Nest site selection in the Common Eider Somateria mollissima: differences between archipelago zones

Abstract

The nest site selection and reproductive traits were studied in Eiders breeding in three zones of the Finnish archipelago . These differed in the degree of isolation (safety from terrestrial predators), the vegetation and human disturbance . The data were gathered in summer 1988 and comprised 52 islands and 1057 Eider nests . Both the numbers of Eiders and their density were higher in the outer zone, where the birds bred on all the islands . In the inner zone, they preferred the smaller and most isolated islands. The nests were located further from the water and more of them were covered in the outer zone than elsewhere. Outer zone Eiders bred earliest and were shyest, and their nests were least frequently robbed . These differences were probably due to the earlier break-up of the ice, better protection from terrestrial predators and weaker human disturbance in the outer zone. In general, covered nests contained more eggs than open ones. The Eiders laid later and more of their nests were robbed on disturbed than on undisturbed islands . On open (well-isolated and less disturbed) islands, the nest density was higher, breeding occurred earlier and fewer nests were robbed. In the principal component analysis, the first principal component (interpreted as safety from predators) accounted for 43% of the variation in nest density and20% of the variation in the distance of the nest from the shore, the laying date and flushing distance . The second principal component (interpreted as island size) explained 40% of the variation in the distance of the nest from the shore and 10% of the variation in nest density . The third principal component (in terpreted as archipelago zone) explained 56% of the variation in clutch size and 20% of the variation in the laying date and flushing distance offemale Eiders . The Eiders preferred well-isolated islands that were not disturbed by humans and had enough vegetation to offer cover for the nest ; the type of island (open vs . wooded) was not important, but the islands fulfilling these conditions were mostly open . Eiders were able to breed successfully in all zones, but the inner zone seemed to be apoorer breeding habitat, as human disturbance and terrestrial predators reduced the number of islands suitable for nesting .