Mergus serrator, commonly known as the Red-breasted Merganser, is primarily found across the Northern Hemisphere. This species has a broad geographical distribution that includes North America, Europe, and Asia. In North America, it is widespread in Canada and the northern United States. In Europe, it ranges from Iceland across to Scandinavia, and in Asia, it is found in parts of Siberia and the Kamchatka Peninsula.

The Red-breasted Merganser typically inhabits cold, freshwater environments and is also found in coastal marine habitats. During the breeding season, it favors freshwater lakes, rivers, and estuaries with abundant aquatic vegetation. In contrast, during the winter, this species migrates to coastal waters, bays, and estuaries, often preferring saltwater conditions.

As a diving duck, the Red-breasted Merganser is well-adapted to an aquatic lifestyle. It primarily feeds on small fish, often hunting in deeper waters. Its serrated bill is specially adapted to help catch slippery prey, making it a proficient hunter in its aquatic habitat. The merganser is part of a unique ecological niche that includes other fish-eating waterfowl, contributing to the health and balance of its ecosystem by controlling fish populations.