Carotenoids and sexual dichromatism in North American passerine birds

Sexual dichromatism and the extent of male carotenoid-derived plumage coloration in breeding season North American passerine birds was analyzed by phylogenetic statistical methods. Passerines as a whole and five passerine subclades were analyzed by both independent contrasts and simulation methods. In passerines as a whole, carotenoids and sexual dichromatism are positively correlated irrespective of analysis method. In three of the subclades, the correlations are significant by a clear majority of analysis methods. Neither melanin nor structurally derived colors show similar significant increases with dichromatism. Carotenoids are obtained by animals solely through the diet, whereas both melanins and structurally derived colors can be synthesized. The relationship between sexual dichromatism and the use of carotenoids in plumage suggests that sexual selection may have promoted the expression of a condition-dependent honest indicator of phenotypic quality.