Delayed timing of breeding as a cost of reproduction

Timing of breeding is a trait with considerable individual variation, often closely linked to fi tness because of seasonal declines in reproduction. Th e drivers of this variation have received much attention, but how reproductive costs may infl uence the timing of subsequent breeding has been largely unexplored. We examined a population of northern wheatears Oenanthe oenanthe to compare three groups of individuals that diff ered in their timing of breeding termination and reproductive eff ort to investigate how these factors may carry over to infl uence reproductive timing and reproductive output in the following season. Compared to females that bred successfully, females that put in less eff ort and terminated breeding early due to nest failure tended to arrive and breed earlier in year 2 (mean advancement  2.2 and 3.3 d respectively). Females that spent potentially more eff ort and terminated breeding later due to production of a replacement clutch after nest failure, arrived later than other females in year 2. Reproductive output (number of fl edglings) in year 2 diff ered between the three groups as a result of group-level diff erences in the timing of breeding in combination with the general seasonal decline in reproductive output. Our study shows that the main cost of reproduction was apparent in the timing of arrival and breeding in this migratory species. Hence, reproductive costs can arise through altered timing of breeding since future reproductive success (including adult survival) is often dependent on the timing of breeding in seasonal systems.