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Red-breasted Merganser

Mergus serrator
Overview
Minnesota Seasonal Status:

A regular breeding resident and migrant; regular in winter months at scattered open-water locations, especially along Lake Superior’s North Shore. The Red-breasted Merganser was a rare species during the Minnesota Breeding Bird Atlas (MNBBA).

North American Breeding Distribution and Relative Abundance:

The Red-breasted Merganser has a global distribution. In North America, northern Minnesota is on the southern edge of the breeding range, which extends from Alaska south and east across Canada, and north to the southern coastal waters of Greenland. In the United States, the merganser breeds locally within the Great Lakes and in northern New England with occasional scattered records farther south. Its distribution and relative abundance in southern Canada and the United States, as depicted by the federal Breeding Bird Survey (BBS), are shown in Figure 1.

Conservation Concern:
Conservation Status Score 11

A game species, the Red-breasted Merganser has been assigned a Moderately Low Continental Priority by the North American Waterfowl Management Plan. It has been assigned a Continental Concern Score of 11/20 by Partners in Flight.

Life History
Migration:

A short- to medium-distance migrant that winters in coastal waters along North America and northern Mexico and in the Great Lakes.

Food:

A diver that feeds primarily on small fish and crustaceans but occasionally takes worms, aquatic insects, frogs, and tadpoles.

Nest:

Usually nests in a small, sheltered depression on the ground near water; occasionally nests in a hollow stump.

Red-breasted Merganser Red-breasted Merganser. Mergus serrator
© Peter Massas
Figure 1.

Breeding distribution and relative abundance of the Red-breasted Merganser in North America based on the federal Breeding Bird Survey from 2011 to 2015 (Sauer et al. 2017).

Minnesota Breeding Bird Distribution*

Restricted largely to the cold waters of Lake Superior’s northern shoreline and other cold, deep waters of the northern forests, the Red-breasted Merganser is the least common of Minnesota’s three merganser species.

As early as the late 1800s, Hatch (1892) wrote that the species was rare throughout northern Minnesota but actually bred “sparingly” as far south as Minneapolis. Roberts (1932), however, seriously questioned Hatch’s assertion that the species occurred anywhere other than the northern forest region. His own files, which included his personal records and those of many contributing observers throughout the state, dated back nearly 50 years and included no evidence that the birds ever occurred south of Lake Mille Lacs. Compared to observations of the Common Merganser, there was even little evidence north of Lake Mille Lacs. Roberts described the species as an uncommon summer resident.

Most reports came from along the North Shore of Lake Superior, where birds were frequently observed at the mouths of many of the region’s fast-moving rivers, including the Pigeon and Temperance. Roberts’s account included several reports of individual Red-breasted Mergansers and pairs sighted along the shoreline from St. Louis Bay in Duluth (1905) to a site 12 miles north of Grand Marias (1928). All accounts presume nesting, but only 1 account by Roberts and his colleague Kilgore came close to confirming nesting. On June 15, 1915, they visited Spirit Island in Lake Mille Lacs. They reported “a pair seen and evidently had a nest; one egg found in a recess among rocks but nest could not be reached.” His account also included his own 1915 observation of a pair “evidently nesting” at Big Oak Island in Lake of the Woods.

Many years later, when Green and Janssen (1975) published their updated accounts of Minnesota birds, the Red-breasted Merganser’s status and distribution were largely unchanged. They described it simply as a resident along the Lake Superior shoreline in Cook, Lake, and St. Louis Counties. Solid breeding evidence remained rare. They wrote, “Good breeding evidence is available for Mille Lacs Lake and for Big Sandy Lake, Aitkin County, but the species is very scarce there.” At the time there were published nesting reports of birds in Beltrami County, inland St. Louis County, and even as far south as Houston County, but the absence of details left open the possibility that female Common or Hooded Mergansers were being confused with female Red-breasted Mergansers, as the confirmation of nesting was often the report of a female with a brood of young (Minnesota Ornithologists’ Union 2017). Female Common and Red-breasted Mergansers can be especially difficult to distinguish from one another, particularly at a distance.

In his updated account of the species’ status in 1987, Janssen reported another new nesting locality, from Leech Lake in Cass County (Oring 1978). Again no nests were found, but the species’ identification was confirmed when a bird was collected. Nevertheless, between 1970 and 1987, there were only 5 counties where breeding was confirmed: Aitkin, Cass, Cook, Lake, and St. Louis. There had been no recent evidence of the birds nesting on Lake of the Woods since the time of Roberts (1932), although Janssen (1987) noted a June 13, 1977, record from the lake that he presumed was a late spring migrant.

Testament to the species’ rarity, the Minnesota Biological Survey has reported Red-breasted Mergansers at only 1 location during their surveys, specifically in central St. Louis County (Minnesota Department of Natural Resources 2016).

During the MNBBA, participants reported 37 Red-breasted Merganser records from 27 (0.6%) of the 4,734 surveyed atlas blocks and from 9 (0.4%) of the 2,337 priority atlas blocks. Breeding was confirmed in 5 surveyed atlas blocks, 4 of which were along the North Shore of Lake Superior (Figures 2 and 3; Table 1). Two of the North Shore blocks were immediately adjacent to one another in Two Harbors. The other block was located in Crow Wing County. All breeding records were of females with broods.

Given the possible misidentification of female mergansers, each of the MNBBA Red-breasted Merganser records were examined closely, particularly those on inland lakes away from the species’ traditional Lake Superior habitats. Four records were invalidated; of the remaining 27 records, all but 3 mentioned the presence of males and/or were on lakes where males had been sighted. The remaining 3 records were in suitable habitat and were observed by experienced observers and accepted.

One confirmed breeding record was from a county where the species had not previously been confirmed nesting. The record was reported from Pelican Lake in Crow Wing County. There were several observations of male and female Red-breasted Mergansers on Pelican Lake during the summer of 2010, lending credence to the nesting report. Clearly, Red-breasted Mergansers, although quite rare, do find suitable habitat in some of Minnesota’s northern lakes away from Lake Superior’s North Shore. But they remain a rare bird and are certainly not a regular species west of Lake Superior. Indeed, their range and abundance have changed little in Minnesota over the past 100 years. The same is true throughout North America (Craik et al. 2015).

*Note that the definition of confirmed nesting of a species is different for Breeding Bird Atlas projects, including the definition used by the Minnesota Breeding Bird Atlas, compared with a more restrictive definition used by the Minnesota Ornithologists’ Union. For details see the Data Methods Section.

Figure 2.

Breeding distribution of the Red-breasted Merganser in Minnesota based on the Breeding Bird Atlas (2009 – 2013).

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Figure 3.

Summary statistics of observations by breeding status category for the Red-breasted Merganser in Minnesota based on all blocks (each 5 km x 5 km) surveyed during the Breeding Bird Atlas (2009-2013).

Breeding statusBlocks (%)Priority Blocks (%)
Confirmed5 (0.1%)1 (0.0%)
Probable5 (0.1%)2 (0.1%)
Possible14 (0.3%)4 (0.2%)
Observed3 (0.1%)2 (0.1%)
Total27 (0.6%)9 (0.4%)
Table 1.

Summary statistics for the Red-breasted Merganser observations by breeding status category for all blocks and priority blocks (each 5 km x 5 km) surveyed during the Minnesota Breeding Bird Atlas (2009-2013).

Breeding Habitat

Primarily a coastal bird, breeding pairs are usually found in backwater wetlands that may be brackish or saltwater. In Minnesota, Red-breasted Mergansers prefer large, deep lakes and rivers bordered by rocky shorelines, gravel outwash, or sandbars (Figure 4). Shorelines with dense, low vegetative cover provide the shelter needed for ground nests. Lakes smaller than 15 ha are selected for nesting only when they are connected to larger, deeper water bodies.

One important factor in selecting suitable breeding habitat is the abundance of small fish that range in size from 10 to 15 cm in length (Craik et al. 2015; Kaminski and Weller 1992).

Figure 4.

Typical breeding habitat of the Red-breasted Merganser in Minnesota (© Gerald J. Niemi).

Population Abundance

Because Red-breasted and Common Mergansers are not distinguished from one another in the annual Waterfowl Breeding Population and Habitat Survey conducted in the United States and Canada, population data are scarce. The survey also does not cover the northernmost reaches of the Red-breasted Merganser’s breeding range. Nevertheless, several biologists and conservation initiatives have estimated that the North American population ranges from a low of 180,000 individuals to as high as 670,000 (Baldassarre 2014). The Sea Duck Joint Venture estimated the population at 350,000 birds (Sea Duck Joint Venture 2007).

Population trends are equally difficult to discern. Data collected by the Waterfowl Breeding Population and Habitat Survey demonstrate a relatively stable population of all mergansers since 1900 (Figure 5; U.S. Fish and Wildlife Service 2016). Data collected by the federal Breeding Bird Survey (BBS) reveal a sharp decline from the mid-1960s until the mid-1980s, followed by relatively stable numbers (Sauer et al. 2017). The data, however, are very sparse and not statistically robust.

The only other long-term data set that is available, which does distinguish between Common Mergansers and Red-breasted Mergansers, is the National Audubon Society Christmas Bird Count. Unfortunately the data have some anomalies in the early years (it began in the early 1900s) that distort recent trends. Yet, when data are examined from 1966 (the same year that the BBS began) to the present time, they demonstrate a small increase beginning in the early 1980s, followed by relatively stable but fluctuating population levels in the following years (Figure 6). An examination of data from years prior to 1966 show that population numbers were actually declining from the late 1950s through the early 1980s (National Audubon Society 2016a).

A similar decline was observed in Common Merganser populations during the same years. Although the factors responsible for the decline in the late 20th century are not entirely understood, contaminants are known to pose a threat to this fish consumer. In particular, White and Cromartie (1977) collected Common, Hooded, and Red-breasted Merganser eggs in the early 1970s on national wildlife refuges throughout the United States. Their results documented “potentially dangerous levels of DDE and possibly PCB’s” in both Common and Red-breasted Merganser eggs. Mercury and a range of other contaminants also were present; eggs also demonstrated significant eggshell thinning. The potential impact of these findings on overall reproductive success, however, was unknown. A second study conducted on Red-breasted Mergansers nesting in Lake Michigan in the late 1970s found no relationship between levels of contaminants measured in eggs and in blood samples collected from incubating females and overall reproductive success (Heinz et al. 1983). Declines in many of the contaminants measured were apparently underway before 1977 (Craik et al. 2015), so the impacts on reproduction may have occurred in earlier years.

In Minnesota, the annual Breeding Population Waterfowl Survey collects data on “large mergansers,” but the survey area largely excludes most of the Red-breasted Merganser’s range in the state, as well as that of the Common Merganser (Cordts 2015).

Other than the potential impact of contaminants, factors that impact local and regional population trends are not well understood but are presumed to be related to food availability and habitat conditions, both of which need considerably more study (Craik et al. 2015).

Figure 5.

Merganser Population Trends from the Waterfowl Breeding and Habitat Survey (U.S. Fish and Wildlife Service 2016).

Figure 6.

Red-breasted Merganser numbers reported on the annual Christmas Bird Count in North America from 1966-2015 (National Audubon Society 2016).

Conservation

Although data on the species’ population size and long-term population trends are scanty, it appears that Red-breasted Merganser populations are relatively stable and secure in their remote, boreal breeding habitats. The North American Waterfowl Management Plan (2004) designated the species a Moderately Low Continental Priority in 2004. More recently, Partners in Flight (2017) assigned it a moderate Continental Concern Score of 11/20 reflecting, in part, concern regarding its small population size and uncertain population trend.

As a fish consumer, the Red-breasted Merganser is not very tasty and so is not a favorite among waterfowl hunters. Its preference for open, coastal waters in the fall also makes it a difficult target. Regardless, it is a harvested species, and from 1999 to 2008 the annual harvest in the United States averaged 14,376 birds; during the same period the annual harvest in Canada averaged 5,253 birds per year (U.S. Fish and Wildlife Service 2017; Baldassarre 2014). The majority of birds are harvested in the Atlantic Flyway.

Like other boreal nesting waterfowl, the Red-breasted Merganser has not been the focus of specific conservation initiatives. Even less is known about this species than its relative the Common Merganser. It is 1 of 15 species of waterfowl that are collectively referred to as sea ducks, species that spend a significant portion of their life cycle in coastal waters. Among Minnesota breeding species, these species include Common Goldeneyes, Buffleheads, and Common and Red-breasted Mergansers. The Sea Duck Initiative and Sea Duck Joint Venture were established specifically to draw more attention to these species in hopes of generating more information about their life history and conservation needs to insure the maintenance of sustainable populations.

The only targeted management measures have taken aim at the species’ consumption of game fish. Efforts to control both Red-breasted and Common Mergansers in localities where young game fish populations are abundant have largely been expensive and ineffective, particularly in the Pacific Northwest. Some efforts on the Canadian Atlantic coast, however, have been locally effective in reducing mortality of young salmon. Nevertheless, biologists caution that such measures warrant careful consideration regarding their cost and their long-term viability and effectiveness (Baldassarre 2014; Craik et al. 2015).

In addition to being targeted for its fish consumption, the Red-breasted Merganser faces potential threats from habitat loss and degradation. Since its breeding range is largely confined to boreal Canada, the species’ future is tied to the future of this region. Although it remains the world’s largest, intact ecosystem, habitat loss and degradation in the face of increasing industrialization and development are major concerns. Numerous conservation organizations have united in an effort to insure that critical breeding areas for all boreal nesting birds are protected.

Climate change is another major concern. Models have predicted variable impacts to the Red-breasted Merganser. The “State of the Birds 2010 Report on Climate Change” considered the species to have a relatively low vulnerability to a warming climate (North American Bird Conservation Initiative 2010). A more recent analysis, however, predicted that only 19% of the species’ current breeding range would remain intact by the year 2080 (Langham et al. 2015; National Audubon Society 2016b). The results led to their classification of the species as “climate threatened.”

At the southern periphery of its breeding range in Minnesota, the Red-breasted Merganser has a secure future in the state in the short term. However, a continued focus by resource agencies and communities on maintaining good water quality in northern lakes and rivers remains essential.

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  • Minnesota Department of Natural Resources. 2016. “Red-breasted Merganser (Mergus serrator).Minnesota Biological Survey: Breeding Bird Locations. http://files.dnr.state.mn.us/eco/mcbs/birdmaps/red_breasted_merganser_map.pdf
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  • Sea Duck Joint Venture. 2007. Recommendations for Monitoring Distribution, Abundance, and Trends for North American Sea Ducks. Anchorage, AK: U.S. Department of the Interior, Fish and Wildlife Service.

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