Little is known about the habitat use patterns and movement ecology of American common eiders (Somateria mollissima dresseri) despite potential negative impacts on habitat and eider populations from anthropogenic sources (e.g., oil spills, hunting, offshore wind energy facilities). We used satellite telemetry to quantify migratory phenology, home range size, winter site fidelity, and resource selection of adult female eiders (n ¼ 24) from December 2011 to July 2013 that were captured during winter in southern New England, USA. Eiders spent 39% of their annual cycle in southern New England. In spring, eiders took an average of 16 days (range ¼ 2-47) in 2012 and 20 days (range ¼ 1-61) in 2013 to migrate from wintering grounds to summer areas, whereas the duration of fall migration averaged 47 days (range ¼ 7-115). Eiders exhibited high site fidelity to wintering areas, with 83% of birds with active transmitters (n ¼ 23) returning to the study area the following winter (2012)(2013). Mean individual core use home ranges on wintering grounds averaged 38.5 km 2 and 95% utilization distributions were 199.3 km 2 . Based on habitat selection models, eiders preferred shallow, nearshore waters that had relatively fine sediments and a high probability of hard bottom. We estimated that only 3% of our 6,212-km 2 study area had a high relative probability of use by eiders. Future development (e.g., offshore wind energy developments) should avoid shallow, nearshore waters with hard bottoms preferred by eiders to minimize potential impacts. Ó 2017 The Wildlife Society.KEY WORDS common eider, migration phenology, satellite telemetry, site fidelity, Somateria mollissima dresseri, southern New England, winter habitat use.

We conducted the first validation of the deuterium dilution method as a nonlethal technique for estimating body composition of a sea duck. We captured male (n = 11) and female (n = 8) American common eiders (Somateria mollissima dresseri) during winters of 2011–2012 and 2012–2013 in southern New England, USA, and compared their directly measured body composition with that estimated using deuterium‐dilution. The best‐supported linear regression models predicted wet lean and fat mass with, on average, 2.0% and 20.2% relative errors, respectively. The deuterium dilution method provides field biologists and managers with a nonlethal method for accurately estimating body composition of common eider during winter. The method is broadly applicable to other migratory birds and can be used to evaluate the effects of ecological and anthropogenic drivers on body composition dynamics. © 2016 The Wildlife Society.

The nearshore and offshore waters of southern New England provide some of the most important wintering habitat for sea ducks in North America. Understanding changes in sea duck body composition during winter could provide insights into current habitat quality and potential effects of anthropogenic disturbances on the body condition of sea ducks. We used the deuterium dilution method, a nonlethal approach, to investigate intraseasonal variation and differences between genders in body composition of 86 adult common eiders (Somateria mollissima) during winter (2011 and 2012) in southern New England. Both male (n = 39) and female (n = 47) eiders increased fat mass by 75% in late winter compared with early winter. Wet lean mass of males remained constant throughout the winter, while that of females decreased by 10%. These seasonal increases in fat stores of eider suggest that birds were able to secure adequate resources to improve their body condition over winter. Our estimates of body composition of eider during winter in southern New England can be used as a baseline when conducting future research to evaluate effects of anthropogenic disturbance and disease. © 2016 The Wildlife Society.