Common Eider Social behaviour
 Eiders are colonial breeders. They nest on coastal islands in colonies ranging in size of less than 100 to upwards of 10,000-15,000 individuals. Chapdelaine, G., P. Dupuis and A. Reed. Canadian Wildlife Service Report Series no. Female eiders often return to breed on the same island where they were hatched. This can lead to a high degree of relatedness between individuals nesting on the same island.