We used implanted satellite transmitters to track the northbound (spring) and southbound (fall) migration and possible breeding locations of three Long-tailed Ducks (Clangula hyemalis) wintering on western Lake Ontario in Ontario, Canada. The birds exhibited short, rapid migration movements punctuated by extended periods of up to 30 days at staging areas. For much of the nesting period (∼10 June to 10 July), the birds remained inland of western Hudson Bay in Nunavut. During fall migration, they circumnavigated Hudson Bay to its eastern coast, opposite the coast they had followed in spring, for a mean travel distance of 6,760 km. Identification of these previously unknown, key migration sites fills some important information gaps on Long-tailed Ducks in eastern Canada, and it augments what is known about important coastal marine habitats in the Arctic.