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 fi rst-year birds. (b) it is related to an impaired fattening, and (c) its ove rlap with migration may involve adverse energetic effects. First-year swall ows tended to moult early in the post-breeding period: the proportion of fi rst-year birds in active moult was highest in July, and it decreased progre ssively until September. On the other hand, adult swallows tended to avoid moulting both in the post-breeding (July) and the migratory period (Septemb er), concentrating their moulting period in August. In all periods, swallow s in active moult stored less fat than non-moulting ones, suggesting a fast er depletion of energy resources by moulting birds. Swallows in active moul t showed a better body condition - measured as body mass relative to body s ize and fat content - than non-moulting ones before migration (coinciding w ith the periods of highest frequency of moulting individuals), but the oppo site was found during autumn migration. Our results support the existence o f a physiological trade-off between body moult and migration, which could h ave fitness consequences; thus, if these activities overlap it would impair migration performance of swallows.