Mergansers are diving ducks found across the Northern Hemisphere, instantly recognizable by their profile on the water. These waterfowl belong to the genus Mergus and are highly adapted to a diet primarily composed of fish. This feeding strategy earned them the common nickname “sawbill” among bird enthusiasts. Although grouped within the seaduck subfamily, most species prefer freshwater habitats like lakes, rivers, and ponds.
The Sawbill: Unique Physical Adaptations
The merganser’s bill is the defining feature that sets it apart from all other duck species, justifying the “sawbill” moniker. Unlike the broad, flat bills of dabbling ducks used for filtering vegetation, the merganser possesses a long, narrow, and straight bill. This slender shape is an adaptation for catching and holding slippery prey underwater.
The edges of the bill are lined with tooth-like serrations, which are horny projections called lamellae, not true teeth. These projections point backward, providing an excellent grip on captured fish and aquatic invertebrates. The bill’s tip also features a slight hook, ensuring that struggling prey cannot escape the duck’s grasp.
Their body structure is optimized for aquatic pursuit. Mergansers have a streamlined shape, and their powerful legs are set far back on the body, functioning like a propeller for propulsion. This leg placement makes them highly efficient swimmers and divers, allowing them to chase fish through the water column. However, this adaptation causes them to appear awkward when walking on land.
Recognizing the Main Merganser Species
North America is home to three commonly observed merganser species, each with distinct plumage and crest characteristics.
Common Merganser (Mergus merganser)
The largest species is the Common Merganser. The male has an iridescent dark green head, often appearing black, paired with a white breast, sides, and a black back. The male’s crest is typically subtle, lying smoothly along the back of the head, giving it a rounded profile. The female is distinguished by her rusty-cinnamon head, which has a sharp boundary with the white chest feathers, and a shaggier crest than the male.
Red-breasted Merganser (Mergus serrator)
The Red-breasted Merganser is slightly smaller than the Common Merganser. Both sexes sport a shaggy, double-pointed crest that gives their head a messy appearance. The breeding male is identified by his dark green head, a white neck band, and a rusty-reddish band across his breast. Females and non-breeding males are grayish-brown overall, with a brownish head and crest. Their bill is thinner than the Common Merganser’s, and the demarcation between the brown neck and white chest is less defined. This species is often found in coastal, saltwater environments during the non-breeding season.
Hooded Merganser (Lophodytes cucullatus)
The smallest species is the Hooded Merganser, known for the male’s flamboyant, fan-shaped crest. This crest is black with a large white patch in the center, and the male can raise or lower the hood to change his head shape. The rest of the male’s plumage is black and white, with contrasting chestnut-colored flanks. The female is a dusky grayish-brown color with a reddish-brown crest that slopes backward. This species prefers smaller, wooded bodies of water and is generally smaller and more compact than its two relatives.
Geographic Range and Nesting Habits
Mergansers are widespread, breeding across the boreal forests of North America and Eurasia where suitable habitats exist. Because they rely on sight for hunting, they prefer clear, unpolluted water bodies, including fast-moving rivers, large lakes, and sheltered estuaries. The Hooded and Common Mergansers favor freshwater, while the Red-breasted Merganser is more commonly found in coastal saltwater areas, particularly in winter.
The species are migratory, moving south from northern breeding grounds as water bodies freeze. They spend winter months wherever open water remains, often near the ice line in large, ice-free rivers and coastal bays. These seasonal movements ensure they always have access to the open water necessary for diving and fishing.
A distinguishing feature of merganser reproduction is their status as cavity nesters, which differs from most other duck species. Both the Common and Hooded Mergansers seek out natural tree hollows or abandoned cavities, often those excavated by woodpeckers, to lay their eggs. They also readily use appropriately sized nesting boxes installed near the water.
The Red-breasted Merganser is an exception, typically constructing its nest on the ground near the water, concealed in dense vegetation. Regardless of the nest site, the young hatch and leave shortly thereafter. The female leads her ducklings to the water to begin feeding themselves almost immediately.