Red-breasted merganser Temporal range: Pleistocene–present PreꞒ Ꞓ O S D C P T J K Pg N ↓
Male in winter, New Jersey, US
Female, New Jersey, US
Conservation status
Least Concern  (IUCN 3.1)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Aves
Order:	Anseriformes
Family:	Anatidae
Genus:	Mergus
Species:	M. serrator
Binomial name
Mergus serrator Linnaeus, 1758
Red-breasted merganser range   Breeding   Resident   Non-breeding   Passage
Synonyms
Merganser serrator (Linnaeus, 1758)

The red-breasted merganser (Mergus serrator) is a duck species that is native to much of the temperate to subarctic Northern Hemisphere. The red breast that gives the species its common name is only displayed by males in breeding plumage. Individuals fly rapidly, and feed by diving from the surface to pursue aquatic animals underwater, using serrated bills to capture slippery fish. They migrate each year from breeding sites on lakes and rivers to their mostly coastal wintering areas, making them the most frequent species in the genus Mergus to frequent saltwater regularly. The worldwide population of this species is stable, though it is threatened in some areas by habitat loss and other factors.

The red-breasted merganser was formally described in 1758 by the Swedish naturalist Carl Linnaeus in the tenth edition of his Systema Naturae under the current binomial name Mergus serrator.^[2][3] The species is monotypic; no subspecies are recognised.^[4] Birds breeding in Greenland were sometimes distinguised as a subspecies M. s. schioleri in the past on the basis of averaging very marginally larger, but this is no longer accepted as distinct.^[5][6][4]

The genus name Mergus is a Latin word used by Pliny and other Roman authors to refer to an unspecified water bird. The specific epithet serrator is Latin for sawyer and is ultimately from serra, meaning saw. It refers to the saw-like projections on the bird's bill, which enable it to hold on to slippery fish, its most frequent prey.^[7]

The adult red-breasted merganser is 51–64 cm (20–25 in) long, has a wingspan of 66–74 cm (26–29 in). Males average slightly larger, weighing 950–1,350 g (34–48 oz), compared to female weight of 800–1,100 g (28–39 oz).^[5][8]

It 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 greyish-brown body. Juveniles look similar to females, but have a shorter crest, and marginally smaller white wing patches.^[6]

The range of the red-breasted merganser broadly overlaps with that of the similar and closely related common merganser. The two species can therefore occur in the same place at the same time, though the species often choose different habitats (red-breasted prefers saltwater, common prefers freshwater, but overlap is frequent).^[6] Breeding male plumages are fairly distinctive, but other plumages such as those born by females, immatures, and non-breeding males can make the two species hard to distinguish. Female common merganser has more contrast between the darker head and lighter breast, and has a light chin patch not seen on the red-breasted.^[9]

During courtship, the female gives a rasping prrak prrak, while the male gives a catlike meow. In flight, the female makes a harsh gruk. At other times this species is largely silent.^[10]

Male red-breasted mergansers show a highly stereotypic metronomic courtship display, in which they stretch, bend and contract their necks, often in synchrony by several males together, showing their crests alternately fanned and staight; females watch from nearby to select their preferred mate, and often chase or even attack the males. Display, which takes place while swimming, starts in mid November, and continues until June or rarely July.^[5]

Red-breasted mergansers dive and swim underwater. They mainly eat small fish, but also consume aquatic insects, worms, crustaceans, and amphibians.^[11]

Its breeding habitat is freshwater lakes and rivers across northern North America, Greenland, Europe, and the Palearctic. It nests in sheltered locations on the ground near water, often, but far from always, close to estuaries or sea coasts.^[6]

It is mostly migratory and northern breeders winter in coastal waters further south. Populations in parts of northwestern Europe with oceanic climates (Iceland, Ireland, Britain, Norway, and Baltic Sea islands) are largely resident or disperse only short distances.^[5] Outside of the breeding season, it forms flocks, which tend to be smaller during spring migration than they are in autumn migration and in winter. ^[12]

The red-breasted merganser is one of the species to which the Agreement on the Conservation of African-Eurasian Migratory Waterbirds (AEWA) applies. The species is also considered a game bird under the Migratory Bird Treaty between the United States and Canada.^[13] This means that the species gets some protection, though hunting it is legal in North America in certain seasons and places determined by local hunting regulations. However, few hunters are interested in the species and relatively few birds are harvested.^[11]

The species is widespread and common enough to be given conservation status of least concern by the IUCN, though populations in some areas may be declining. Threats include habitat loss through wetland destruction, exposure to toxins such as pesticides and lead, and becoming bycatch of commercial fishing operations.^[11] Anglers and fish farmers have also persecuted the species, which they regard as a competitor, though the impact of this on the species' population overall is not known;^[12] illegal persecution by fisheries interests is however significant in Britain, particularly Scotland.^[14]

• 
  Male in display posture, Baltic Sea coast of Germany
• 
  Male in display posture, Baltic Sea coast of Germany
• Courtship display, Illinois, USA
• 
  Male in eclipse plumage, Cemlyn, Wales
• 
  Male in flight, New Jersey, USA
• 
  Female in flight, New Jersey, USA
• 
  Egg, Collection Museum Wiesbaden

Wikimedia Commons has media related to Mergus serrator.

Wikispecies has information related to Mergus serrator.

• "Red-breasted merganser media". Internet Bird Collection.
• Red-breasted merganser photo gallery at VIREO (Drexel University)