Helpers during replacement clutches buffer the impacts of late breeding on a cooperative bird

Abstract

Renesting is an important strategy employed by many bird species to compensate for nest failure. However, although replacement clutches may increase an individual's annual reproductive success, they impose substantial energetic and fitness costs. In some cooperatively breeding species, helpers lighten breeders' workloads thereby facilitating a second clutch within a season or increasing their survival prospects. Alternatively, renesting may increase the chances of having help in replacement clutches as some individuals that have lost their own clutch could redirect their parental effort to another nest. Herewe used a 20-year dataset of a facultative cooperative breeder, the Iberian magpie, Cyanopica cooki, to investigate how helping and renesting behaviours combine to affect reproductive output. After controlling for clutch size and laying date, we show that helpers had a positive effect on the reproductive success of pairs, particularly in replacement clutches. Indeed, our study revealed that replacement clutches were much more common than previously thought (observed in three-quarters of the females that lost the first clutch), and that the probability of receiving help was higher in replacement clutches. As helpers increased in number over the breeding season, they could contribute to buffering the negative effects of breeding late, when environmental conditions can be more adverse, and the probability of successful independent reproduction is lower. This study indicates that species with cooperative breeding and a short lifespan can prioritize current over future reproduction by renesting at the end of the breeding season, when the benefits of having help outweigh the costs associated with deteriorating environmental conditions.We propose that renesting can be a context-dependent strategy (mediated by social and environmental factors) that may increase reproductive success in avian cooperative breeding systems.