Would you rather be sexy or healthy? relationships between the immune system and coloration in Common Redpolls (Acanthis flammea)

Abstract

Birds have long been studied as examples of sexual selection. There are many traits that can point to a bird’s quality as a mate, such as song, dance, and elaborate plumage features. Each of these traits has an energetic cost, and carotenoid-based plumage is one such trait. Carotenoid pigments are responsible for most yellow, orange, and red feathers, and almost a quarter of all bird species display carotenoid-based coloration. However, because carotenoids are only directly synthesized by plants, algae, and some fungi, birds must accumulate them in their diets before depositing them into their feathers. The pathway to produce red feathers, such as those displayed by Common Redpolls (Acanthis flammea), is more recently evolved and includes more carotenoid modification steps—and a potentially higher energetic cost—than yellow plumage. If a bird has enough energy to both produce these feathers and remain healthy, the plumage is an honest signal of quality. Conversely, if the bird is putting more energy towards plumage and gaining a mate, there may be a cost that affects other systems, such as the function of the immune system. Although limited, there have been studies conducted linking carotenoids and coloration to a stronger immune system and better condition. This work examines trends between coloration, body metrics, and immune system function in Common Redpolls to determine if plumage production is a direct signal of health. Ultimately, we found that the plumage coloration of redpolls is an honest signal of their health. Cap size in both males and females demonstrates immune health, where larger caps correlate with a stronger immune system. Also, redder breast coloration in mature males shows stronger humoral immunity.