The Goosander (Mergus merganser), is a large sea duck, which is distributed over Europe, North Asia and North America. Although belonging to the seaducks, It is most common on lakes and rivers. Its nests can be found in treeholes. Males and females are easily distinguished, since the male Goosander has a dark green head, while the head of the female is reddish-brown. These large fish-eaters have serrated edges to their bills to help them grip their prey. Along with the Smew and the Hooded Merganser, they are therefore often known as "sawbills". They also eat mussels and shrimps; young birds mainly eat aquatic insects. North American birds migrate south in small groups to the southern United States; on the west coast, some birds are permanent residents. Scandinavian and Russian birds also migrate southwards, but western European birds are largely resident.