Carotenoid access and intraspecific variation in plumage pigmentation in male American Goldfinches (Carduelis tristis) and Northern Cardinals (Cardinalis cardinalis)

Abstract

1. Behavioural ecologists have shown much interest recently in the factors that promote variation in carotenoid-based colour displays. Because vertebrates cannot synthesize the pigments needed to attain this form of bright coloration, most research has focused on the effect of dietary access to pigments on carotenoid ornamentation. 2. However, few studies have considered experimentally how the availability of carotenoid pigments in the diet influences variability in the expression of carotenoid-based coloration. 3. Dietary access to carotenoids was manipulated in captive groups of male American Goldfinches (Carduelis tristis L.) and Northern Cardinals (Cardinalis cardinalis L.) to assess the role of pigment acquisition in shaping intraspecific variation in carotenoid-based plumage pigmentation. 4. Substantial variation was detected in the plumage displays of captive male goldfinches and cardinals within a diet treatment, such that birds with unlimited access to the same diet type displayed carotenoid-based plumage variation that did not differ significantly from that typical of wild males from each species. Additionally, the natural plumage brightness that male goldfinches displayed at capture predicted the degree to which they grew colourful plumage under controlled conditions in our experiments. 5. These results suggest that carotenoid-based plumage variation in male goldfinches and cardinals is less sensitive to pigment availability in the diet than colour variation observed in other studied taxa. Differences in ornamental coloration among males from these two passerine species may instead reflect their ability to utilize ingested pigments physiologically.