Hi Ken,

Harlequins pair up on the wintering grounds and return to the female's natal stream usually by mid-May, but are preceded by unpaired males who I guess want to make a last ditch effort to find (or in this case, steal) a mate.
Apparently there are more males than females, due to decreased survivorship of females. Some immature males and females oversummer in the wintering areas, and most males are not able to pair up until they are about three years old.

Perhaps you saw young males who were fully mature but not able to find mates?

(P.S. This information was gleaned from an article on Harlequin Ducks in the April issue of Birding magazine.)

Mike Donahue
Seattle