No Apparent Immediate Reproductive Costs of Overlapping Breeding and Moult in a Mediterranean Great Tit Population

SIMPLE SUMMARY: The annual breeding and moulting calendar in birds is affected by global warming and is leading to an increased overlap between these two activities in some populations. Allocation of resources for two energy-demanding activities could negatively affect performance in one or both of them. In this paper, we examined whether the overlapping of breeding and moulting has negative effects on the reproductive performance of a population of great tit (Parus major) in eastern Spain. We found that, in pairs where both parents overlapped breeding and moulting, clutch size was smaller, fewer eggs hatched, and fewer fledglings in poorer body condition left the nest. However, these differences disappeared when the seasonal trend in breeding performance was taken into account, i.e., the poorer reproductive performance of pairs that overlap moulting and breeding was mainly due to the fact that they breed later, and reproductive performance declines as the season progresses. Thus, we conclude that the overlap of breeding and moulting does not impose additional immediate reproductive costs in this population. ABSTRACT: Some phenological events in birds, such as breeding and moulting, are being affected by rising temperatures due to global warming, and many species have undergone temporary changes in these energetically demanding phases that are often separated in time. This has led to an increased overlap between breeding and moulting in some populations. This overlap causes conflicts in resource allocation and may impose fitness costs that could affect immediate reproductive performance. We tested whether this occurs in a great tit (Parus major) population in eastern Spain. In 71% of 390 pairs, in which both parents were captured during the period of overlap between moulting and breeding, at least one parent was moulting when feeding the chicks of its second brood. Later breeders were more likely to overlap breeding and moulting, and when both parents overlapped, clutch size was smaller, fewer eggs hatched and fewer fledglings in poorer body condition were produced. Some results were intermediate when only one parent moulted. However, all these differences between moulting and non-moulting pairs disappeared when the seasonal trend in reproductive parameters was taken into account, as moulting birds bred later and reproductive performance decreased seasonally. Therefore, the overlap of breeding and moulting does not impose additional reproductive costs in this population.