This note is a response to a recent posting on the inland-nw-birders email list reporting a sighting of Harlequin Ducks.
Male Harlequins typically leave the breeding grounds for the coast at about this time. We have seen a few on the Lochsa into mid June but that is probably the latest. The males are often in small "loafing" groups before heading for the coast.
The breeding males apparently migrate with their mates and then reunite with them later when they also return to the coast. Some studies have shown that breeding populations usually winter together in the same location. The second reference below indicates there is evidence that Harlequins breeding in Idaho winter in the San Juan Islands, WA. Harlequin Ducks are still relatively poorly studied in the northern Rockies.
More information here, plus check out the BNA account for Harlequin Duck (available at good libraries).