Harlequin Ducks (Histrionicus histrionicuS) arrive along the eastern coast of the United States at traditional wintering grounds from late September to December through early January. Counts in January and February best represent wintering bird numbers. Spring migrants depart from April to May, although from more southerly wintering areas, birds appear to depart in March. Breeding has never been confirmed in the eastern United States. Molting females have been documented in Maine during the fall, but the extent of molting has not been assessed. It is estimated 1,575 to 1,800 Harlequin Ducks wintered in the eastern United States from Maine to North Carolina between the winters of 1997-98 and 2001-02. Over 75% of the birds wintering in the eastern United States were in Maine, primarily in the vicinity of Isle au Haut. Smaller concentrations representing about 18% of the wintering birds in the eastern United States were at locations in Massachusetts and Rhode Island, primarily the Cape Anne region and Martha's Vineyard, Massachusetts and the Sachuest Point region, Rhode Island. South of Rhode Island, Barnegat, New Jersey is the only area that regularly reports counts of over 25 birds. In the Isle au Haut region of Maine, numbers along survey routes have not shown a linear change from 1989-2002, but show a pattern of decline from 1989 to 1993 followed by increases since then. In eastern Maine, population increases are greatest and birds are expanding their winter range into new areas. All other areas in the eastern United States south of Isle au Haut that regularly report over 50 birds show significant population increases since the 1980s, north-eastern US.