Low costs of salt turnover in Common Eiders Somateria mollissima

The energetic costs of salt turnover were measured in Common Eiders Somateria mollissima by respirometry. Rates of salt intake in the field were estimated from salt content of their chief prey, the mussel Mytilus edulis. Metabolic rate in Eiders increases in response to salt intake and may double for a short time following a load of 5 g NaCl. With a load of 2.5 g and 5 g NaCl the metabolic rate remains elevated for about one h. Overall costs of salt turnover are estimated at 1.3 kJ g-1 NaCl. From the proportion of water content to flesh content of mussels collected in the Wadden Sea, the costs of salt turnover are calculated at 2.0-2.4% of metabolizable energy intake. In the field, salt intake per foraging bout reaches 4.3 g and salt excretion, as inferred from the frequency of head shaking, continues for over one h. The direct salt intake in wintering Eiders reaches about 50 g per day leading to a mass-specific salt intake of 24 mg g-1 body mass. However, total intake is somewhat higher as Eiders drink some water after swallowing a mussel. The mass-specific salt intake is higher than reported for other marine birds. The maximum rate of salt excretion in Eiders, as calculated from published data, hardly matches the rate of salt intake of feeding Eiders. It is concluded that the capacity for salt excretion may limit the rate of food intake in birds feeding on whole-shelled bivalves in times of high food demands and low flesh contents of the bivalves.

Nehls G., ARDEA 84 (1): 23-30