Globally distributed across the northern hemisphere, on both freshwater (lakes, rivers) and saltwater (coastal bays, estuaries) environments. Often found in mixed flocks of waterfowl.

Pay close attention to head shape, crest prominence, and the contrast between the head and neck. Common Merganser females have a sharper demarcation between brown head and white chest; Red-breasted Mergansers have a more diffuse boundary and often a more pronounced, shaggy crest. Male IDs are generally easier due to distinct plumage.

Year-round in many parts of their range. More easily observed in winter when they congregate on larger, ice-free bodies of water, or during spring and autumn migration.