Bregnballe T. & Frederiksen M. (2021) Conspecific density as a driver of offspring body condition in three Cormorant colonies in Denmark. ARDEA 109 (3): 593-608

Density-dependent depletion of prey during the breeding season may affect breeding performance and colony development in seabirds and colonial waterbirds. Breeding Great Cormorants Phalacrocorax carbo sinensis are central place foragers, so parental flight distances to profitable foraging grounds and thus parental provisioning of offspring are likely affected by local food availability. Chick body condition in a given colony may therefore be influenced by the distance to feeding areas, changes in fish stocks and fish distribution as well as the number of Cormorants from neighbouring colonies using the same feeding areas. At three Danish Great Cormorant colonies with overlapping foraging ranges and variable access to shallow marine areas, Vorsø (VO), Mågeøerne (MA) and Stavns Fjord (SF), we investigated variation in mean chick body condition (n = 9697) in relation to colony location, year, colony size and estimated density of foraging conspecifics during 1993–2007. Foraging areas available to VO colony parents had the highest theoretical densities of conspecifics and the VO chicks were invariably in poorer condition than the chicks at the other two colonies. Chick body condition was negatively correlated with estimated foraging density, especially within a foraging range of 20 km. Body condition between all three colonies was weakly correlated over the years, suggesting that changes in macro-environmental conditions (e.g. fish density) only had moderate impacts on food availability, independently of Cormorant numbers. We conclude that both the geographical location of colonies, the size of the colony itself and the nearest neighbouring colonies and the associated variation in density of foraging Cormorants were major drivers of variation in chick body condition between colonies and years.