Rebecca E. Koch and Geoffrey E. Hill
Most red, yellow, and orange colouration in birds is produced via carotenoid pigments. Ornamental colouration resulting from carotenoids has been shown to serve as a signal between potential mates or rivals, and individuals with more brilliant colors tend to be healthier than their duller counterparts. The mechanisms that link the internal health of a bird to its external colouration have been the focus of investigation over the past several decades. The most widely accepted—but still debated—explanation for how carotenoid-based colons can reflect a bird’s health is the “resource tradeoff hypothesis,” which assumes that carotenoid pigments are important not only to colouration, but also to internal processes like immune defence. In this review paper, we consider the key assumptions of the resource tradeoff hypothesis and why we do not yet have definitive answers for how carotenoid-based colouration is linked to health. Assimilating information from a literature of nearly 180 studies of carotenoids in birds, we weigh the evidence for each of the main assumptions of the resource tradeoff hypothesis. We uncover uneven support for key assumptions of the hypothesis and point to areas where more research is needed to properly assess the hypothesis. Our review is a key step toward uniting the expansive field of carotenoid colouration research under a unifying idea, and driving the field toward productive answers to longstanding questions. By better understanding the role of carotenoid pigments in animal physiology, we can better understand the function of ornamental colouration, but also how carotenoids may (or may not) be important dietary supplements.