Survival of female common eiders Somateria m. mollissima in a declining population of the northern Baltic Sea.

In long-lived species, adult survival is the population parameter having the highest elasticity, and therefore, it can be expected to be least affected by climatic variations. We studied the dynamics and survival of breeding female common eiders Somateria mollissima mollissima in the Baltic Sea from 1960 to 2007. Using nest censuses and capture-recapture methods, we investigated: (1) the annual apparent survival (phi) of breeding females, (2) the survival-mediated population fluctuation, (3) weather effects on survival, and (4) long-term population trends. Based on capture histories of 6,393 females, average phi was 0.882 (95% confidence interval 0.864, 0.899). We found no relationship between population growth rate and survival. Furthermore, the highest ranking models, based on Akaike's information criterion, indicated no effect of the North Atlantic Oscillation index on the phi of eider females. Population size, assessed from nest counts, has been steadily declining since 1985. Despite the long time series (48 years), the overall variation in the phi rates remained comparatively narrow, at maximum ranging only 10% between 2 consecutive years. Results imply that declining female survival is not the driving force behind the population decline, and we hypothesize that the overall poor fledging success and the consequent low recruitment explain the decreasing trend of nest densities since 1985.