The Red-breasted merganser (Mergus serrator) is a diving duck, one of the many bird species originally described by Carl Linnaeus in his landmark 1758 10th edition of Systema Naturae, where it was given the binomial name Mergus serrator. The Red-breasted merganser is also notable for being the fastest duck ever recorded. It attained a top airspeed of 100 mph (160 km/h) while being pursued by an airplane. This eclipsed the previous speed record held by a canvasback clocked at 72 mph (116 km/h).
The Red-breasted merganser has a spiky crest and long thin red bill with serrated edges. The male has a dark head with a green sheen, a white neck with a rusty breast, a black back, and white underparts. Adult females have a rusty head and a grayish body. Juveniles look similar to females, but lack the white collar and have smaller white wing patches.
Red-breasted mergansers breed across northern North America, Greenland, Europe, and the Palearctic. They are migratory and spend winter in coastal waters in the south. Red-breasted mergansers prefer to breed on freshwater lakes and rivers and also can use brackish water of estuaries, shallow bays, or inlets.
Outside of the breeding season, Red-breasted mergansers are social and form flocks that can reach 100 individuals. However, these flocks are smaller during spring migration than they are in autumn migration and in winter. Red-breasted mergansers feed during the day in shallow waters. They dive from the surface to pursue aquatic animals underwater, using serrated bills to capture slippery fish. When not breeding, Red-breasted mergansers are mostly silent but during courtship, the female gives a rasping 'prrak prrak', while the male gives a catlike 'meow'. In flight, the female makes a harsh 'gruk'.
Red-breasted mergansers are carnivores (piscivores) and mainly feed on small fish. They may also add to their diet aquatic insects, worms, crustaceans, and amphibians.
Red-breasted mergansers are serially monogamous and form pairs only for a period of one breeding season. They start to breed between April and June. Pairs can nest singly or in loose colonies. The nest is usually located near water; it is a shallow depression made with grass and lined with soft down. The female then lays 8-24 eggs and incubates them for about 28-35 days. The chicks hatch precocial; they are covered with down, their eyes are open and they can leave the nest within 24 hours. Fledging typically occurs at 60-65 days of age and the young become reproductively mature when they are 2 years old.
Although not considered threatened, populations of Red-breasted mergansers in some areas may be declining. The main threats include habitat loss through wetland destruction, exposure to toxins such as pesticides and lead, and becoming bycatch of commercial fishing operations. Anglers and fish farmers have also persecuted mergansers, which they regard as a competitors.
According to the IUCN Red List, the total population size of the Red-breasted merganser is around 495,000-605,000 individuals. The European population consists of 70,100-120,000 pairs, which equates to 140,000-240,000 mature individuals. Currently, this species is classified as Least Concern (LC) on the IUCN Red List, and its numbers today are stable.
