Breeding synchrony through social stimulation in a spatially segregated population of European starlings

Breeding synchrony occurs when individuals simultaneously perform portions of the reproductive cycle more often than expected by chance. We tested for the role of social stimulation in driving breeding synchrony of European starlings, Sturnus vulgaris, in a spatially-structured population in New Zealand over 11 years. Natural spreads in laying in groups were consistently tighter relative to aggregations of individuals chosen at random. Rather than a decrease in synchrony with increasing density, as would be expected if competition reduces breeding condition, we found that birds in dense breeding areas showed tighter synchrony. As the number of active nests within 500 m of a breeding location (i.e. human-constructed concrete bunkers) increased, the spread in clutch initiation date decreased. There was a correlation between synchrony and fledging success; less synchronous locations had lower fledging success. These results indicate that local and larger-scale levels of social stimulation may help coordinate reproductive timing in group-living birds. While the link between fitness and fledging success may not be causal, our results suggest that factors driving synchrony may also be related to individual quality and reproductive success.