Red-breasted Merganser Mergus serrator Scientific name definitions

The Red-breasted Merganser is a medium-sized “saw-bill” duck with a wide distribution in the Northern Hemisphere, breeding up to 75°N. An expert diver, it usually runs across the water to become airborne and can fly low and noiselessly over water at considerable speed. Its serrated mandibles enhance its ability to capture underwater prey. It is similar in distribution and ecology to the Common Merganser (Mergus merganser) but, in contrast, it occurs more frequently in salt water and estuaries, typically nests on the ground, and has dark-colored down.

Its spectacular asymmetrical nuchal crest appears incongruous with symmetry elsewhere in nature. Odd social courtship displays draw attention to males in late winter and spring. This duck does not acquire full breeding plumage until the second year, and it first breeds when at least 2 years of age. Seasonally it is a notably late breeder, having young that often do not fledge until September. Young are adapted to cool northern waters and quickly become independent. Females are left to incubate and care for young as males depart during early incubation for poorly known molting areas. Large creches are a feature of coastal areas with high densities of breeders.

The Red-breasted Merganser breeds and winters in cool northern environments with abundant small fish. It is a bird of the tundra and boreal forest zones, inhabiting coastal bays and large water bodies or rivers. In coastal areas, Red-breasted Mergansers are often observed at river mouths where, on islands, they breed in loose colonies and in association with terns and gulls. There are remarkable records of distribution of this species at high latitudes.

Much of what we know about the Red-breasted Merganser comes from studies conducted in Iceland and Scandinavia by researchers examining other species of diving ducks concurrently (Bengtson Bengtson 1970b Bengtson, S. A. (1970b). Location of nest-sites of ducks in Lake Myvatn area, north-east Iceland. Oikos 21:218-229. Close , Bengtson 1971c Bengtson, S. A. (1971c). Variations in clutch-size in ducks in relation to the food supply. Ibis 113:523-526. Close , Bengtson 1971a Bengtson, S. A. (1971a). Food and feeding of diving ducks breeding at Lake Myvatn, Iceland. Ornis Fennica 48:77-92. Close , Bengtson 1971b Bengtson, S. A. (1971b). Habitat selection of duck broods in Lake M?yvatn area, north-east Iceland. Ornis Scandinavica 2:17-26. Close , Bengtson 1972b Bengtson, S. A. (1972b). Reproduction and fluctuations in the size of duck populations at Lake Myvatn, Iceland. Ornis Scandinavica 2:17-26. Close ; Hildén 1964 Hildén, O. (1964). Ecology of duck populations in the island group of Valassaaret, Gulf of Bothnia. Annales Zoologici Fennici 1:153-279. Close , Nilsson Nilsson 1965 Nilsson, L. (1965). Observations on spring behaviour of the Red-breasted Merganser. Var Fagelvarld 24:244-256. Close , Nilsson 1970a Nilsson, L. (1970a). Food-seeking activity of south Swedish diving ducks in the non-breeding season. Oikos 21:145-154. Close , Nilsson 1970b Nilsson, L. (1970b). Local and seasonal variation in sex ratios of diving ducks in south Sweden during the non-breeding season. Ornis Scandinavica 1:115-128. Close , Nilsson 1980 Nilsson, L. (1980). Winter diving duck populations and available food resources in the Baltic. Wildfowl 31:131-143. Close , Joensen 1973 Joensen, A. H. (1973). Moult migration and wing-feather moult of sea-ducks in Denmark. Danish Review of Game Biology 8 (4). Close , Sjöberg Sjöberg 1985 Sjöberg, K. (1985). Foraging activity patterns in the Goosander (Mergus merganser) and the Red-breasted Merganser (M. serrator) in relation to patterns of activity in their major prey species. Oecologia 67:35-39. Close , Sjöberg 1988a Sjöberg, K. (1988a). Food selection, food-seeking patterns and hunting success of captive Goosanders Mergus merganser and Red-breasted Mergansers M. serrator in relation to the behaviour of their prey. Ibis 130:79-93. Close , Sjöberg 1989 Sjöberg, K. (1989). Time-related predator/prey interactions between birds and fish in a northern Swedish river. Oecologia 80:110. Close ). Early North American studies of food habits stemmed from attempts to measure impacts of mergansers on salmon stocks, and to protect fish (White White 1936 White, H. C. (1936). The food of kingfishers and mergansers on the Margaree River, Nova Scotia. J. Biol. Board Can. 2:299-309. Close , White 1937 White, H. C. (1937). Local feeding by kingfishers and mergansers. J. Biol. Board Can. 3:323-338. Close , White 1939a White, H. C. (1939a). Bird control to increase the Margaree River Salmon. Bull. Fish. Res. Board Canada 58. Close , White 1939c White, H. C. (1939c). The food of Mergus serrator on the Margaree River, Nova Scotia. J. Fish. Res. Board Canada 4:309-311. Close , White 1957 White, H. C. (1957). Food and natural history of mergansers on salmon waters in the Maritime provinces of Canada. Bull. Fish. Res. Board Canada 116. Close , Munro and Clemens 1939 Munro, J. A. and W. A. Clemens. (1939). The food and feeding habits of the Red-breasted Merganser in British Columbia. Journal of Wildlife Management 3:46-53. Close , Elson Elson 1950 Elson, P. F. (1950). Increasing salmon stocks by control of mergansers and kingfishers. Fish. Res. Board Can. Atl. Prog. Rep. 51:12-15. Close , Elson 1962 Elson, P. F. (1962). Predator-prey relationships between fish-eating birds and Atlantic Salmon. Bull. Fish. Res. Board Can. 133. Close ).

More recently, efforts have been made to examine the level of competition between foraging Red-breasted Mergansers and sport fish in the Great Lakes region (Bur et al. 2008). Reproductive biology is known primarily from studies in eastern Canada, and remains largely unknown elsewhere. Work proceeding at Kouchibouguac National Park, New Brunswick, since 1984 is attempting to elucidate reproductive strategies that involve habitat selection, brood parasitism, and survival, in association with a large colony of terns (Young and Titman Young and Titman 1986 Young, A. D. and R. D. Titman. (1986). Costs and benefits to Red-breasted Mergansers nesting in tern and gull colonies. Canadian Journal of Zoology 64:2339-2343. Close , Young and Titman 1988 Young, A. D. and R. D. Titman. (1988). Intraspecific nest parasitism in Red-breasted Mergansers. Canadian Journal of Zoology 66:2454-2458. Close , Titman 1997 Titman, R. D. (1997). Population dynamics and social system of reproductive Red-breasted Mergansers in Kouchibouguac National Park, New Brunswick. Paper read at Proceedings of a workshop on Research, Monitoring and Natural Resource Management in the Greater Kouchibouguac Ecosystem., at Richibucto, N.B., 23-24 Nov, 1995. Close , Craik and Titman 2008, 2009). Only recently have key post-breeding molting sites been identified and the biology of flightless birds been documented (Craik et al. 2009, 2011).

Red-breasted Mergansers exhibit lower levels of breeding population differentiation than the Common Merganser, possibly because of range expansion to more northerly latitudes following the last glacial maximum in North America. Additionally, costs of dispersal from breeding sites may be lower for the ground-nesting Red-breasted Merganser than for the Common Merganser, which requires a cavity for nest placement (Pearce et al. 2009).

Overall, the Red-breasted Merganser remains one of the least understood species of waterfowl in North America. Size and trends in continental populations are not reliably known because surveys do not typically differentiate between Red-breasted and Common mergansers, large portions of the species' range are not surveyed, and most counts occur too early to provide adequate estimates for this late-breeding duck. Aspects of migration and connectivity between breeding, molting, and wintering sites remain largely unknown.