The effects of territory quality on age-dependent reproductive performance in the northern wheatear, Oenanthe oenanthe

Abstract

Age differences in access to high-quality resources and its relation to nest predation risk have generally been neglected in efforts to explain why old individuals have a higher reproductive performance than young ones. I used long-term population data to investigate age- and habitat-specific nest predation risk and reproductive success of male northern wheatears breeding in a heterogeneous agricultural landscape. Old (≥2 years) male wheatears had higher reproductive success than yearling males because their nests were less likely to be predated, they produced more fledglings from successful broods and they were more likely to start a new breeding attempt following a complete failure. Also, old males arrived and bred earlier than yearlings. Old wheatears were more likely to breed in habitats with a permanently short field layer, while yearlings mainly bred in habitats with a field layer gaining in height during the incubation–nestling period. Nest predation risk was significantly lower and fledgling production among successful breeders was higher in territories with a permanently short as compared to growing field layer. Data on order of habitat occupation suggested that yearlings were not as good as older males at identifying territories with a permanently short field layer (as reflected by a significant interaction between male age and order of occupancy). Both cross-sectional and longitudinal data suggest that an age difference in territory field layer height was a major cause of age differences in reproductive performance in wheatears. Other factors, such as differential survival, breeding time, local familiarity and increased foraging and breeding skills either had small effects or did not significantly explain the observed age differences in reproductive performance. I conclude that nest predation and an age-related access to high-quality habitats are likely to be of great importance for the observed age differences in reproductive performance in the northern wheatear and possibly many other bird species.