Although the Eider generally prefers
more open and salty areas, since the late 80´s a breeding population has
spread throughout the fjord, and it now breeds even on the southernmost islets
in the fjord. In the mid 90´s a population was growing on Eskilsø
but they were driven away by Foxes. The island will probably be repopulated
if and when Eskilsø is free of foxes. The only protection for the Eider´s
nest is the female´s camouflage.
Even in the colder half of the year a lot of Eider stay in the fjord –
mainly in the northern part.
The Eider´s increasing population is due partly to a general increase
in the species in the Baltic area (until recently, when it has begun to decrease),
partly to a decrease in pressure from hunters, and also probably to the improvement
in the fjord´s water quality. The Eider´s most important foodstuff
is (just like the Tufted Duck) the common mussel, which is found in large numbers
in most of the fjord. It is able to eat bigger mussels than the Tufted Duck
In the southern end of Isefjord there are very big colonies of Eider on the
islands south of Orø. In 2001 and 2002 they suffered from avian cholera,
and a large fraction of them lost their lives. Avian cholera has not been observed
in Roskilde Fjord.