Testing the resource trade-off hypothesis for carotenoid-based signal honesty using genetic variants of the domestic canary

Carotenoid-based coloration in birds is widely considered an honest signal of individual condition, but the mechanisms responsible for condition dependency in such ornaments remain debated. Currently, the most common explanation for how carotenoid coloration serves as a reliable signal of condition is the resource trade-off hypothesis, which proposes that use of carotenoids for ornaments reduces their availability for use by the immune system or for protection from oxidative damage. However, two main assumptions of the hypothesis remain in question: whether carotenoids boost the performance of internal processes such as immune and antioxidant defenses, and whether allocating carotenoids to ornaments imposes a trade-off with such benefits. In this study, we tested these two fundamental assumptions using types of domestic canary (Serinus canaria domestica) that enable experiments in which carotenoid availability and allocation can be tightly controlled. Specifically, we assessed metrics of immune and antioxidant performance in three genetic variants of the color-bred canary that differ only in carotenoid phenotype: ornamented, carotenoid-rich yellow canaries; unornamented, carotenoid-rich 'white dominant' canaries; and unornamented, carotenoid-deficient 'white recessive' canaries. The resource trade-off hypothesis predicts that carotenoid-rich individuals should outperform carotenoid-deficient individuals and that birds that allocate carotenoids to feathers should pay a cost in the form of reduced immune function or greater oxidative stress compared with unornamented birds. We found no evidence to support either prediction; all three canary types performed equally across measures. We suggest that testing alternative mechanisms for the honesty of carotenoid-based coloration should be a key focus of future studies of carotenoid-based signaling in birds