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Technical Memoranda 4
Recher HR and Holmes RT
Supervising Scientist, 1982
ISSN 0810-9532
ISBN 0 644 01216 1
Five species of diurnal herons (Ardeinae) are common on the Magela Creek flood plain and forage along the edges of natural and artificial waterbodies both inside and outside the Ranger Uranium Project Area. Insects, frogs and fish are the most commonly taken prey. The species of heron differ in the kinds and sizes of prey they take, their foraging location, degree of sociality and foraging behaviour. The range of foraging behaviours is similar to that described for North American herons, but the Australian birds differ in ways that may be related to the generally drier conditions that prevail over the Australian continent.
Little Egret Egretta garzetta and Pied Heron Ardea picata were the most active hunters. The Great Egret E. alba, Plumed Egret E. intermedia, and White-necked Heron A. pacifica foraged predominantly by 'standing and waiting' or by 'walking slowly'. The Pied Heron foraged mostly on dry land and took small insects from low vegetation. The Plumed Egret and White-necked Heron foraged in shallow water where there was dense emergent vegetation. Mostly they took small fish, tadpoles and frogs. The Little Egret and Great Egret hunted in open water habitats. The Little Egret was restricted to shallow water in drying pools or edges along channels and took small fish and insects. The Great Egret hunted in deeper water where it took relatively large fish. All the herons congregated where prey were concentrated by the receding waters of the flood plain.
Because it takes relatively large fish, the Great Egret is most likely to be affected by any contamination of the aquatic environment by heavy metals or radionuclides. The Nankeen Night Heron Nycticorax caledonicus hunts at night and was not studied. However, it is also abundant on the Magela Creek flood plain and probably feeds on large fish and frogs. It would also be at risk from any contamination of the aquatic environment. The other herons take smaller or immature prey or hunt mostly in terrestrial habitats. They are therefore less likely to be affected by contamination of the aquatic environment.
Our observations were confined to a small part of the seasonal cycle and all species probably hunt in different ways and places and take different kinds and sizes of prey during the different times of the year. It is necessary to obtain information on foraging habits and diets throughout the year before the risk to all species of ardeids can be fully ascertained.