As shown here, some species, such as the great-tailed grackle on the left, are highly dimorphic in plumage color. In other species, such as the common grackle on the right, males and females are more similar in color. Such differences in dimorphism are t ypically attributed to the process of sexual selection, the evolution of traits that enhance mating success through mate attraction or competition for mates. Sexual selection in these species is predicted to act primarily on males, since the number of of fspring they have depends primarily on mate attraction.
The results I have found contradict this theory as an explanation of the evolution of dimorphism. When closely related species are compared, males have relatively similar coloration; differences in dimorphism have apparently resulted from the evolution o f FEMALE, not male, brightness. This result is supported by an analysis of the evolutionary relationships among blackbirds. It suggests that to understand the evolution of dimorphism in this, and possible other, groups of birds, we need to consider why females would evolve bright or dull coloration as well as why coloration evolves in males.
reference: Irwin, R.E. 1994. The evolution of plumage dichromatism in the New World Blackbirds: social selection on female brightness? The American Naturalist 144:890-907.