Long-tailed manakins (Chiroxiphia linearis) and swallow-tailed manakins (C. caudata) are closely related, sexually dichromatic, lek-breeding species in which male mating success is highly skewed. Males of both species delay plumage maturation. Before reaching the definitive state, they wear a sequence of feather coats less conspicuous than that of the adult. Nondefinitive plumages probably enhance male survival in the two species; in C. caudata they may also enhance breeding success of young males, who may be fully reproductively mature their first year. In C. linearis testicular development is retarded along with that of plumage, although males may be physiologically capable of breeding prior to the acquisition of the definitive plumage. This difference probably reflects differences in the social systems of the two species. Five hypotheses have been proposed to explain the evolution of delayed plumage maturation. The sexual-selection, crypticbreeder, and winter-adaptation hypotheses suggest that it functions primarily to enhance survival of young males. The juvenile- and female-mimicry hypotheses emphasize enhancement of immediate mating success. Support is provided for all but the female-mimicry hypothesis; it is argued that data are more consistent with juvenile mimicry and a neotenic origin of nondefinitive plumages.