Inconsistent findings of ageing across different feather-quality indices in a wild passerine

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In most animals, individuals tend to decline in performance in later life, known as ageing. In birds, studies of ageing have traditionally concentrated on metrics of survival and reproductive success, whereas morphological traits have received comparatively little attention. Feather quality is a key morphological trait for passerines as it contributes to flight performance, camouflage and thermoregulation, and is, therefore, important for survival. However, there are, as of yet, very few studies on how feather quality changes with age. In this study, we use a closely monitored natural population of long-lived Seychelles warblers to investigate whether and how three different indices of feather quality change with age within individuals. We found that feather mass-to-length ratio increased with age (indicating improving quality with age), while rachis width declined with age (indicating declining quality), and barbule density showed no ageing pattern. All feather quality indices showed no sex differences in ageing rates. The inconsistent findings of ageing across the three feather quality indices may suggest that life history trade-offs between feather quality and survival or reproduction are not as strong, and feather quality may not be a costly trait to maintain. Age-related improvements of mass-to-length ratio but not the other indices may also suggest that certain aspects of feather quality are more important than others to maintain. Opposite patterns of ageing in rachis width and mass-to-length ratio may also suggest possible trade-offs between different feather quality metrics. Our study highlights the need for further investigation on energetic trade-offs between maintenance of different phenotypic traits and fitness