SummaryThe Magpie GooseAnseranas semipalmatashows substantial sexual dimorphism in body size, with males being 34% heavier. In at least one habitat (homogeneous sedge swamps) this dimorphism appears to be associated with slight divergence in foraging niche. The size distribution ofEleocharis dulcistubers ingested by Magpie Geese is similar to their availability in swamp sediments, but there are substantial differences between sexes and suggestions of variation between age groups. Males take more of the larger tubers than females, and juvenile (first-year) birds may ingest a greater proportion of small tubers than birds more than one year old. We suggest that this divergence arises through a mix of size, social and behavioural factors. Within family groups larger males may gain prior access to undisturbed foraging sites, and be better able to exploit cues to tuber size that may be associated with deeper-rooted larger plants. Juveniles accustomed to foraging for seeds in breeding areas may be slow to adopt their parents' foraging strategies after moving to dry season habitat.