Quantitative color variation within and across morphs of the polymorphic White-throated Sparrow

Coloration has evolved to serve diverse functions, including communication. In species with discrete color polymorphisms, the extent to which color variation exists within morphs and communicates multiple messages often remains unclear. We employed reflectance spectrometry to study variation in coloration in the dimorphic White-throated Sparrow (Zonotrichia albicollis), which exhibits a "white'' and "tan'' morph in both sexes. We explored whether distinct color traits distinguish between morph and sex classes, and whether color variation exists within classes that might reflect differences in individual quality. Further, we asked whether sexual dichromatism is more pronounced in the white morph, in which males display greater promiscuity and aggression and, thus, may be under stronger sexual selection for conspicuous coloration. Distinct aspects of crown plumage coloration differentiated the two morphs versus the two sexes and multiple types of coloration were associated with a morph, suggesting both multiple and redundant messaging functions of coloration. The brightness of white coloration and yellow carotenoid-based coloration differentiated the morphs, whereas the brightness and saturation of brown to black melanin-based pigmentation differentiated the sexes within morphs. However, coloration also varied considerably within morph and sex classes, potentially reflecting differences in individual quality. Finally, more sexual dichromatism existed within white morph than within tan morph birds. White morph males and females differed in white and yellow coloration, which also differentiated the morphs, and in melanin-based coloration. By contrast, tan morph males and females differed only marginally in coloration, and only in terms of melanin-based coloration. Results suggest that crown coloration is a multifaceted signal, and that selection has acted differently on coloration in both the morphs and the sexes. Our study suggests that multifaceted coloration can play multiple and redundant messaging functions, shows that color variation in polymorphic species can communicate more than morph, and suggests that morph-specific reproductive strategies alter selection on coloration.