Body moult and autumn migration in the barn swallow Hirundo rustica: is there a cost of moulting late?

The timing of moult in birds is thought to be constrained by physiological trade-offs. We studied barn swallows in central Spain to assess whether (a) body moult tends to be temporally separated from migration in adult and first-year birds, (b) it is related to an impaired fattening, and (c) its overlap with migration may involve adverse energetic effects. First-year swallows tended to moult early in the post-breeding period: the proportion of first-year birds in active moult was highest in July, and it decreased progressively until September. On the other hand, adult swallows tended to avoid moulting both in the post-breeding (July) and the migratory period (September), concentrating their moulting period in August. In all periods, swallows in active moult stored less fat than non-moulting ones, suggesting a faster depletion of energy resources by moulting birds. Swallows in active moult showed a better body condition-measured as body mass relative to body size and fat content-than non-moulting ones before migration (coinciding with the periods of highest frequency of moulting individuals), but the opposite was found during autumn migration. Our results support the existence of a physiological trade-off between body moult and migration, which could have fitness consequences; thus, if these activities overlap it would impair migration performance of swallows.

Perez-Tris, J., de la Puente, J., Pinilla, J. & Bermejo, A. 2001, Ann. Zool. Fennici 38: 139-148