Winter extra-tropical cyclones as a driver of seabird survival: variation between and within Common eider populations

Mid-latitude atmospheric variability is mainly driven by a type of cyclone, the extra-tropical cyclones (ETCs) that have a primary role in determining local weather and its variation, inducing strong winds, precipitation, and temperature changes. 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. Indeed, only few studies have examined the impact of tropical cyclones, another kind of cyclones, on the temporal variation of seabird survival and no study has explored the potential impact of ETCs, although the latters could be potential mechanisms behind some winter NAO-survival relationships highlighted in previous studies. A fortiori, very little has been done to study the potential heterogeneity within or between populations with different winter migratory tactics and undergoing different winter environmental conditions. We used capture-mark-recapture (CMR) data sets collected in two arctic (northern Canada and Svalbard) and one subarctic (northern Norway) populations of Common eider, Somateria mollissima, over periods of 19, 16 and 30 years, respectively and corresponding datasets of winter ETCs in each wintering area to explore their link with the temporal variation of adult annual survival. We found significant and negative correlations between ETC activity and eider survival but different mechanisms seemed to be involved among the studied populations and could explain part of observed winter NAO effects. The number of ETCs, extreme or not, was directly linked to survival in the Canadian population, whereas the wind speed of the strongest ETC impacted adult survival with time lags for the Svalbard and northern Norway eider populations. We suggest that climatic shelters found on the wintering grounds, such as fjords, could provide natural protection and partly explain inter-population heterogeneity.