| Summary: | In the northern Baltic Sea, Common Eiders Somateria mollissima feed mainly on mussels, taken exclusively by diving. By comparing the rate at which food is ingested (prey mass taken per time unit) with the rate of digestion (gut contents divided by known transit time), we found that the digestion rate was the major constraint on energy assimilation of female Common Eiders in the western Gulf of Finland (Tvärminne), whereas in the central Gulf of Finland (Söderskär), the limiting factor was the ingestion rate. This indicates that females at Söderskär had to work harder to secure one meal. The density of Blue Mussels Mytilus edulis of suitable size for Common Eiders was significantly lower at Söderskär compared to Tvärminne and likely the main cause for the difference. We conclude that hyperphagia at the breeding grounds was possible among females at Tvärminne, but not among females at Söderskär. Nevertheless, these differences in feeding behaviour did not translate into differences in body weights at incubation onset or clutch size. This may be due to the insensitiveness of these parameters to the ambient conditions measured, or to the fact that 1997 was in fact a benign feeding year at Söderskär. Our results suggest that females of a migratory Common Eider population to a great extent rely on reserves accumulated on the wintering grounds, but that nutrients derived from the breeding grounds may be important for maintenance of body condition and for the overall health status of a laying female.
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