The Lac Du Der has become one of the most important roosting sites in Western Europe for the Common Crane during the migration periods. In November and in februari-march, up to 70.000 Common Cranes come to sleep at this huge artificial lake, which serves as a drinking water reservoir for the Isle de Paris. During the previous decades, the number of wintering birds kept on increasing. Nowadays the site holds also important numbers of wintering Cranes. Up to 10.000 Common Cranes are feeding during most of the winter in the fields and meadows around the lake.