Mate fidelity, senescence in breeding performance, and reproductive trade-offs in the barn swallow

1. Senescence is defined as the decline in performance with age, and can be determined by diverse mechanisms such as accumulation of mutations, 'wear and tear' and antagonistic pleiotropy. Animals can mitigate the negative effects of senescence on reproductive performance by acquiring experience as they age. 2. We analysed the effect of age on clutch size, breeding date, fledging success and offspring quality (morphology and T cell-mediated immunity) of barn swallows (Hirundo rustica, Linnaeus) by comparing the progeny of individuals of known age during consecutive years. 3. Reproductive performance had no significant effect on probability of divorce, and adults that re-mated with each other had no reproductive advantage. 4. Body size, feather development and T cell-mediated immunity declined with paternal and/or maternal age already after the first breeding season. Body mass declined after paternal age of two years. T cell-mediated immunity of nestlings in one year covaried negatively with that of their siblings the previous year. 5. Hence, barn swallows suffer an age-dependent decline in offspring quality, and apparently trade quality of current offspring against that of subsequent reproductive events, although causality of this relationship could not be assessed in this correlational study.