Variation in egg size of Common Eiders

Egg size of Eiders Somateria mollissima was studied in the colony on Vlieland, The Netherlands, during six consecutive years. Mean length and breadth of 2476 eggs was 77.84 and 51.25 mm, respectively. Mean volume measured of 1882 of these eggs was 104.03 cm-3. A large variation in egg size was found, the volume of the smallest egg being only 53 of the largest one. Major factors influencing the size variation appeared to be differences between females and the position of the egg in the laying sequence of the clutch. Differences in egg size between females were not related to their body size. The first egg in a clutch was smaller than the second, but the last egg was the smallest. Also clutch size and age of the female contributed to the variation. Females produced smaller eggs as they grew older. Age affected the last eggs in the clutch more than the first ones. Mean egg size varied only slightly among years, though temperatures in winter and food conditions in spring varied considerably during the study.

Swennen C. & Van Der Meer J., ARDEA 80 (3): 363-373