Dietary carotenoid pigments and immune function in a songbird with extensive carotenoid-based plumage coloration

Carotenoid pigments can directly enhance the immune responses of vertebrates, and they are used by many animals to create ornamental color displays. It has been hypothesized that these two functions of carotenoid pigments are linked: animals must trade off use of carotenoid pigments for immune function versus ornamental display. We tested two key predictions of this hypothesis with captive American goldfinches, Carduelis tristis, a species with extensive carotenoid-based plumage coloration. First, we tested whether the immune systems of male goldfinches are carotenoid limited during molt by supplying treatment groups with low, approximately normal, or high dietary access to lutein and zeaxanthin. Dietary treatment had a significant effect on plumage and bill color but not on immunocompetence. We compared the cell-mediated and humoral immune responses and the course of disease after infection for males in the different treatments. We observed no significant effect of the carotenoid content of diet on immune response or disease resistance. Second, we tested whether there was a positive relationship between immune function and expression of ornamental coloration by comparing both the pre- and posttreatment plumage coloration of males to their immune responses. We failed to find the predicted trade-off between ornament display and immune function. These findings do not support the hypothesis that songbirds with extensive carotenoid-based plumage displays trade off the use of carotenoids for ornamentation versus immune function.