Age-related change in carotenoid-based plumage of the American redstart (Setophaga ruticilla)

Plumage colouration serves a variety of functions for birds, including signalling conspecifics, crypsis, and predator-prey interactions. Though much research has been conducted on colour change in species which exhibit delayed plumage maturation, where birds do not exhibit definitive adult plumage until their second breeding season or later, relatively few studies have examined how colour changes once definitive adult plumage has been attained. Studies that have focused on changes in definitive plumage colour have proposed two non-mutually exclusive hypotheses to explain colour change: age-related colour change occurs within individuals, and colour biased survival of more or less colourful individuals. Few studies have examined how carotenoid-based plumage colour changes, and those that have found conflicting results as to whether age-related colour change is occurring. In this study I examined the carotenoid-based plumage colour of the American redstarts, a migratory passerine that exhibits delayed plumage maturation where males obtain definitive plumage after their second year. Using reflectance spectrometry on feathers collected over a ten year period from a breeding population in Ontario, Canada, I quantified the colour characteristics of each feather and compared the intermediate (juvenile) colour to the definitive plumage colour, and tracked how the definitive adult plumage colour changed over time. As expected, due to delayed plumage maturation, male redstarts had definitive plumage that was significantly more orange and saturated in carotenoid chroma than their intermediate plumage. Additionally, males were most orange-shifted (reflected light at the longest wavelengths) during their first breeding season with definitive plumage, and became significantly more yellow-shifted (reflected light at shorter wavelengths) in the subsequent season. Female redstarts displayed significantly brighter and more orange-shifted plumage during their second breeding season compared to their first, but did not undergo any further colour change in subsequent years. The results of this study support the hypothesis that age-related colour change occurs within individuals, however I cannot rule out the possibility that differential survival also acts at the population level. Plumage colour is a complex trait and further research is needed to understand the relationship between change and reproductive effort.