Seaducks (including mergansers) moult their flight feathers during the summer. The males initiate the moult usually when the females are incubating their eggs. The females moult later when their chicks are fledged, or in case of nesting failure they join the males. For some species the moulting areas are located far from the breeding areas, and the males perform a moult migration to reach their moulting grounds.

As the birds shed their flight feathers almost simultaneously, they loose their flying abilities. The flightless period is usually about three weeks.

Some of the seaducks assemble in large flocks in restricted areas when they moult, and as their only way of escape is by swimming, such flocks are vulnerable to disturbance and oil spills. Large oil spills have the potential to hit large and significant population segments, where many birds are assembled.

In West Greenland following seaducks occur: king eider (Somateria spectabilis), common eider (Somateria mollissima), long-tailed duck (Clangula hyemalis), harlequin duck (Histrionicus histrionicus) and red-breasted merganser (Mergus serrator). And among these particularly king eiders, harlequin ducks and red-breasted mergansers assemble in certain areas to moult.

A pair of harlequin ducks, the male is coloured in

bluish grey and reddish brown.

NERI has in relation to mapping of oil spill sensitive areas, surveyed moulting seaducks in West Greenland.

• Go to the page: Oil spill sensitive areas

King eiders do not breed in West Greenland, and the major part of the birds moulting in West Greenland are from population breeding in high Arctic Canada. The primary moulting areas are in NW Greenland mainly at the island of Disko and in the Upernavik municipality. Males arrive from late July, later followed by the females and juvenile birds. The primary moulting habitat is remote fjords with soft button, where the king eiders feed down to about 50 meters depths. NERI has surveyed moulting king eiders in 1993, 1994 and 1995. In late August when the numbers of moulting birds peak, we estimate that 30,000-40,000 birds are assembled at the moulting grounds. However, the total numbers of birds moulting in West Greenland are much higher, and the late August figure represent just a snapshot from a very long moulting period extending from July well into October. Recently NERI has initiated satellite tracking of males tagged at the moulting grounds.

The harlequin duck is a scarce breeder at rivers in West Greenland. When the females initiates brooding the eggs, the males assemble along the coast in exposed archipelagos. There they moult, and later females and juveniles arrive. In recent years Canadian biologists have tracked by satellite males both from breeding and wintering sites in eastern Canada and USA to Greenland moulting grounds.

• More info: Canadian Wildlife Service

The density of moulting harlequin ducks in Southwest Greenland in July 1999.

A flock of moulting red-breasted mergansers in a Greenland fjord.

In July 1999 NERI conducted a special aerial survey aimed at the moulting harlequin ducks. The result was that 3500 males were counted mainly in the area between Nuuk and western Qaqortoq municipality. However, many more males are moulting in West Greenland, because the small harlequin ducks are very difficult to detect from an airplane, and because all moulting areas could not be surveyed. The number of moulting males are more likely to be at 5000-10,000 birds. The highest densities were found to the south of Nuuk, and this may be due to the Canadian contingency.

Red-breasted mergansers breed in small numbers at coast and lakes in West Greenland. They are usually seen in pairs and small flocks, but in two particular areas moulting males occur in large flocks. These sites were surveyed from airplane in July 1998 and July 1999, and in total these two areas hold about 1300 birds.

Moulting long-tailed ducks and common eiders do not assemble in similar large aggregations as the above mentioned species. The males usually moult rather close to their breeding sites in small and dispersed flocks.

The two areas in Southwest Greenland where NERI has located large flocks of moulting mergansers.

Additional information:

• Contact: David Boertmann, dmb@dmu.dk
• Contact: Anders Mosbech amo@dmu.dk
• NERI technical report 345: "Important summer concentrations of seaducks in West Greenland"  (PDF-format 3.2 MB)