Common Mergansers dive underwater to catch fish. After the chicks leave the nest in summer, the female stays with them as they grow up while males gather in flocks. In winter, mergansers form large flocks on inland reservoirs and rivers. They stay in these tight flocks to feed and court during the cold months. In migration and winter, they mix with other fish-eating, diving ducks such as Bufflehead, goldeneyes, and other species of mergansers.