A healthy family of common mergansers (Mergus merganser) indicates an abundance of small fish in the Tuul River on the outskirts of Ulaanbaatar, Mongolia.

Mergansers are large diving ducks that subsist primarily on fish, although the young also fill up on aquatic insects such a the larva of mayflies and stoneflies. They make their nests in cavities in trees, sometimes a good distance from water. Less frequently, mergansers nest in holes in cliffs or high banks. They can be found on open water throughout the Northern Hemisphere, and are a good indicator of clean water and healthy populations of small fish.

If you look closely, you can make out the serrations on her bill – the perfect adaptation for holding onto fish. In contrast to the females, the drakes’ heads are dark green, their flanks are white and their backs are black. Although common mergansers are usually encountered on freshwater lakes and rivers, they are frequently seen in coastal bays and estuaries as well.

A baleful eye tells us we’ve approached close enough. Undisturbed this family went about their business, dipping their heads underwater in search of food as they paddled along the river’s current breaks.