The red-breasted merganser is the most widespread of the mergansers, a group of fish-eating ducks also known as saw-bills due to their long, narrow, serrated beaks (3)(4)(5). These slender seabirds are also renowned for their marked sexual dimorphism, with the more elaborately coloured male decorated with glossy sheens of green and blue to attract the somewhat duller female during the breeding season (6). At this time, the male red-breasted merganser has a dark, metallic-green head with a shaggy, double-pointed crest, a white collar around the neck and, as its common name suggests, a red breast. The upperparts are otherwise black except for large white patches on the wings, while the bill and legs are a starkly contrasting bright orange-red. Outside of the breeding season, the male has a largely slaty-grey, duller plumage, with a rusty brown head and crest. Year-round the female is similar in appearance to the non-breeding male, but with more white on the chin, a lighter back and smaller white patches on the wing (3)(4). The mergansers are strong fliers and among the fastest of ducks, capable of achieving speeds of up to 80 miles per hour. When taking off from the water’s surface, they use rapid-wing beats and thrusts of the webbed feet to take to the air, and once airborne, typically fly low and fast (3).
These birds return to the breeding grounds from April to June when groups of males immediately begin unusual courtship behaviours to attract a mate (2)(3). Swimming in synchrony with a characteristic posture, the males tuck their head into the shoulders, raise the crest and point the bill slightly upwards. The head is shaken from side to side in a rapid flicking motion and the head and neck are quickly raised and lowered towards the water with the crest erect (3). Once partners have paired up, the female will begin searching for a nest site, often amongst groups of other ducks, gulls or terns (3). A simple nest is scratched into the bare ground in a concealed location or in a natural cavity or burrow, typically within 25 metres from water, and gradually the female lines the nest with feathers and vegetation. Usually 8 to 10 eggs are laid and incubated for 31 to 32 days by the female. During this time, the male leaves the breeding site and travels along the coast in small groups to moult. The chicks fledge after some 60 to 65 days in the nest before reaching maturity at 2 years of age (2)(3).
The red-breasted merganser largely feeds on small fish, including juvenile fish in nurseries and salmon streams, although it may also eat aquatic invertebrates, such as crustaceans and insects, as well as some plant material (2). It may forage alone or in small groups, with individuals cooperating to trap fish. This bird typically swims along the water’s surface, repeatedly submerging its head to scan for its prey using lenses specially adapted for underwater sight. Diving to depths of up to five metres, the red-breasted merganser catches its prey after an underwater pursuit, or by probing at rocky crevices with its elongated bill (3).
The red-breasted merganser breeds throughout much of North America, as far south as the Great Lakes, as well as Greenland, Iceland and most of northern Europe and Asia, southwards to the United Kingdom, northeast China and Japan. However, outside of the breeding season, the range of this highly migratory species greatly expands to include both the Pacific and Atlantic coastlines of North America, down to Mexico, much of Central Europe and the Mediterranean, the Black Sea, southern Caspian Sea, the south coasts of Iran and Pakistan, the east coast of China, and the Korean Peninsula (2)(7)(8).
The red-breasted merganser breeds along the wooded shorelines of deep lakes, rivers and streams, as well as shallow bays and estuaries with a sandy bottom, preferring narrow channels with small rocky islands and grassy banks rather than large, open expanses of water. At other times of the year, it is largely found at sea, inhabiting deeper offshore waters as well as inshore areas (2)(7).
In the absence of any significant threats, the red-breasted merganser is not currently considered threatened with extinction. Despite some declines in parts of Europe, the species’ breeding range appears to be expanding, while there have been notable increases in some populations, such as that in Finland (2)(7)(9). However, although it is not usually considered a popular game species, this species is occasionally hunted in Denmark and North America, with 3,000 to 4,000 shot annually in Quebec, while its eggs may be eaten in Iceland (2)(3). Birds foraging near salmon rivers and fish farms may also be persecuted, often illegally, due to perceived predation upon fish stocks. In some areas, the red-breasted merganser is further threatened by pollution, the alteration of its habitat by dam construction and deforestation, as well as accidental drowning and entanglement in fishing nets (3)(7).
Although the red-breasted merganser is generally not the target of any specific conservation measures due to it still being relatively abundant across its wide range, it has benefited in many areas from the creation of artificial nest boxes, which increases the availability of nesting habitat. The removal of alien predators, such as the American mink (Mustela vison), has also allowed some threatened populations to make recoveries, including those on islands south-west of Finland (7).
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Diverse group of animals with jointed limbs and a hard chitinous exoskeleto) characterised by the possession of two pairs of antennae, one pair of mandibles (parts of the mouthparts used for handling and processing food) and two pairs of maxillae (appendages used in eating, which are located behind the mandibles). Includes crabs, lobsters, shrimps and barnacles.
To keep eggs warm so that development is possible.
Animals with no backbone, such as insects, crustaceans, worms, molluscs, spiders, corals and others.
When males and females of the same species differ in appearance.
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