Change in body mass during breeding in Barn Swallows Hirundo rustica

Abstract

Capsule The body mass of female Barn Swallows (Hirundo rustica) varies cyclically in relation to breeding attempts within breeding seasons. Aim To determine whether body mass declines in relation to the progress of incubation in female and male Barn Swallows and whether this differs between breeding attempts within a season. Methods Adult Swallows were caught and ringed at breeding sites as part of a long-term study to estimate survival rates. Measurements of body mass and size at the time of ringing were analysed in relation to the start of incubation, defined from detailed observations of the progress of breeding at each site. Results Wing length, hour of weighing, and time from the start of incubation contributed significantly to variation in female body mass. Body mass peaked 3 days before the start of incubation and declined linearly by 0.115 g per day during incubation and brood rearing in both breeding attempts within a season. For males, body mass declined by 0.026 g per day from the start of incubation during the first breeding attempt, but this was not apparent for the smaller sample of males in the second breeding attempt, possibly because such breeding-related changes may be masked by premigratory increases in mass later in the breeding season. Conclusion The body mass of female Swallows varied cyclically during the breeding season in relation to breeding attempts and there were clear differences between male and female Swallows in the magnitude of body mass changes as breeding progressed.