Silver gull breeding at two colonies in the Sydney-Wollongong regio, Australia

Abstract

Silver gulls have increased in abundance in Sydney-Wollongong this century. Both the number of colonies and their sizes have grown. The largest colony of 43 000-50000 pairs is on Big I., off Wollongong. Breeding was compared at this colony with that at a satellite colony on artificial structures in Rozelle Bay, Sydney Harbour. Breeding occurred from July to February at Big I., a lengthening of the season since the 1960s. At Rozelle Bay, breeding occurred almost all year round. Egg and clutch size varied within and between years at Big I. First-eggs of clutches, laid during the first breeding peak, were significantly larger than those laid later in each breeding season. The size of third-eggs in clutches differed between years. It is proposed that these disparities were a result of differences in food availability between years. Clutch sizes were significantly larger during the first breeding peak than later clutches of each season. Clutch size was larger at Rozelle Bay. Variation in clutch size was linked to quality of individuals and food availability. Breeding success at colonies was low, indicating that carrying capacities have been reached.