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

Plumage colouration serves a variety of functions for birds, including conspecific signalling, crypsis, and predator–prey interactions. Though much research has been conducted on colour changes in species with delayed plumage maturation, where birds do not exhibit definitive adult plumage until their second breeding season or later, relatively few studies have examined how plumage colour changes once definitive adult plumage has been attained. In this study on male and female American Redstarts (Setophaga ruticilla), which exhibit male delayed plumage maturation, we used reflectance spectrometry to evaluate carotenoid-based tail colour changes over 11 breeding seasons, examining both within-individual and population level changes. At a population level, males in their first breeding season in definitive adult plumage had a feather hue that was more orange-shifted than birds in their second year in adult plumage, and marginally, but not significantly, more orange-shifted than birds in their third year in adult plumage. Within-individual analysis of males recaptured in subsequent seasons also revealed a shift away from orange towards a more yellow feather hue as individuals aged. Within individuals, red chroma was highest for males in their second year in adult plumage, but it showed no population-level effects. At the population level, female redstarts in their first breeding season displayed plumage with a higher red chroma and a lower brightness than birds in their second breeding season, potentially as a result of differences in the timing and conditions of moult (first-year bird tail feathers were grown in the nest). For adult males and females, there was no difference in plumage colouration between birds that returned to the study site compared to those that failed to return. Together, our results suggest that within-individual change rather than differential survival best explains our findings. We suggest that studies examining age-related colour changes are critical for understanding the evolution of complex signalling systems, such as that of American Redstarts.