Individual quality and double-brooding in a highly synchronous songbird population

Multiple brooding, the production of more than one set of offspring per breeding season, is a life-history trait that potentially doubles or triples fecundity, but the factors responsible for variation in the occurrence of multiple brooding within species remain poorly understood. We investigated the potential causes and consequences of double-brooding in the highly synchronously breeding European Starling (Sturnus vulgaris), for which we predicted that clutch initiation date would have little effect on double-brooding propensity compared with individual quality. Double-brooding effectively doubled annual fecundity in European Starlings (based on the annual number of chicks fledged), but on average only 38% of individual females had a second brood. Furthermore, 39% of females that initiated a second clutch experienced total failure of their second brood, and thus accrued no advantage in fecundity from their decision to double-brood. As we predicted, variation in the propensity for, and success of, double-brooding was independent of laying date, but also of other putative measures of individual quality (clutch size, egg mass, relative female age, and nestling provisioning rate). However, we found no evidence of a cost of double-brooding; females that double-brooded had significantly higher return rates and similar breeding productivity in the year after double-brooding compared with single-brooding females. Thus, a small proportion (∼20%) of 'high-quality' female European Starlings effectively double their potential breeding productivity through double-brooding without apparently paying a cost or experiencing any simple tradeoffs.