Early Growth of Magpie Geese, Anseranas-Semipalmata - Sex-Differences and Influence of Egg Size

Abstract

The magpie goose, Anseranas semipalmata, is the most conspicuous and among the most numerous of waterfowl inhabiting the seasonal wetlands of tropical northern Australia. Modelling of juvenile growth rates of captive birds with the logistic equation showed that magpie geese grow at rates similar to the fast-growing anserine geese. High juvenile growth rates permit exploitation of seasonally favourable environments that, although highly productive, may deteriorate rapidly and unpredictably. Sex differences in juvenile growth patterns are consistent with pronounced sexual dimorphism in adult body size (larger males), which is associated with an unusual polygynous mating system. Although smaller, females grow relatively faster than males, so that at a given age they have completed a greater proportion of the pre-fledging growth phase; wing growth is particularly advanced and females fly earlier. Slower development may increase mortality among juvenile males when family groups are forced to abandon drying swamps, and contribute to skewing of the sex ratio towards females. Larger hatchlings from large eggs tend to maintain a size advantage, at least until fledging.