The Tufted Duck is, together with the Coot, the most common bird in the fjord in the winter. It comes here in October, when the Siberian winter starts to freeze the water in its breeding grounds. In the fjord the Tufted Duck lives well on the very dense populations of Common Mussel, which cover large parts of the fjord bed. These mussels are too small for human eating, but just the right size for a Tufted Duck.



The most important roosting place is Kattinge Sø, where there are often about 20,000 birds packed together in one dense flock. They sit passively on the lake all day long, but about an hour after sunset, when it is properly dark, the whole flock takes to the air as one and goes out onto the fjord to feed. They prefer the shallow areas with moving currents around Eskilsø, but use more or less every part of the fjord except the middle of Bredningen. In the morning they return to the lake as soon as it begins to grow light but this return takes place in a more haphazard manner.

Kattinge Sø is not the only roosting site on Roskilde Fjord; Selsø, near Skibby, is also home to many Tufted Duck. The total winter population on the fjord is about 35,000. This is 3-5% (1 in 25) of all Tufted Duck on the northwest European migration route, which means that from the point of view of conservation this area is of international significance for the Tufted Duck. If Kattinge Sø is frozen the ducks have to move elsewhere.

The cooling-water outlet by the nuclear reactor at Risø, and Roskilde harbour, were formerly important roosting places, but now that the reactor and the sewage outlet in the harbour have both been closed there are only a few plcaes left where there is moving water when the fjord is frozen, so the Tufted Duck has to move on to Holland if the ice lasts for more than a short time. Many other water birds winter just far enough south to avoid the problem of ice covered water



The Tufted Duck has also recently developed a small breeding population on the islets in the southernmost part of the fjord. In 2002 about 40 pairs bred on the islets in Lejre and Kattinge Vig.

The best way to see the Tufted Duck is to go to Kattinge Sø just before the ice comes. There is an excellent observation post for bird watchers, and for a wonderful - if shortlived - experience, wait until an hour after sunset and hear the whirr of wings as a flock of 20,000 ducks passes just above your head.