Breeding Success of Egrets Related to Rainfall: A Six-Year Australian Study

The breeding success of four egret species (Egretta alba modesta, E, intermedia Plumifera, E. garzetta immaculata and Ardeola ibis coromandus) was monitored over six seasons at two colonies in the Hunter Valley, New South Wales, Australia, For all species the mean number of young fledged per successful nest fluctuated among breeding seasons, with the greatest success in wet years. In decreasing order, the greatest fluctuations occurred in E. garzetta, E. alba and E. intermedia. The breeding success of A. ibis did not vary consistently with wet and dry breeding seasons. The three Egretta species are, to varying degrees, aquatic feeders, whereas A. ibis feeds mostly in dryland habitats and is less dependent on wetlands than the other three species. We postulate that food availability is the ultimate factor controlling breeding success.