Condition dependent strategies of egg size variation in the Common Eider Somateria mollissima

Abstract We analysed intraclutch egg-size variation in relation to clutch size and to female body condition in the Common Eider Somateria mollissima during an 8-year period. The aim was to assess if eiders adaptively adjusted egg size within the laying sequence in response to different clutch sizes, which potentially could optimise reproductive success through a size advantage in hatchlings. The analyses were performed on both population and individual level using data from recaptured females that changed clutch size between seasons. Based on 1,099 clutches from 812 individual females, population clutch size averaged 4.13 eggs (range: 1-6), with marked annual variation in the dominance of 4- and 5-egg clutches, which constituted c. 70% of all clutches. Clutch size was positively related to female pre-laying body condition at both the population and individual level. Egg size varied significantly within and between clutch sizes and changes were significantly related to the laying sequence. First eggs were significantly larger in 4-egg clutches and second eggs smaller (marginally insignificant) than in 5-egg clutches, a pattern also found among individual females changing clutch size between seasons. The relationship between female pre-laying body condition, clutch size and the intraclutch egg-size pattern indicates that both clutch size and egg size is actively adapted to the pre-breeding body condition of the female. We suggest that the observed pattern of intraclutch egg-size variation reflects a finely tuned conditional dependent mechanisms that enable females in a suboptimal condition to optimize reproductive output.