proximate control of plumage dichromatism in a phylogenetic context suggests that estrogen-dependent plumage dichromatism, found in the avian orders Struthioniformes, Galliformes, and Anseriformes, is likely to be ancestral in extant birds, while plumage dichromatism dependent on testosterone, luteinizing hormone, or nonhormonal factors is a more derived condition. An examination of the possible pathways leading to estrogen-dependent plumage dichromatism suggests that the fewest evolutionary steps are to begin from a condition in which both sexes are more brightly colored, followed by selection for duller coloration in one sex. The fact that estrogen-dependent dichromatism is ancestral in extant birds suggests that more brightly colored monochromatism may have been ancestral in modern lineages of birds.