Magpie Goose, Anseranas-Semipalmata, Nesting on the Mary River Floodplain, Northern-Territory, Australia - Extent and Frequency of Flooding Losses

Magpie geese, Anseranas semipalmata, nest synchronously in large colonies on the seasonally inundated floodplains of the Northern Territory. Erratically variable wet season rainfalls may cause rises in floodplain water levels that destroy many nests. We describe such an event on the Mary River foodplain in which all of the nests within a monitored study area were drowned. Vulnerability to flooding (as indicated by variation in the depth at which eggs first submerged) varied significantly among vegetation types. Mean rises in water levels causing inundation of eggs varied from 67 cm in nests constructed on a base of Oryza rufipogon to 118 cm on nests constructed on Eleocharis brassii. Nests close to major river channels are more vulnerable to flooding. Analysis of patterns of river level rises since 1959 indicate that such catastrophic nest losses are relatively common, perhaps occurring as often as one year in seven. Lesser flooding losses probably occur in most years. Nesting colonies on the Mary River are likely to suffer higher flooding mortalities than on many other river systems.