Abstract

The effect of supplementary feeding and artificial twinning on nestling growth and survival in Australasian Gannets (Morus serrator) breeding at Pope’s Eye Marine Reserve in Port Phillip Bay, Victoria, was investigated in 1997–98 and 1998–99. Australasian Gannets were capable of raising additional young; however, twin young were generally of poorer quality, had slower development, attained lower maximum weights, and had lower survival rates. Chicks given supplementary food attained maximum weights greater than both single and twin chicks. The reduction in breeding performance of gannets provided with additional offspring, coupled with the greater fledgling masses of supplementary-fed chicks, suggests that parents were unable to provide sufficient food for two young, indicating that this population is potentially limited by food availability. That clutch size in this species is limited to a single egg is supported by their apparent inability to successfully incubate two eggs. Recent increases in the size of the Australasian Gannet population in Victoria indicate that other factors, such as natural or anthropogenic perturbations in environmental conditions may also be important in mediating population changes.