Bias in Food Habits of Australian Waterfowl.

Abstract

Most food studies of Australian waterfowl have relied on gizzard analyses. This introduces bias because of differential digestion rates. Oesophageal and gizzard contents collected from feeding grey teal (Anas gibberifrons) and pink-eared duck (Malacorhynchus membranaceus) at 2 sites in south-western New South Wales were determined. Pink-eared duck ate 99.6% animal food (mainly chironomid larvae and ostracods); grey teal ate 74.2% animal food (mainly corixids and dipteran larvae) at one site but 63.9% plant food (mainly grass seeds) at the other. Both species contained higher proportions of animal material in their oesophagi than in their gizzards. Rank correlation analyses showed no significant relationships between oesophageal and gizzard contents in the 2 species in this study, nor in other Australian and overseas waterfowl reported in the literature. It is suggested that gizzard analysis has led to overestimation of the importance of seeds in the diet of Australian waterfowl and underestimation of the importance of invertebrates. It is recommended that future food habits studies of waterfowl use oesophageal contents from feeding birds, and results from previous gizzard studies be treated with caution.