The heat increment of feeding (HIF), including heat from digestion, assimilation, and nutrient interconversion, may substitute for thermogenesis and reduce thermoregulation costs. HIF and its substitution have been measured mainly in animals fed single large meals with high protein content, but many species such as some dabbling ducks (Anatini) feed more continuously in intermittent small meals with low protein content. We measured HIF in seven mallard ducks (Anas platyrhynchos) eating mixed grain (corn, wheat, milo) ad libitum while floating on water at 23 degrees C (thermoneutral) and 8 degrees C. HIF was calculated as the difference in oxygen consumption between fed and fasted birds, correcting for costs of behavior, heat storage (change in body temperature), and heating food. Substitution occurred if HIF was lower at 8 degrees C than at 23 degrees C. Food intake of mallards averaged 83% of that required for maintenance (zero energy balance) at 23 degrees C, and 68% of maintenance at 8 degrees C. Mean HIF (+/-1 SE) was 1.59+/-0.61 l O(2) at 23 degrees C and 1.48+/-0.68 l O(2) at 8 degrees C. These values were 4.9% and 3.9% of metabolizable energy intake, consistent with values expected for grain. HIF did not differ between temperatures (ANCOVA, birds as blocks, intake as covariate, P=0.51), indicating no measurable substitution at these intake levels in intermittent meals. For these large birds that feed on low-protein foods in intermittent small meals, the ecological importance of HIF substitution appears negligible during periods when food intake is below that required for energy balance.