Body cooling and its energetic implications for feeding and diving of tufted ducks

Wintering in a temperate climate with low water temperatures is energetical ly expensive for diving ducks. The energy costs associated with body coolin g due to diving and ingesting large amounts of cold food were measured in t ufted ducks (Aythya fuligula) feeding on zebra mussels (Dreissena polymorph a), using implanted heart rate and body temperature transmitters. The effec ts of diving depth and food ingestion were measured in two sets of experime nts: we measured body cooling and energy costs of six tufted ducks diving t o different depths in a 6-m-deep indoor tank; the costs for food ingestion and crushing mussel shells were assessed under seminatural winter condition s with the same ducks feeding on mussels in a 1.5-m-deep outdoor pond. Body temperature dropped during feeding bouts and increased gradually during in termittent resting periods. The temperature drop increased linearly with di ve duration. The rate of body cooling increased with feeding depth, but it was lower again at depths below 4 m. Half of the increment in energy costs of diving can be attributed to thermoregulatory heat production, of which a pproximately 50% is generated after diving to warm up the body. The excess costs for ducks feeding on large-sized mussels could be entirely explained by the estimated energy cost necessary to compensate the heat loss followin g food ingestion, suggesting that the heat production from shell crushing s ubstituted for thermoregulation. Recovery from heat loss is probably a majo r component of the activity budget of wintering diving ducks.