Life-history trade-offs result in evolutionary optimization of feather quality

Resource allocation for body tissues is under strong selection pressures because allocating too many resources may waste both energy and essential nutrients, and allocating too few resources can result in functional disability. We thus hypothesized that feather quality is the outcome of life-history trade-offs that determine feather resource allocation and that these trade-offs are determined by species-specific annual routine and life-history attributes. We accordingly predicted that certain species will develop a more durable plumage when this plumage needs to function over a longer period as compared with species that replace their feathers soon after their creation. We tested this prediction using plumage quality indices and moult timing data of nest-grown juvenile feathers from 49 passerine species. By using feather length, mass and melanization measurements, we found that the lifespan of nest-grown feathers until projected replacement during the next moult is a strong predictor of feather quality. This was found in both an inter-specific analysis and when comparing different feathers of an individual. We suggest that the maintenance of functional feathers throughout the lifespan of a bird while avoiding excessive deposition of resources in feathers that are replaced soon after their growth represents an evolutionary process of resource allocation optimization.