Understanding how environmental factors aVect ecological parameters is important to understanding and predicting impacts of environmental change. Given evidence and anticipated impacts of climate variability, this is especially true with respect to sea ice and its role in animal life history in northern regions. We examined relationships between the extent of consolidated spring ice cover (pack and landfast), nest initiation and clutch size in common eiders (Somateria mollissima) in northern Labrador, a sub-Arctic region on the east coast of Canada. Our initial prediction was that eiders would delay nesting and have smaller clutches in years with more extensive spring ice cover. Between 1998 and 2003, we surveyed coastal islands for breeding eiders and collected information on nest age and clutch size. For those years, we estimated ice cover based on Radarsat-1 images supplied by the Canadian Ice Service during the spring period (approximately June 7-12). We found that spring ice cover was a signiWcant positive predictor of nest initiation date, and the regression equation indicated that if the average extent of ice cover around nesting islands increased by 18 ha, average nesting date was delayed by approximately 1 day. Nest initiation date was a signiWcant negative predictor of clutch size, and the regression equation indicated that a 20 day delay in nesting reduced average clutch size by approximately 1 egg. However, ice cover itself was not a signiWcant predictor of clutch size. Our Wndings suggest that eiders breed when ice is present, but ice extent may negatively inXuence aspects of their breeding ecology.

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