This guide is based on wikipedia's list of birds of Germany with accidentals removed to keep the guide under 500 entries.
The Common Merganser (North American) or Goosander (Eurasian) (Mergus merganser) is a large duck, of rivers and lakes of forested areas of Europe, northern and central Asia, and North America. It eats fish and nests in holes in trees. John James Audubon called this bird the "Buff-breasted Merganser" in his book The Birds of America.
The Red-breasted Merganser (Mergus serrator) is a diving duck, one of the sawbills.