Feather mass and winter moult extent are heritable but not associated with fitness-related traits in a long-distance migratory bird

In birds, the allocation of resources to plumage production may have important fitness consequences. However, we have only a limited understanding of how plumage traits respond to natural selection, making it difficult to predict how variation in plumage traits may contribute to the adaptation of birds to environmental change. In this study, we collected plumage-related data in a pedigreed population of a long-distance migratory bird (the Pied Flycatcher Ficedula hypoleuca) to estimate the heritability of two plumage traits: feather mass (as a proxy of feather quality) and the extent of winter moult. We further explored whether these plumage features were associated with some fitness-related traits. Variation in plumage characteristics could be explained by differences in sex, age and year, which indicates a high degree of plasticity in these traits. After controlling for these effects, however, feather mass and winter moult extent were highly repeatable (r = 0.58–0.82) and heritable (h² = 0.59–0.65), suggesting that additive genetic variation accounts for a significant proportion of the residual phenotypic variation of plumage traits in this population. Although the studied characteristics showed evolutionary potential, we did not find any relationship between plumage features and fitness-related traits like spring arrival date, egg-laying date, mating success or mating-time. We conclude that current selection on feather mass and moult extent, if existing, is weak, and that these moult-related traits are currently of minor importance for the adaptation of our study population to global warming.