Habitat selection by common eiders in winter and its interaction with flock size

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We studied habitat selection in relation to prey density and water depth in the common eider, Somateria mollissima L., wintering in the Gulf of St. Lawrence, eastern Canada. In this region, eiders are confronted with low temperatures, ice cover, and reduced day length. We predicted that they should select feeding habitats characterized by high prey density and shallow water to minimize the time and energy spent while diving. About 1000 flocks were localized by triangulation on our study site (20.5 km2). We inferred the diving depth and the habitat being used from the position of eiders on bathymetric and community maps. The highest density of prey occurred in shallow water reefs where there were patches of blue mussels, Mytilus edulis L., and green sea urchins, Strongylocentrotus droebachienensis (Müller). Despite the fact that eiders can dive to depths as great as 42 m to feed, they strongly aggregate in shallow water, and their distribution closely coincides with the highest density of prey. The degree of selection for the reef habitat varies with seasonal variations in the size of flocks and in the total number of eiders present. Although flocking as an antipredator behaviour cannot be rejected, we interpret the high degree of flocking by eiders in our study area as a strategy to facilitate feeding in winter.