LOW COSTS OF SALT TURNOVER IN COMMON EIDERS SOMATERIA-MOLLISSIMA

The energetic costs of salt turnover were measured in Common Elders So materia mollissima by respirometry. Rates of salt intake in the field were estimated from salt content of their chief prey, the mussel Mytil us edulis. Metabolic rate in Elders increases in response to salt inta ke 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 a bout 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 muss els collected in the Wadden Sea, the costs of salt turnover are calcul ated at 2.0-2.4% of metabolizable energy intake. In the field, salt in take per foraging bout reaches 4.3 g and salt excretion, as infered fr om the frequency of head shaking, continues for over one h. The direct salt intake in wintering Elders 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 Elders drink some water after swallowing a mussel. The mass-specific salt intake is higher than reported for ot her marine birds. The maximum rate of salt excretion in Elders, as cal culated from published data, hardly matches the rate of salt intake of feeding Elders. It is concluded that the capacity for salt excretion may limit the rate of food intake in birds feeding on whole-shelled bi valves in times of high food demands and low flesh contents of the biv alves.