Surf scoter (Melanitta perspicillata) populations wintering in the Chesapeake Bay primarily prey on two food items, the hooked mussel (Ischadium recurvum) and dwarf surfclam ( Mulinia lateralis). The decline of oyster reefs (Crassostrea virginica) has decreased availability of mussels inducing surf scoters to switch to a more opportune food item, the dwarf surfclam. The objectives of this study were: (1) to determine the comparative nutrient composition of these prey items; (2) to evaluate the energy assimilated by surf scoters from these prey items; (3) to determine the functional responses of scoters foraging on four different ecologically relevant densities (30, 100, 1000, and 3000 m^-2) of each prey item; and (4) to model the foraging value (costs - benefits) of both prey items for surf scoters. I. recurvum contained higher ash, protein, lipid, and energy per item than M. lateralis. Metabolizable energy from each prey item by surf scoters was 83% for M. lateralis and 87% for I. recurvum . The shell strength of I. recurvum was significantly stronger than M. lateralis. For scoters foraging in a large diving tank 2 m deep, intake (# s^-1) for M. lateralis was significantly higher than I. recurvum at high densities, but lower at the low densities. Gross energy intake (kJ s ^-1) and metabolized energy intake (kJ s^-1) were significantly greater for I. recurvum than M. lateralis. Based on nutrient content, metabolizability, behavior and intake rates, and energy expenditure at naturally occurring densities, the foraging value for M. lateralis was significantly lower than I. recurvum. Despite higher ash content and harder shell, which would partly offset the apparent energetic advantages of I. recurvum, greater foraging value of I. recurvum than M. lateralis provides a more beneficial prey item for wintering surf scoters. Therefore, wintering surf scoters must adapt in order to maintain their daily energy requirement. If surf scoters are forced to feed primarily on M. lateralis, the most advantageous and available prey in the Chesapeake Bay, instead of I. recurvum; there may be insufficient energy for them to build fat reserves needed to make migration.