Molt, feather growth rate and body condition of male and female Barn Swallows

Molt and ecdysis are costly activities crucial to several animals. Most birds completely molt their plumage once per year. The diverse energy and aerodynamic costs of molt are expected to shape the evolution of molt schedules and rate of feather renewal. Differences in molt strategies are also predicted between the sexes because of sex-related variation in fitness payoffs of rapid and early feather renewal. Feathers show barring patterns that reflect daily growth pulses. Ptilochronological analysis of growth bar width (GBW) reflecting condition-dependent feather growth rate can provide important, though overlooked information on molt strategies, and on the effects of body condition on feather renewal and subsequent migration or breeding. In this study of migratory Barn Swallows (Hirundo rustica) wintering in Nigeria, for the first time we analyzed sex-related variation in molt schedules and feather growth rate using molecular sexing. We found no sex difference in molt schedules but faster feather growth in females. Hence, protandry in arrival of swallows to European breeding grounds does not depend on sex-specific timing of molt or feather renewal rate. Individuals with advanced molt had larger GBW and muscle mass, suggesting that individuals in good condition complete molt earlier thanks to faster feather growth. Arrival date on the breeding areas may thus reliably signal male quality, if molt completion constrains migration. Finally, the frequency of low GBW values among wintering males was larger than during breeding in swallows that winter in Nigeria, suggesting selection against low GBW. This study provides the first unequivocal data on sex-related variation in molt schedules in a species with no discrete sexual dimorphism, and emphasizes the potential of ptilochronology in ecological-evolutionary studies, as it shows that feather growth rate covaries with timing of molt but may not mediate protandrous migration.