During winter time, 100,000s of rooks (corvus frugilegus) come to Vienna from the east, in order to spend the cold season in a more moderate climate with a better food supply. During the day, they spread all over the city to search for food, in the afternoon, they return to their sleeping trees, which are mainly in bit parks like the Prater. When the rooks leave again for the East, this is an unmistakable sign that winter is over.