The effects of captivity on the bill coloration of male Northern Cardinals (Cardinalis cardinalis)

Abstract

Male Northern Cardinals (Cardinalis cardinalis) possess two distinct carotenoid-based signals, bright red plumage and orangish-red bills. The red ketocarotenoids necessary for both of these color ornaments cannot be synthesized de novo but are instead produced by metabolizing yellow carotenoids obtained from the diet. Carotenoid-based ornaments have been shown to be negatively affected by environmental stressors and the health state of birds and thus can serve as signals of individual quality. Carotenoid pigments necessary for bill coloration are continuously produced and replenished. In this study, we tested the hypothesis that holding wild-caught male cardinals in captivity, which previous studies have shown increases the stress level of wild-caught songbirds, would impair metabolism of yellow dietary carotenoids, thus negatively affecting the carotenoid coloration of bills. We captured two groups of male Northern Cardinals. One group was held in cages until the onset of molt, and the others were free-living birds, captured at the onset of molt. For each individual, we measured bill coloration, bill carotenoid content, and circulating carotenoids in the plasma. We found that the bills of both wild and captive cardinals were colored with astaxanthin, α-doradexanthin, canthaxanthin, lutein and zeaxanthin. Free-living birds had more total ketocarotenoids in their bills than those that had been held in captivity. The measured ketocarotenoid content in the bills did not correlate to plasma ketocarotenoid content in either captive or free-living birds. Lastly, bill hue before captivity was predictive of bill hue after captivity, with redder males maintaining more colorful bills in captivity. These data support the hypothesis that red carotenoid-based bill coloration is a condition-dependent trait.