Events during one stage of the annual life cycle of migratory birds can have lasting (i.e. carry-over) effects that influence demographic parameters in subsequent seasons. We studied migratory connectivity and potential carry-over effects in a declining population of sea ducks. We measured stable isotope values of carbon (δ¹³C) and nitrogen (δ¹⁵N) in head feathers to assign breeding White-winged Scoters (Melanitta fusca; hereafter scoters) to either Atlantic or Pacific winter populations. The discriminant function for δ¹³C and δ¹⁵N correctly classified 93% of scoters sampled from these 2 winter areas. We then applied this classification scheme to head feathers of females breeding at Redberry Lake, Saskatchewan, and Cardinal Lake, Northwest Territories, to stratify each breeding population by winter provenance. We evaluated carry-over effects associated with winter location of females breeding in Saskatchewan by testing for differences in (1) nesting phenology, (2) clutch size, (3) mid-incubation body mass, (4) nest success, and (5) concentrations of trace elements contaminants of cadmium (Cd), mercury (Hg), selenium (Se), and lead (Pb) in blood, between strata of putative winter origin. Breeding females from the Atlantic coast had later dates of nest initiation, greater mid-incubation body mass, and also had higher concentrations of Cd (one year only), Pb, and Se, relative to birds from the Pacific. Neither nest initiation date nor mid-incubation body mass, however, were related to contaminant concentrations in blood. We found no differences in clutch size or nest success between putative winter strata. Our study detected carry-over effects in the Saskatchewan population that merit further attention.