The Audobon website reports:
This fish-eating duck is the typical merganser of freshwater lakes. Its flocks are usually small, but these may combine into big concentrations sometimes at large reservoirs. Common Mergansers living along rivers may spend hours resting on rocks or on shore. The British call this bird the “Goosander.” In some parts of Europe, with artificial nesting sites provided, the species has become a common nesting bird along city waterfronts; this has not yet happened in North America.