Unlike most other birds, waterfowl pair on their wintering grounds, not the breeding grounds. Across waterfowl species, larger dabbling ducks pair the earliest (starting in November), smaller dabblers and diving ducks pair closer to the spring. Very little is known about the pairing chronology of sea ducks. A small (60 -100 birds) moulting and wintering population of Harlequin ducks was studied from June to November 1995 to assess the moulting and pairing chronology of this population. Males returned from the breeding grounds in June and July, and immediately began the pre-basic moult. Most males were in the basic plumage and flightless by late July through August. The pre- alternate moult began in September and most males were back in full atternate plumage by October 1. Courtship and pairing began immediately after the finish of the pre- alternate moult. Through August and September the sexes tended to segregate into same sex groupings. Additionally through this period males clumped into progressively larger and larger groupings, possibly interacting to determine a dominance hierarchy. In October mixed sex groups were more prevelent and all the ducks were much more dispersed. As males are moulting immediately after arrival on the moulting grounds and begin to pair immediately after they have finished their moult, it is very likely that there is strong sexual selection for early pairing in Harlequin, and probably other, sea ducks.