Australian Waterfowl Do Not Necessarily Breed on a Rising Water Level

Abstract

Nest sites of waterfowl (pacific black duck, grey teal, pink-eared duck, maned duck and Australian shelduck) in 200 tree holes were monitored monthly over 4 years in a swamp complex near Booligal, N.S.W. The most nests occurred in 1978 following a complete drawdown and reflooding of the swamps. A rising water level on an already full swamp did not result in significant nesting activity. The response to the drawdown and filling was not a simple matter of more aquatic vegetation and invertebrates being produced at this time. Samples of nekton were taken by net and quadrats, and plankton was taken by nets. The results indicated that there were more of the common invertebrates in other seasons than in that following the filling, and vegetation did not develop fully until the 1978 breeding season was nearly finished. It is postulated that the high nesting activity related to a massive production of invertebrate detritivores, particularly Chironomus tepperi, which was stimulated by the decaying organic material from the dead aquatic vegetation (e.g. Vallisneria spiralis, Carex sp., Elaeochoris sp. and algae) of the previous full period, and the flooded dead dryland vegetation (e.g. grasses). A lack of these chironomids is suggested as the cause of the low nesting rates in other seasons, when the swamps were full and food apparently was available. It is also postulated that these and other detritivores are efficiently captured and provide a superabundant food source, at the water surface and in very shallow water, for both adults and chicks. Other invertebrates are either not available to chicks in these microhabitats or are harvested too inefficiently or slowly by adults. The concept that a rising water level per se is central to the breeding ecology of some species of ducks should be abandoned. Rather, the significant phenomena are the drawdown, and the production of detritus and detritivorous invertebrates when the wetland basin is refilled.