The number of nesting eiders was obtained from direct total counts of nests or incubating females. Observers usually visited the islands once or twice during the incubation period. Some larger islands were not completely covered. The number of nests was in those cases obtained from extrapolations of nest counts of selected parts of the islands. The counting methods used were constant over years. The number of nests found in a study region is dependent on both population size, i.e. the numbers of adult females in the region, and the frequency of non-breeding (Coulson, 1984 and Coulson, 2010). Mean egg clutch size values were obtained from samples within colonies at Ertholmene, Gotland and Tvärminne.