Diving ducks face the strongly cooling properties of aquatic environments. In experiments with Tufted Ducks Aythya fuligula feeding on zebra mussels Dreissena polymorpha in outdoor cages, we measured changes in food consumption and diving behaviour in relation to water temperature (3-22°C). Water efflux and daily energy expenditure (DEE) were measured using doubly-labelled water. Tufted Ducks swallow mussels whole and crush the shells in the gizzard. Due to the low flesh content of mussels, daily mussel consumption was extremely high in winter (up to 3 times the duck body mass of 600 g). Water efflux was 6.5 to 13 times higher than allometrically predicted, with a maximum of 1.11·d-1. Because of high water turnover rates, DEE estimates from doubly-labelled water measurements were variable, but on average agreed with DEE estimates obtained from food consumption and time-energy budgets (ca. 5% higher values). DEE increased with lower temperatures from 2.5 (summer, 20°C) to more than 4 times the basal metabolic rate (winter, 3°C). Costs of thermoregulation and heating up ingested mussels (as estimated from the theoretical caloric heat) primarily explained the high DEE in winter. At lower temperatures, dive duration was shorter and less time was spent selecting small mussels at the bottom, while more larger mussels were brought up and were ingested at the water surface. Thereby, the energy costs of crushing shells probably increased in favour of reducing diving costs. The narrow margins of the energy budget, wherein costs must be balanced, imply that food availability and water temperature are important attributes to winter distributions of diving ducks.