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 potentially doubling or tripling fecundity, but the factors responsible for variation in 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 starlings (Sturnus vulgaris), where we predicted that ‘date’ (clutch initiation) would have little effect on double-brooding propensity compared to individual ‘quality’. Double-brooding effectively doubled annual fecundity in European starlings (based on annual number of chicks fledged), but on average only 38% of individual females was double-brooded. Furthermore, 39% of females that initiated a second clutch experienced total failure of their second brood, thus accrued no fecundity advantage from their decision to double-brood. As we predicted variation in 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 age, and provisioning rate). However, we found no evidence of a cost of double-brooding; double-brooded females had significantly higher return rate, 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 simple trade-offs.