Broedvogelstand en reproductie van de Eidereend Somateria mollissima op Vlieland in 1994 en 1995

The number of breeding pairs and breeding success of Eiders nesting on Vlieland (Dutch Waddenzee) was assessed in 1994 and 1995. The breeding numbers were assessed on the basis of a count of Eiders at and around the island in the beginning of the breeding season (early May; table 1). The next step was a nest search, during which information on the clutch size and the amount of predated nests was obtained. In 1995, a considerably smaller number of nests was found than in 1994 (table 2), and a greater proportion appeared to have been predated (table 3). Damage on several of the egg shells observed indicated predation by rats which only very recently became established on the island and the population of which increased dramatically. Breeding success was based on the number of roosting ducklings which gathered on the shore in early July. After two years without any reproductive success, small numbers of ducklings were observed in 1994 (138 pulli), and some 747 were observed in 1995. The latter year can be regarded as moderately successful, the first year was rather poor. The two census methods had a remarkably different outcome. Future studies will need to focus on the reliability of either method.