This heavy-weight of the duck world is a surprising resident of south Cumbria, the Walney area having the southernmost breeding colony in Britain. The Eider has arctic associations because of the superior insulating qualities of its down - 1.5kg in a sleeping bag is said to provide comfort for the occupant down to -35C.

The first nest was recorded on Walney in 1949 (but not on Foulney until 1962), with the number of nests increasing steadily to over 200 by the early 70's, to over 500 by the early 80's, and reaching a peak of about 1500 nests in the early 1990's. Since then, numbers of nests have declined seriously to between 200 and 300 in recent years.

Colonies of Eider need high concentrations of shellfish (especially mussels) in accessible waters, which Morecambe Bay can provide. But breeding success seems to be due largely to the supply of food and the amount of protection from predators. In Iceland, where eiderdown is an economic crop, breeding sites are managed and in Greenland Eiders have taken to nesting near tethered husky dogs, which keep predators away. Surprisingly, it is the predatory gulls on Walney which provide the protection, the Eiders being robust enough to keep them away and smart enough to produce their off-spring a week or two before the gulls produce their voracious young. Such is the density on Walney, it is not unknown for Eider eggs to be found alongside gull eggs, being incubated by the gull!

February and early March are good times to see large rafts from the Morecambe Bay coast; these can cover vast areas of the sea. These spectacular sights can be seen off Baycliffe and Newbiggin as the tide falls, with numbers that can be as high as 1500 and 2000.

Courtship begins in March, which can sometimes be very vocal as many males woo one female with their far-carrying coo...ooo

The nest, usually built in the second half of April, is a shallow hollow made with grass or any material close by, which is then lined with down plucked from the duck's breast. When away from the nest the female covers the eggs with the down to protect them from cold and dehydration and to provide camouflage. Despite this, females leave the nest less often than other wildfowl and are remarkably stubborn in remaining on the nest when approached.

By late May, eggs are starting to hatch and from mid to late June creches form off the west coast of Walney, tended by the ducks. The drakes gather at this time in large flocks to moult, usually along the coast between Foulney Island and Baycliffe.

In recent years numbers nesting at other sites in Morecambe Bay have increased, with more birds being seen in the Leven estuary and especially on Chapel Island. Eiders are now a common sight for rail travellers across the Leven viaduct.

The recent decline in numbers in the area is thought to be due to falling stocks of mussels and particularly predation by foxes. Mussel stocks (and their age profile) vary from year to year, with Eiders preferring the younger mussels. Unregulated commercial harvesting is believed to have depleted the Morecambe Bay beds and attempts are now being made to establish a "Fisheries Order" for the Bay.

Prior to 1990 foxes were rarely seen on Walney, but they are now an established breeding species. Predation also occurs on Foulney, as it is visited by foxes from the surrounding area. In 2005 there were reports of a possible mink attack on Chapel island, claiming some nests.

Eiders are long-lived species and can withstand poor breeding years. If feeding conditions are poor it seems many do not attempt to breed, or fail to complete breeding. Spring estimates of the total number of Eiders on the north side of Morecambe Bay can be as high as 2000 in recent years (down from typical numbers 0f 4000). With only 300 or so breeding pairs now on Walney, it is clear that many are not breeding.

There is little apparent movement of Walney's Eiders, although they may be joined in winter by others moving down from the north. Although this is the world's most abundant sea duck, there can be few better places than Walney and the Furness coastline to provide such a superb opportunity to observe the behaviour of this species. The male's expressive double-coo carries far and a group in full voice is a sound not to be forgotten.