Work against buoyancy during submergence is a large component of the energy costs for shallow diving ducks. For penguins, buoyancy is less of a problem, however they still seem to trade-off levels of oxygen stores against the costs and benefits of buoyant force during descent and ascent. This trade-off is presumably achieved by increasing air sac volume and hence pre-dive buoyancy (Bpre) when diving deeper. Tufted ducks, Aythya fuligula, almost always dive with nearly full oxygen stores so these cannot be increased. However, the high natural buoyancy of tufted ducks guarantees a passive ascent, so they might be expected to decrease Bpre before particularly deep, long dives to reduce the energy costs of diving. Body heat lost to the water can also be a cause of substantial energy expenditure during a dive, both through dissipation to the ambient environment and through the heating of ingested food and water. Thus dive depth (dd), duration and food type can influence how much heat energy is lost during a dive. The present study investigated the relationship between certain physiological and behavioural adjustments by tufted ducks to dd and food type. Changes in Bpre, deep body temperature (Tb) and dive time budgeting of four ducks were measured when diving to two different depths (1.5 and 5.7 m), and for two types of food (mussels and mealworms). The hypothesis was that in tufted ducks, Bpre decreases as dd increases. The ducks did not change Bpre in response to different diving depths, and thus the hypothesis was rejected. Tb was largely unaffected by dives to either depth. However, diving behaviour changed at the greater dd, including an increase in dive duration and vertical descent speed. Behaviour also changed depending on the food type, including an increase in foraging duration and vertical descent speed when mussels were present. Behavioural changes seem to represent the major adjustment made by tufted ducks in response to changes in their diving environment.