JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. Wiley and Nordic Society Oikos are collaborating with JSTOR to digitize, preserve and extend access to Ornis Scandinavica. P. 1993. Determinate laying and egg attendance in Common Eiders. -Ornis Scand. 24: 48-52.Egg laying, egg attendance by females, and egg loss were studied in the Common Eider Somateria mollissima. Clutch size fixation appeared to occur before the first egg was laid (determinate laying). Intervals between the laying of eggs were usually about 24 h. Permanent egg attendance started before clutch completion (4 to 6 eggs), usually from the second egg on, and was independent of the actual number of eggs in the nest and final clutch size. Egg loss during inattentive periods was 80 times higher than during attendance. Spontaneous breaks in incubation occurred at intervals of 1-3 days, but sometimes a female left 2 or 3 times a day. Breaks were used for drinking and lasted between a few min and 2.5 h. Females left most frequently in the first few hours after sunset. The ducklings hatched nearly synchronously except the last egg, which hatched on average 8 h later than the others. Early egg attendance, fasting during incubation, and leaves during night hours will have evolved for clutch protection.