Department of Biology, Trent University, Peterborough, Ontario K9J 7B8, Canada.
Conspicuous ornamentation has been linked to immunological and physiological condition in males of many species. In species where both sexes are ornamented, it is unclear whether the signal content of ornaments differs between males and females. We examined the immunological and physiological correlates of carotenoid-based bill and plumage ornamentation in American goldfinches Spinus tristis, a species in which bright orange bills are sexually monomorphic but yellow plumage is sexually dimorphic during the breeding season. Because bill color is dynamic over short periods while plumage color is static over longer time frames, we tested whether these signals have the potential to provide temporal information about immunity and condition. In both sexes, bill color (but not plumage color) was negatively related to leukocyte differential, a measure of recent stress, while plumage color (but not bill color) was positively related to resting metabolic rate. In females, bill color also positively correlated with immunoglobulin Y, a component of acquired immunity, while plumage color positively predicted natural antibody levels, a component of innate immunity. In males, neither bill color nor plumage color predicted immune function, suggesting that the mechanisms underlying these signals vary with sex. Our results demonstrate that dynamic signals such as bill coloration do not merely reflect the same information provided by static signals but that these two classes of signal provide information about different temporal aspects of phenotypic quality. Furthermore, our results indicate that a signal expressed in both sexes has the potential to provide different information depending on the sex of the bearer.