Plumage color as a composite trait: developmental and functional integration of sexual ornamentation

Most studies of condition-dependent sexual ornaments have treated such ornaments as single traits. However, sexual ornaments are often composites of several components, each produced by partially independent developmental pathways. Depending on environmental and individual condition, components of these ornaments may reflect different behavioral or physiological properties of an individual. One of the best-known, condition-dependent ornaments is carotenoid-based plumage coloration, which has at least four distinct components: pigment elaboration, patch area, pigment symmetry, and patch area symmetry. Here we examined fitness consequences of variation in individual components of carotenoid ornamentation in male house finches (Carpodacus mexicanus). Over 5 yr and several selection episodes, we studied variation in the plumage components in a large sample (n = 498) of males from a Montana population. The ornament components were partially independent of each other and had distinct fitness consequences. Selection for higher fecundity favored an increase in redness of coloration and a decrease in pigment asymmetry and patch area asymmetry but did not act on patch area itself. In contrast, viability selection favored larger and more symmetrical ornamental patches but did not act on pigment elaboration. Developmental and functional interrelationships among individual components of ornamentation strongly differed between house finch populations. Distinct patterns of selection on individual components of condition-dependent ornaments, combined with partially independent development of components, should favor the evolution of composite sexual traits whose components reliably reflect condition across a wide array of environments.