Common Mergansers are are found on open inland waters throughout the year. In contrast Red-breasted Mergansers prefer quiet salt-water bays. They usually stick to the coasts during migration from their breeding grounds in the northern boreal forest and tundra. Infrequently do they appear on inland lakes and rivers.
This pair ventured inland. I found them on a quiet backwater of the CT River. In basic winter plumage the male and female look very much alike, except for the male being perhaps a bit darker. The two photos below, taken in Gloucester Harbor in MA, show them in breeding plumage.
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