Long-term fitness consequences of breeding density in starling colonies: an observational approach

Abstract

Understanding the evolution of gregarious breeding in birds necessitates assessing how fitness components vary in relation to conspecific density. So far, the ultimate causes of breeding density remain contentious, owing to the logistic challenges in both obtaining and interpreting long-term effects of density on fledgling survival, recruitment rates or natal dispersal in wild populations. Here, we used observational data from a 13-year study on the reproduction of a facultatively gregarious bird, the Spotless Starling, Sturnus unicolor, to investigate the consequences of breeding density for fitness and offspring natal dispersal. We compared breeding performance, recruitment success and dispersal distances of offspring arising from nest-boxes placed in either a high (HD) or a low density (LD). We found that although neither clutch size nor fledgling survival varied in relation to density, HD nests produced significantly fewer recruits. Moreover, recruits from HD nests settled closer to their natal territories than those from LD nests. These findings suggest that the proximity to neighbours incurs delayed fitness costs in the Spotless Starling. Furthermore, the negative density-dependent dispersal may result from the increased mortality of individuals in worse condition and with lower ability to compete for breeding vacancies in the vicinity of their natal territories. With this study, we provide long-term observational results that may be a basis for further experiments intended to unravel proximate mechanisms underpinning density-dependent patterns in wild breeding colonies.