This medium-sized sea duck breeds in boreal forest lakes of northern Canada and Alaska, and during nonbreeding periods is widely distributed in nearshore marine habitats along both coasts of North America. Satellite telemetry and banding indicate that individuals are faithful in consecutive years to sites used throughout the annual cycle and that timing and routes of migration vary between the sexes, and often among individuals that co-occur in the same general wintering or breeding area.
There are few studies of breeding ecology in this species. Telemetry results indicate that birds wintering from Baja, Mexico to southern British Columbia intermix on breeding areas that occur mainly in the Northwest Territories. Pairs migrate together to breeding sites, although after just a few weeks males depart for coastal molting sites that are typically distinct (and may lie in a different direction) from their ultimate wintering sites. Only a few nests have been described to date, but appear to occur many meters inland from freshwater lakes where they are well concealed by vegetation, downed logs, or rock outcroppings. Limited evidence suggests reproductive success can vary widely among years owing to weather conditions.
Surf Scoters nest later than most waterfowl, and likely have limited opportunity to re-nest. Females are solely responsible for incubation, and after synchronous hatching of ~6–9 eggs the female leads her precocial young to water where they feed on aquatic invertebrates. Females with broods are not territorial, and on crowded lakes accidental exchanges of young among broods and brood amalgamation occur. Unsuccessful females depart breeding sites well ahead of successful females, which in turn depart before young of the year.
Relative to males, molting sites of females appear to lie closer to their breeding or wintering sites, or en route between the two. Further research on the breeding period is especially needed to assess: (1) age of reproductive maturity and frequency of breeding attempts across years (which are assumed to be delayed and low, respectively, in sea ducks relative to other waterfowl species); (2) factors constraining breeding success; and (3) annual movements of sub-adult and nonbreeding birds.
Outside the breeding range, Surf Scoters occur almost exclusively at sea where they spend most of the year. During this period they occur in flocks that can vary greatly in size, as well as in sex- and age-composition. Flock size may number in the many thousands in wintering sites with highly profitable feeding, but aggregations throughout the annual cycle are typically greatest at their coastal molting and spring staging sites. At wintering sites, feeding is mainly diurnal and occurs closer to shore than nocturnal roosting sites. Surf Scoters dive to feed on a variety of benthic invertebrates, and feeding occurs mainly in shallow water but can extend to subtidal sites of at least 25 m depth. Marine feeding habitats are diverse, ranging from rocky- to soft-bottom substrates that may or may not be vegetated. Pairs form in fall or early winter, and then paired males spend appreciable time defending a moving area around the female from several competing males.
Surf Scoters and the congeneric White-winged Scoters (Melanitta fusca) have been combined and treated identically in most population and conservation assessments because they often co-occur on both wintering and breeding sites, and they are difficult to distinguish in aerial surveys. Although both species often feed heavily on bivalves in wintering sites, recent study indicates marked differences in their seasonal patterns of habitat use. Relative to White-winged Scoters, Surf Scoters are more likely to experience overwinter declines in body reserves and increased feeding effort. These changes appear to underlie the greater tendency of Surf Scoters to shift feeding sites in late winter and spring to take advantage of seasonal foods such as herring spawn, epifaunal crustaceans in seagrass beds, aggregations of reproducing polychaetes, and infaunal prey made available by the feeding activity of Gray Whales (Eschrichtius robustus). Declines in Pacific herring (Clupea pallasii) may threaten Surf Scoter populations because herring spawn appears to be a critical food throughout spring migration on the Pacific Coast.
Although trend data are sparse in e. North America, continental numbers of the three scoter species combined appear to have declined appreciably during the past 30 to 60 years. Depending on timeframe and location, estimates of decline range from about 40–80% with some evidence that declines have slowed or stabilized since the 1990s. Reasons for these declines are mostly unknown; possible factors include climate change impacts on boreal nesting habitats (e.g., by advancing seasonal phenology, and by drying wetlands and altering their chemistry and prey availability), or cyclic shifts in oceanic regimes (e.g., by altering marine prey availability). On the Pacific Coast, preliminary studies indicate that survivorship is high at major molting sites, but survivorship can be markedly lower during winter depending on location, age, and body mass. During nonbreeding periods, Surf Scoters aggregate in coastal sites where they are often exposed to a suite of human impacts. The role of these impacts on scoter population processes requires further study, but important impacts likely include: degradation and loss of nearshore feeding habitats (e.g., effects of shoreline armoring, and loss of critical habitats such as seagrass beds); developmental and reproductive consequences of elevated contaminant exposure, including both acute and chronic affects of oil exposure (scoters are one of the species most frequently caught in oil spills); changes in predator communities; disturbance by watercraft; and introductions of invasive prey items.
Surf Scoters were until recently one of our least-known sea ducks (Bellrose 1980). Particularly in western North America, however, increased study beginning in the early 2000s has clarified many aspects of this species' ecology. Key wintering studies have been conducted on diet (Anderson et al. 2008), feeding behavior (Kirk et al. 2007a; Lewis et al. 2005, 2007b), habitat use and feeding strategies (Žydelis et al. 2006, 2009; Kirk et al. 2007b, 2008; Lewis et al. 2007a; Anderson et al. 2009b; Zipkin et al. 2010; Anderson and Lovvorn 2011, 2012; Lovvorn et al. 2013; De La Cruz et al. 2014), survival (Anderson et al. 2012, De La Cruz et al. 2013, Uher-Koch 2013), exposure to contaminants (Hoffman et al. 1998, Barjaktarovic et al. 2002, Elliott et al. 2007, Harris et al. 2007, Eagles-Smith et al. 2009, Wilson et al. 2010, Bendell 2011), and distribution (Iverson et al. 2004, Lukeman et al. 2010).
Key studies of spring migration include Anderson et al. (2009b), De La Cruz et al. (2009), and Lok et al. (2011, 2012); fall migration (including molt migration) has not been critically assessed using available telemetry data. Studies of the breeding period remain few, but key research includes that of Décarie et al. (1995), Morrier et al. (1997), Lesage et al. (2007), Savard et al. (2007), Takekawa et al. (2011), Brook et al. (2012), and Drever et al. (2012). Key studies of the wing molt period include those by Dickson (2011), Dickson et al. (2012), and Uher-Koch et al. (2014).