Migratory Behavior and Differential Resource Allocation between Wing and Tail Feathers in a Passerine Bird

Temporal and energetic constraints associated with migration may compromise plumage quality and, ultimately, flight ability in migratory birds. As a consequence, migrants may invest more resources in parts of the plumage that are essential for long, sustained flight (such as the primary wing feathers) than in less important feather tracts. We used migratory and sedentary Blackcaps (Sylvia atricapilla) to analyze within- and between-individual variation in the mass and quality of wing and tail feathers. Migratory Blackcaps in both adult and juvenile plumage had lighter tail feathers than sedentary Blackcaps, but the primary feathers were of similar mass. Interestingly, the quality of primary and tail feathers (estimated from the mass of the feather in relation to its size) were positively correlated within individuals. However, migratory individuals had higher-quality primary feathers than sedentary individuals, given the quality of their tail feathers. Therefore, migratory Blackcaps appeared to preferentially allocate limited resources to primary feathers at the expense of the quality of the less important tail feathers. We suggest that this represents an adaptive mechanism to reduce the costs of migration constraints on plumage functionality.