The American common eider Somateria mollissima dresseri is a sea duck of coastal mid-Atlantic North America, and breeding colonies in the southern part of its range have been in decline. To better understand threats faced by the subspecies, we used satellite telemetry to track 46 eiders through their annual cycle in four years from three regions in the southern part of the range, to identify key locations and migratory corridors. Female eiders exhibited highly variable movement phenology within and among colonies, but coastal Maine and Massachusetts were consistent, important moulting areas for males and females from all breeding colonies. Most birds wintered in coastal waters around Cape Cod and Nantucket Sound, meaning that threats in this region (industrial development, disease outbreak, harvest) could have deleterious effects on much of the population.
Understanding annual movements of migratory wildlife is critical for sound management of populations, especially those that are at risk from anthropogenic activities (Petersen and Savard 2015, Hays et al. 2019). This knowledge allows us to identify locations where a species may experience threats (Amélineau et al. 2018, Mason et al. 2018), as well as where and when dispersed breeding individuals aggregate, which might be critical for determining exposure to contaminants and disease (Ballard et al. 2017). The common eider Somateria mollissima is a large, long-lived sea duck with a circumpolar breeding distribution (Goudie et al. 2000). Common eiders are colonial nesters with high philopatry to breeding colonies (Mallory 2015), and are gregarious at most stages of their annual cycle, notably in the winter and during post-breeding remigial moult (Merkel 2004a, Milton et al. 2006). These eiders have high cultural significance to sport hunters and indigenous peoples due to their long-held importance to people for food and down, but in many parts of the range, abundance is in decline (Suydam et al. 2000, Merkel 2004b, Descamps et al. 2009, Gilliland et al. 2009, Ekroos et al. 2012, Milton et al. 2016), which is attributable to a suite of factors (overharvest, changing food supplies, habitat change and/or predator increases at colonies, disease, poor recruitment; see Goudie et al. 2000, Koneff et al. 2017, Allen et al. 2019).
The American common eider, Somateria mollissima dresseri, Sharpe, 1871, is the subspecies that breeds on coastal islands from Labrador, Canada to New York, USA (Goudie et al. 2000, Chaulk et al. 2005, Waltho and Coulson 2015). There are an estimated 240 000 individuals in the population (Canadian Wildlife Service Waterfowl Committee 2017), but more southern breeding numbers are in decline (Bowman et al. 2015), and S. m. dresseri is listed as an international priority species for the Sea Duck Joint Venture (2017). Banding efforts have suggested that colonies in Maine, Massachusetts and Nova Scotia were part of the same subpopulation through affinities to breeding and wintering areas (Reed and Erskine 1986, Krementz et al. 1996). However, banding efforts in some regions have declined in the last decade (Milton et al. 2016), and surveys suggest eiders are changing their nonbreeding locations (Milton unpubl.). Adult female survival in this subpopulation is variable. Female survival among breeding colonies in Nova Scotia is low, likely due to increased predation and habitat changes at colonies (Milton et al. 2016), while survival rates among Maine adult female eiders were notably higher (Allen et al. 2019). However, other threats to this subpopulation include windfarm and aquaculture development in key habitats (Žydelis et al. 2006, Langston 2013), oil spills in a coastal area of high shipping traffic (Sperduto et al. 2003, Thieltges et al. 2006), declining food resources (Sorte et al. 2016), contaminants (Meattey et al. 2014, Pratte et al. 2015), and outbreak of a novel disease (Ballard et al. 2017).
To better understand how common eiders in this subpopulation move and use different locations through the year, as well as the extent to which they may overlap in habitat use, we tracked American common eiders with satellite transmitters from several breeding colonies in the southern part of the S. m. dresseri range. Earlier banding data suggested that the Cape Cod region was an important wintering area for this subpopulation (Krementz et al. 1996), so we expected high spatial overlap in birds from different colonies in that area. We also predicted that birds from the northernmost colony would be the first to depart for autumn migration and the first to depart for their breeding grounds in the spring. Female eiders have sole responsibility for rearing young and failed or non-breeders may help rear eider crèches (Goudie et al. 2000), so we expected males to depart breeding areas and reach wintering areas earlier than females.