Date of Award

1-1-2001

Document Type

Thesis

Publisher

Edith Cowan University

Degree Name

Master of Science

Faculty

Faculty of Communications, Health and Science

First Supervisor

Dr Mark Lund

Second Supervisor

Dr Allan Burbidge

Third Supervisor

Professor Harry Recher

Abstract

A colony of Great Egrets (Ardea alba) and Nankeen Night Herons (Nycticorax caledonicus) has existed at the Perth Zoo in Metropolitan Perth for over 25 years. The colony is particularly significant for the conservation and management of Great Egrets in Western Australia as it is the only colony located in the Metropolitan area. Baseline information of their breeding biology was needed to facilitate the development of management guidelines for the zoo colony. Foraging behaviour was used to highlight specific adaptations in hunting strategies and diet. However, it was not possible to observe foraging Nankeen Night Herons as they forage at night. Therefore, another species, the Little Egret, was selected to highlight specific adaptations. From 1997 to 1999 the reproductive biology of the Great Egret and Nankeen Night Heron was assessed. The number of Nankeen Night Herons nesting at the Perth Zoo from 1996 to 1998 increased, while the number of Great Egrets declined. Both species nested in tall trees but only Great Egrets were specific in their choice of nesting tree species. Horizontal nest placement appeared to be influenced by body size. Great Egrets had a larger clutch size than the Nankeen Night Heron, and a slightly higher offspring mortality rate. There was some indication that Great Egrets may use the colony as an information centre about productive feeding grounds. Foraging behaviour of Great Egrets and Little Egrets was recorded at six wetlands in the Perth Metropolitan area. Great Egrets were found to be mainly searchers using ‘stand and wait’ and 'walk slowly’ foraging behaviour, while the Little Egret was a ‘pursuer’ hunting by 'walking slowly', 'walking quickly' and 'pursuing prey'. Great Egrets caught a greater number of prey per attempt at capture, feeding on larger sized prey, mostly fish. Little Egrets fed on smaller sized prey, mostly invertebrates. Habitat type and wind speed had a significant effect on striking success of Great Egrets. Cloud cover, wind speed and direction had a significant effect on striking success of Little Egrets. The larger body size of the Great Egret allowed them to forage in deeper water than the Little Egret. Baseline information provided by this study has assisted in the development of management recommendations for the zoo colony and for Great Egrets and Little Egrets in the Perth Metropolitan area. To provide long-term information on overall population trends for the colony, regular counting and population distribution mapping of Great Egrets and Nankeen Night Herons is needed. To prevent Nankeen Night Heron numbers elevating and possibly encroaching on the nesting habitat of the Great Egret, food available in the zoo grounds should be reduced by covering caged animals food. Planting of nesting trees within the existing colony may be required to enable the number of Great Egrets nesting in the Perth Zoo to increase. To prevent disturbances to birds when foraging within Perth wetlands, sites that are reachable by humans should be fenced off, or access restricted.

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