Breeding success, but not reproductive investment, increases with female age in Tachycineta leucopyga (Chilean swallow)

Abstract Life-history theory predicts a trade-off between current and future reproductive effort and, in this context, the terminal investment hypothesis proposes that reproductive investment, and consequently breeding success, should increase at older ages. We analyzed how relative minimum age related to breeding investment and success in Tachycineta leucopyga (Chilean Swallow) females across 6 breeding seasons in northwestern Argentine Patagonia. We also examined age associations between laying date and physical condition. Our results showed no evidence that reproductive investment increased with age. However, older females laid earlier and achieved higher breeding success than younger ones, which may be related to age-associated differences such as experience or skill. We propose that older females may refine foraging efficiency, increasing the quality or quantity of food delivered per visit, while maintaining self-care; physical condition slightly improved from 1- to 2-yr-old females but remained similar in older females. Given that migratory aerial insectivores are particularly vulnerable to global climate change, understanding how reproductive investment and success vary with age provides essential life-history data to identify critical demographic constraints. Such information is particularly scarce in South America, where information on breeding costs and survival remains limited and hinders the development of effective conservation strategies.