Social-Behavior and Colony Formation in a Population of Crested Terns, Sterna-Bergii, in Southwestern Australia

Abstract

In a breeding population of crested terns near Fremantle, W.A., social behaviour involved complex patterns of social displays and paired behaviour. The social phase was probably truncated over much of the extended breeding season by the presence of incubating conspecifics, and may serve to synchronise laying in their absence. The responses of pre-laying crested terns to incubating conspecifics were studied by means of artificial 'colonies' of polyurethane models. These proved to be most attractive to prebreeding pairs searching for nest sites, and the earliest eggs were invariably laid among the decoys. Pre-breeding terns in the social phase were not attracted to the decoys. It is suggested that loose groups of incubating conspecifics acted as a key stimulus, releasing settlement and laying in birds in an advanced state of reproductive readiness. This key stimulus could change the learned location of colonies but during late summer and early autumn other factors, probably related to food availability, controlled the onset of laying. Small, spatially distinct, nesting groups were less synchronous in their laying than larger colonies. Such small groups are thought to result from discontinuities in reproductive phase between groups of terns, which are not apparent when the number of pre-laying birds is large.