Common Mergansers are numerous in summer along rivers in northern North America, and many a canoe trip will turn them up without much trouble. Look for them sitting on rocks in midstream, disappearing around the next bend, or flying along the river, when their white wing patches and heavy bodies make them easy to identify. In winter, seek Common Mergansers on large rivers and lakes; look for them in large flocks mixed with other ducks such as Common Goldeneye and Bufflehead. Look for the sharp dark-and-white contrast of the snazzy males and the crisply defined, rusty heads of females.
Common Mergansers are streamlined ducks that float gracefully down small rivers or shallow shorelines. The males are striking with clean white bodies, dark green heads, and a slender, serrated red bill. The elegant gray-bodied females have rich, cinnamon heads with a short crest. In summer, look for them leading ducklings from eddy to eddy along streams or standing on a flat rock in the middle of the current. These large ducks nest in hollow trees; in winter they form flocks on larger bodies of water.
Information from the All About Birds website, , © Cornell Lab of Ornithology.