Title: Evolution of breeding plumages in birds: A multiple-step pathway to seasonal dichromatism in New World Warblers (Aves: Parulidae)

Bird feathers serve multiple functions through their physical structure and coloration, but the evolution of functional novelty in bird feathers remains poorly understood. We investigated how selective pressures gave rise to seasonal coloration change in the feathers of the New World Warblers (Aves: Parulidae), a family with a remarkable diversity of plumage, molt, and life history strategies. Seasonal color changes in the plumages of migratory warblers are hypothesized to reflect a tradeoff between natural and sexual selection on the breeding and non-breeding distributions. We used comparative methods including phylogenetic path analysis to examine nested hypotheses relating to the evolution of seasonal dichromatism (i.e. breeding and nonbreeding plumages) and the molts that produce these plumages. We found that biannual molts likely evolved in response to increased feather wear and that changes in feather coloration evolved after the biannual molt itself. These results demonstrate that structural needs, not seasonal selection on coloration, drive the evolution of molt strategies in Parulidae. Importantly, once a biannual molt evolves, it served as a preadaptation for seasonal changes in plumage color. These results reveal how life history strategies act upon multiple and separate feather functions to drive the evolution of feather replacement patterns and bird coloration.