Ecological Studies of the Black Swan IV.* The Timing and Success of Breeding on Two Nearby Lakes on the Southern Tablelands of New South Wales

The breeding of black swans on two nearby lakes, 18 km apart, was documented over a 5-year period. The lakes differed in physiography and in botanical features. The numbers of swans breeding and the timing of breeding differed markedly between the lakes and coincided with gross differences and changes in observed abundance in aquatic plants known to be important as food for swans. Important factors affecting the selection of nest sites were social factors, security, the presence of an adequate substrate and of materials to construct a nest, and ease of access to water. Within each breeding season the timing of laying often differed among nesting colonies on the one lake, apparently reflecting an influence from social factors in synchronizing breeding within a colony. Breeding success was poor, due evidently to failure of cygnets to survive in the first weeks after hatching and to desertion of eggs in nests. Failure of food supplies may be the main cause. The reproductive strategy of the black swan appears to be essentially opportunistic; the female is capable of laying repeatedly over an extended period, so that eggs are often produced at times that do not necessarily predict suitable conditions for either the survival of cygnets or even the completion of incubation.