The evolution of bill coloration and plumage dimorphism supports the transference hypothesis in dabbling ducks

Bright coloration in male birds is typically thought to be driven by sexual selection (female choice or male–male competition). Bird species often vary in the intensity of bright coloration, but few studies have addressed this cross-species variation. Potentially this variation could result from either variation in female preferences or in the relative costs of male traits. Species of dabbling ducks vary in the presence of bright male plumage and bill coloration. I tested the transference hypothesis for ornament evolution in dabbling ducks using a phylogenetic study of character evolution. The transference hypothesis makes three predictions: (1) a costly male ornamental trait is the ancestral condition, (2) a less costly male ornamental trait is the derived condition, and (3) gains in the less costly trait are associated with losses or absence of the more costly male trait. All three of these predictions were satisfied in this study of the evolution of plumage dimorphism and bright bill coloration in the dabbling ducks, given that bright plumage coloration is more costly than bright bill coloration.