Observations on a Great Egret Ardea alba and Nankeen Night Heron Nycticorax caledonicus colony at the Perth Zoo, Western Australia

A colony of Great Egrets Ardea alba and Nankeen Night Herons Nycticorax caledonicus has existed at the Perth Zoo, Western Australia for over 25 years. The colony of egrets is one of very few in the region and hence is significant for the conservation and management of Great Egrets in Western Australia. From 1996 to 1998, surveys were conducted to determine the number of breeding pairs, clutch size, breeding success and nest site selection of birds in the colony. Most observations were ground based, but a 30-metre cherry picker was used to inspect nests and determine clutch size. One hundred and thirty night heron and 49 egret nests were found in 1996; 92 night heron and 41 egret nests in 1997; and 153 night heron and 36 egret nests in 1998. Nesting commenced in September, with peak numbers in early November. Both species nested in tall trees well above zoo visitors and animals. Egrets nested only in pines, whereas night herons nested mainly in figs. Great Egrets had and an average clutch size of 2.6-2.7 by early November compared with a clutch of 1.6-1.8 for Nankeen Night Herons. It was estimated that 95 per cent of egret eggs and 89 per cent of night heron eggs laid by early November in 1997 hatched and that 53 percent of egret eggs (1.4 per nest) and 65 per cent of night heron eggs (1.2 per nest) survived to fledging. Incomplete data precluded estimates of breeding success for 1996 and 1998. The colony appears viable, but its long-term survival depends on the on-going provision of nest trees within the zoo and the conservation of habitat within the Perth metropolitan area.

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