text: The ducks are divided between several subfamilies in the Anatidae family ; they do not represent a monophyletic group but a form taxon , since swans and geese are not considered ducks . Ducks are mostly aquatic birds , mostly smaller than the swans and geese , and may be found in both fresh water and sea water .
simplified: Ducks are closely related to swans and geese , which are in the same family .