The Red-breasted Merganser nests in holes and crevices in the lavafields. Newly hatched young are taken to the Ytrifloi where large creches are formed each one normally accompanied by one female. The Goosander Mergus merganser is uncommon on M vatn. Only 2-3 pairs nest there. On Lax the Goosander is quite common, 6-10 pairs occur down to the hydropower station at Br ar.
Males disappear before moulting begins to an unknown moulting ground.
During the winter, however, tens or hundreds of Goosanders gather at M vatn, probably depending on the stock size of Arctic Char and Brown Trout in the lake. The Wigeon Anas penelope is the most common among the dabbling ducks at M vatn. Between 500 and 1900 pairs arrive in spring. First they graze on the hayfields but as soon as the midges begin to emerge they move out on the lake to pick up the flies as they come to the surface. On windy days flocks of Wigeon feed on material drifting ashore.