Common Merganser

The common merganser (Mergus merganser), also known as the goosander in Europe, is a large fish-eating duck found across much of the Northern Hemisphere. It breeds throughout northern Europe, Asia, and North America, particularly in forested regions with clean rivers and lakes. During winter, many populations move farther south to ice-free lakes, rivers, reservoirs, estuaries, and coastal waters.

It is the largest of the mergansers, measuring around 58–72 cm (23–28 in) in length, with a wingspan of 78–97 cm (31–38 in). Males are generally slightly larger than females and can weigh between 0.9 and 2.1 kg (2–4.6 lbs).

Adult males in breeding plumage are striking birds, with a glossy dark green-black head, bright red bill, white body, pale salmon-tinted underparts, and black-and-white wings. Females and non-breeding males are more subdued, with grey bodies and a warm reddish-brown head with a shaggy crest. Both sexes have narrow, serrated bills adapted for catching slippery fish underwater. These serrations have earned mergansers the nickname “sawbills.”

The common merganser can be confused with the red-breasted merganser (Mergus serrator), but several features help distinguish them. Common mergansers are larger and bulkier, with a thicker bill and cleaner white flanks. Breeding male red-breasted mergansers have a spiky crest and a rusty-colored breast band, while females have a more sharply defined border between the brown head and pale neck. Unlike the red-breasted merganser, which is strongly associated with coastal habitats, the common merganser is primarily tied to freshwater lakes and rivers.

The common merganser is strongly associated with clear freshwater habitats rich in fish. It is most commonly found on deep lakes, broad rivers, reservoirs, and forested waterways, especially in northern boreal and temperate regions. During winter, it may also occur in estuaries, sheltered coastal bays, and large ice-free bodies of water. Breeding birds usually prefer habitats with mature trees nearby, as they rely heavily on tree cavities for nesting.

Fish make up the majority of the diet, though amphibians, aquatic insects, crustaceans, and small aquatic invertebrates may also be taken opportunistically. The species hunts primarily by diving underwater and actively pursuing prey with remarkable agility. Its streamlined body, strong feet, and narrow hooked bill make it highly specialized for underwater hunting. Juveniles initially feed more on aquatic insects before gradually shifting toward a fish-based diet as they mature.

Common mergansers often hunt alone or in small groups, though larger flocks may cooperate to herd schools of fish into shallow water. In rivers, they are frequently seen drifting downstream before flying back upstream to repeat feeding runs.

The common merganser is generally a cautious and alert species, often staying far from shore on open water. It is an excellent swimmer and diver, capable of remaining underwater for extended periods while chasing fish. In flight, it appears fast and powerful, flying low over the water with rapid wingbeats.

Outside the breeding season, common mergansers can be quite social and may gather in loose flocks, especially during migration and winter. However, breeding pairs become territorial during the nesting season, particularly around rivers and suitable nesting sites.

Courtship behavior involves elaborate displays by males, including head tossing, neck stretching, and synchronized swimming. Males may also produce low croaking or growling sounds during displays. Females are generally quieter but can give harsh alarm calls when disturbed.

Ducklings are surprisingly independent soon after hatching and often leap from nest cavities high above the ground to reach the water below. Females sometimes form “crèches,” where several broods combine into one large group supervised by one or more adult females. These groups can occasionally contain dozens of ducklings.

The common merganser is partially migratory, though migration patterns vary depending on latitude and climate. Northern populations breeding in Scandinavia, Siberia, Canada, and Alaska typically migrate south in autumn when freshwater begins to freeze. More southern populations may remain resident year-round or move only short distances.

Autumn migration generally occurs between September and November, while spring migration takes place from February to April. Wintering birds are commonly found farther south in temperate Europe, the northern United States, East Asia, and along unfrozen coastal areas.

Unlike the red-breasted merganser, which commonly winters along marine coastlines, the common merganser usually favors inland freshwater habitats during winter whenever open water is available. They are among the last waterfowl species to leave northern freshwater systems as ice forms.

Breeding season typically begins in spring, with nesting occurring between April and June depending on latitude and climate. Common mergansers are cavity nesters and usually breed in holes in large trees near water, though they may also use rock crevices, abandoned buildings, nest boxes, or occasionally ground sites.

The female lays around 6–17 creamy white eggs, with incubation lasting approximately 28–35 days. Incubation is carried out solely by the female, while males usually leave shortly after egg-laying begins and gather elsewhere to molt.

After hatching, ducklings leave the nest within a day or two. In tree nests, they instinctively jump to the ground from heights that can exceed 10 meters (33 ft), bouncing surprisingly well due to their light weight and soft down. The female then leads them to nearby water, where they begin feeding immediately.

Young birds grow rapidly and become capable of flight after roughly 60–70 days. Females provide protection and guidance during this time, though ducklings largely feed themselves. In the wild, common mergansers can live well over 10 years, with some individuals reaching ages of around 13–15 years.

The common merganser is currently classified as least concern on the IUCN Red List due to its extremely large range and overall stable global population.

Despite this, local populations face several threats. Habitat degradation, river pollution, wetland destruction, deforestation around breeding areas, and the loss of suitable nesting cavities can negatively affect breeding success. Water pollution and acidification may also reduce fish availability in some regions.

The species has also historically faced persecution from anglers and fish farmers who viewed it as competition for fish stocks. Illegal shooting and disturbance still occur in some areas, particularly where fishing interests are strong.

Interestingly, the common merganser has expanded parts of its breeding range in Europe over the past century, including into Scotland and England, likely aided by improved water quality and legal protection.