Material and Methods
Among the five species of the genus swan (Cygnus), the mute swan Cygnus olor Gmelin (1789) is the best known in northern European latitudes. Together with the geese the swans make up a subfamily (Anserinae) which, with the ducks and their relatives are again classified as the family of Anatidae. This family, together with that of the game-birds, makes up the order of Anseriformes (Grzimek et al. 1968).