Effect of food shortage on the reproductive output of Common Eiders Somateria mollissima breeding at griend (Wadden Sea)

Following a food-induced major mortality of Common Eiders Somateria mollissima in the Dutch Wadden Sea in winter and spring 1999/2000, the reproductive output at Griend was evaluated in comparison with previous seasons. In 2000, the number of breeding pairs showed a decline of 38% and female Common Eiders commenced breeding some 2-3 weeks later than in three previous years. Mean clutch size in 2000 (4.6 eggs clutch-1) was similar to 1999 (4.9 eggs clutch-1), but the hatching probability declined from 0.41 in 1999 to 0.18 in 2000. In 2000, the majority of the nests were deserted prior to hatching, often a few weeks after incubating had started. The number of chicks hatched in 2000 was only a quarter of that in 1999. Observations elsewhere in the Dutch Wadden Sea indicated similar poor breeding results. We suggest that a food shortage in winter and spring was the principal cause for the low reproductive output of Common Eiders in 2000. Adult females probably failed to accumulate sufficient energy stores needed for their prolonged fast during laying and incubation.