Winter extratropical cyclone influence on seabird survival: variation between and within common eider Somateria mollissima populations

Extratropical cyclones (ETCs) play a primary role in determining the variation in local weather and marine conditions in the mid-latitudes. ETCs have a broad range of intensities, from benign to extreme, and their paths, frequency, and intensity may change with global warming. However, how ETCs, and cyclones in general, currently affect marine wildlife is poorly studied and remains substantially unexplored. To understand how winter ETCs affect the inter-annual variability of adult seabird survival, we used capture-mark-recapture datasets collected in 2 arctic (northern Canada and Svalbard) and 1 subarctic (northern Norway) breeding populations of common eider Somateria mollissima over periods of 19, 16 and 30 yr, respectively. We found significant negative correlations between winter ETC activity and female eider survival, but different mechanisms appear to be involved in the different studied populations. The number of winter ETCs, extreme or not, was linked to survival without lags in the Canadian population, whereas amplitude and duration of extreme winter ETCs (with time lags) impacted female adult survival in the Svalbard and northern Norway eider breeding populations. We hypothesise that fjords in the wintering grounds of some populations act as climatic shelters and provide natural protection, and hence could partly explain inter-population heterogeneity in the response to ETCs. We suggest that ETCs represent a likely mechanism behind the frequently reported relationship between North Atlantic Oscillation and seabird survival in the North Atlantic.