Impact of Water Diversions on Colonially-Nesting Waterbirds in the Macquarie Marshes of Arid Australia

Abstract

-The building of dams and subsequent diversion of water from the Macquarie River have had a significant impact on the breeding of colonial waterbirds in the Macquarie Marshes, New South Wales, southeastern Australia. Annual flows (1978, 1986-1996), measured at Oxley where the Marshes begin, were significantly related to total colony size (number of nests) and sizes of six nesting Ciconiidae (Intermediate Egret Ardea intermedia, Rufous Night Heron Nycticorax caledonicus, Glossy Ibis Plegadisfalcinellus, Straw-necked Ibis Threskiornis spinicollis, Australian White Ibis Threskiornis mollucca and Royal Spoonbill Platalea regia) colonies. We used these results and available historical flow data extending back to 1944 to demonstrate the probable impact of water diversions on breeding in these colonial nesting species. The impact of Burrendong Dam, opened in 1967, and Windamere Dam, opened in 1984, on the numbers of nests would have been significant. Instead of a colony of more than 100,000 nests in 1969, a colony of less than 10,000 probably established. Similarly, in 1984, a colony of more than 100,000 could have been established with no water diversions, but in all likelihood the colony was less than 5,000. Generally colony sizes were significantly less (100,000 over 11 years) than would be expected without diversions of water upstream. Colony sizes of more than 100,000 nests estimated during the floods of the 1950s are unlikely to occur again. Numbers of annual breeding events also declined with water diversions from ten to seven (1963-1973); eight to seven (1974-1984) and eight to five (1985-1995). Received 18 December 1997, accepted 18 May 1998.