Food resources available to diving ducks wintering on the Great Lakes have changed dramatically since the introduction of dreissenid mussels (Dreissena bugensis and D. polymorpha). We investigated the diets of Buffleheads (Bucephala albeola), Common Goldeneyes (B. clangula), and Long-tailed Ducks (Clangula hyemalis) during winter, 2002–2004, on northeastern Lake Ontario and determined the levels of dietary overlap. Dietary niche-breadth values were low, and dietary overlap values (prey size and type) were high for all species. Ducks primarily consumed high-quality, energy-dense prey (Amphipoda, Chironomidae), which were abundant. Our results highlighted three patterns: (1) dreissenid mussels constituted 85% of the macroinvertebrate community in Lake Ontario but were consumed in relatively low amounts during winter, (2) foods of high energy-density such as Amphipoda and Chironomidae were likely abundant enough for ducks to selectively feed on them, and (3) some constraint caused ducks to select energy-dense prey instead of the most available items (dreissenid mussels). Although the abundance of prey may have allowed numbers of diving ducks to increase in the past few decades on the Great Lakes, the long-term implications of high levels of dietary overlap among diving ducks is relatively unknown and warrants continued monitoring.