Mergansers are fish-eating ducks. They are closely related to Buffleheads and goldeneyes, but because they diverged sharply from their invertebrate-eating relatives, their anatomy has diverged sharply as well. The main point of divergence is in the bill. Buffleheads have a normal duck-shaped bill, only 2-3x as long as wide.
A merganser bill, on the other hand, may be 4x as long as wide. But more than that, the structure of the bill has changed dramatically. Duck bills have plates or lamellae, fine transverse ridges on the cutting edges that let water escape from the bill when prey is brought to the surface. In mergansers, these plates have been modified to produce a saw-toothed effect. These aren’t true teeth, lacking in birds, but they are very toothlike, analogous to the teeth of barracudas, needlefish and dolphins, other fish-catching vertebrates.
We have three kinds of mergansers in the Pacific Northwest. Two of them are in the genus Mergus, the Common Merganser (M. merganser) and Red-breasted Merganser (M. serrator). Both of them are large ducks, the Common one of our largest. Both have relatively long, slender bills the edges of which look like saw blades. These toothy bills are perfectly adapted for capturing slippery fish, and mergansers feed only on fish.
The Common breeds in rivers all over the Northwest and extends its range in winter to lakes and marine environments, especially deep channels with swift currents. The Red-breasted breeds in the Arctic and Subarctic and descends on the PNW in the winter, primarily on salt water. It is one of the common and widespread wintering ducks on Puget Sound.
Both of these mergansers move around the landscape looking for fish, especially fish in large schools that are easier to capture than the individual fish that the ducks encounter most of the time. Thus herrings and sand lance, two of our common schooling fishes, are often prey. Like other ducks, these mergansers may be in flocks.
The least modified and smallest of the mergansers is the Hooded (Lophodytes cucullatus). It is close enough to the goldeneyes (Bucephala) to have hybridized with both Common Goldeneye (B. clangula) and Bufflehead (B. albeola) and is considered intermediate between the goldeneyes and the other mergansers.
All three mergansers have nesting habits like the goldeneye group. Hooded Mergansers nest in tree holes like Buffleheads and goldeneyes, Common Mergansers, much larger, need much larger crevices in trees, but like Hooded, sometimes uses old Pileated Woodpecker holes. On the other hand, Red-breasted Mergansers nest on the ground, often in the shelter of rocks or fallen trees adjacent to their preferred breeding wetlands. With their northerly breeding range, there aren’t many trees big enough for a nest hole!