Common Merganser

A regular breeding species and migrant; observed during the winter months in open-water areas, especially along the open-water stretches of the Mississippi River from central Minnesota south to the Iowa border. The Common Merganser was an uncommon species during the Minnesota Breeding Bird Atlas (MNBBA).

Distributed across the boreal forests of North America and Eurasia, the Common Merganser’s breeding range in North America extends from southeastern Alaska across Canada and as far south as the northwestern and west-central United States, the northern Great Lakes, and New England. 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.

A game species, the Common Merganser has been assigned a Moderately Low Continental Priority by the North American Waterfowl Management Plan and has been assigned a Continental Concern Score of 7/20 by Partners in Flight.

A short- to medium-distance migrant that winters across the central and southwestern United States, south to northern Mexico. Some populations are year-round residents.

A diver that feeds primarily on small fish but occasionally consumes aquatic invertebrates, small vertebrates, and plants.

Primarily a secondary-cavity nester but may nest in rock crevices, in holes under tree roots, along an eroded shoreline, or in nest boxes.

The Common Merganser is a breeding resident across Minnesota’s northern forest landscape. In the first comprehensive account of Minnesota’s avifauna, Hatch (1892), a physician in Minneapolis in the late 1800s, wrote that he would find young Common Mergansers “on the ponds and small grassy lakes” of “his country” in the early days. Presumably he was remembering birds he saw in the lakes and ponds surrounding Minneapolis when he first moved to the area in 1858.

Many years later, when Roberts (1932) wrote his two-volume treatise on Minnesota birds, he included this note from Hatch, as well as several other reports that mention the species’ presence and presumed breeding status in southern Minnesota. One account was from Dr. Hvoslef, an avid student of Minnesota’s bird life and a physician who lived and practiced in Lanesboro. In 1888, Dr. Hvoslef wrote that he “saw a female and I think 2 very small young close by.” Combined with 2 other accounts from Iowa and Heron Lake, the reports led Roberts to suspect that Common Mergansers once nested in limited numbers as far south as the Iowa state line.

At the time of his writing, however, there were no recent records from southern Minnesota. Instead, the species was considered a common summer resident in the northern forested regions of the state, particularly in the “triangle formed by the Rainy and Pigeon rivers on the north and Lake Superior on the south” (Roberts 1932).

Whether the Common Merganser ever nested south of its primary range in northern Minnesota is unclear. In their updated account of the species in 1975, Green and Janssen describe the species as a resident in the north-central and northeastern regions of the state. They noted the three 19th-century records mentioned by Roberts (1932) in Fillmore (1888), Hennepin (Hatch’s account in 1892), and Jackson (1887) Counties but remarked that there has never been further information to suggest that the species ever nested in the central and southern regions. Yet recent summer observations west of the species’ primary breeding range, in Clearwater and Otter Tail Counties, prompted them to encourage observers to look for the species in these areas. The species is strongly associated with deep, clear lakes and rivers, and thus the shallower and often turbid waters of southern Minnesota’s lakes and rivers seem ill suited for this diving bird. Furthermore, it would not be unusual for the female Common Merganser and young to be confused with the female Hooded Merganser, which is a regular breeding species in southern Minnesota. On the other hand, there are other historical accounts of a more southerly breeding distribution in Pennsylvania and, in the past 50 years, within Virginia (Pearce et al. 2015). Indeed, the species declined as a breeding species in these areas in response to hunting pressure and water-quality degradation. With more regulations and community efforts to improve water quality, Common Mergansers are now recolonizing some portions of their former range, where some of the largest gains have been seen in New York, Pennsylvania, and Vermont (Pearce et al. 2015).

In 1987, Janssen delineated 6 northern counties where breeding had been confirmed since 1970: Beltrami, Cook, Koochiching, Lake, Lake of the Woods, and St. Louis. By 1998, Hertzel and Janssen had added 2 additional counties: Cass and Itasca. Several years later, in 2006 and 2008, nesting was confirmed in Mille Lacs and Crow Wing Counties, respectively (Minnesota Ornithologists’ Union 2016).

The Minnesota Biological Survey reported 48 breeding season locations for the Common Merganser through 2014. All but 1 of the records were from 5 northern counties: Beltrami, Cook, Itasca, Lake, and St. Louis; 1 record was from southern Pine County (Minnesota Department of Natural Resources 2016).

During the MNBBA, participants reported 397 Common Merganser records from 6.2% (294/4,740) of the surveyed atlas blocks and from 5.8% (135/2,337) of the priority blocks. Breeding was confirmed in 88 (1.9%) of the surveyed blocks (Figures 2 and 3; Table 1). The birds were reported from 27 of Minnesota’s 87 counties and were confirmed breeding in 15 counties. In addition to the counties identified by Hertzel and Janssen in 1988, breeding evidence was confirmed in Aitkin, Hubbard, Itasca, Morrison, Sherburne, Stearns, and Wabasha.

The Landcover Suitability Model generated using MNBBA data clearly suggests the preponderance of highly suitable habitat in the Laurentian Mixed Forest Province (Figure 4) but also suggests scattered pockets of suitable habitat are present in the central and northern regions of the Eastern Broadleaf Forest Province, including the Twin Cities, and in the Tallgrass Aspen Parklands. Summer observations reported to the Minnesota Ornithologists’ Union in these regions certainly confirms the potential suitability of habitat in these localities (Minnesota Ornithologists’ Union 2016).  Small pockets of suitable habitat also are predicted to occur further south in the state.

As the MNBBA did for the Common Goldeneye, it documented two centers of the Common Merganser’s breeding distribution in northern Minnesota. The first is located in the Arrowhead region, and the second in the north-central counties of Aitkin, southern Beltrami, Cass, Crow Wing, Itasca, and northern Mille Lacs. Records are sparse in the intervening industrialized Iron Range.

Perhaps more interesting are the scattered records farther south in east-central, southern, and western Minnesota. Most notable are the confirmed breeding records from Sherburne, Stearns, and Wabasha Counties. All of these records were examined carefully; a few records outside the core northern range were deleted due to coding errors. But the remaining records all were from qualified observers. The recent movement of birds from their core breeding range to areas farther west and south has been noted by observers submitting summer records to the Minnesota Ornithologists’ Union as well. From 1930 to 1979, there were summer records outside of the species core breeding range in only 6 years; from 2000 to 2014 there were summer records outside of the core breeding areas in 12 of the 15 years, including every year since 2006 (Minnesota Ornithologists’ Union 2016). Although some of these reports may be of late migrants, there is a clear trend of increasing summer reports in southern Minnesota. Even more significant is the number of confirmed breeding reports in central and southern counties. Consistent with the observations reported by Pearce and his colleagues (2015), the Common Merganser may be expanding or reoccupying areas of its former breeding range farther to the south in Minnesota. Roberts’s (1932) original speculation about the species’ former status in this region may indeed have been correct.

Further evidence of the species’ occurrence in more southern regions of the Great Lakes states was documented by atlas work conducted in Wisconsin and Michigan. Wisconsin’s first atlas (1995–2000) confirmed a breeding record as far south as Waushara County in the central region of the state, east of La Crosse (Cutright et al. 2006). They noted that although the Common Merganser is still an uncommon breeder in the state, atlas data indicated the species is “more plentiful during the summer in Wisconsin than previously thought.” During the first two years of the second atlas, breeding was confirmed also in Marquette County, which is located immediately south of Waushara County (eBird 2017). During Michigan’s second atlas (2002–2009), records documented the Common Merganser’s expansion in the Lower Peninsula. “Particularly notable is confirmed nesting on the Wayne/Monroe County line, a breeding location unknown in historic times” and located along the Lake Erie shoreline south of Detroit (Chartier et al. 2013).

*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.

Common Mergansers are most commonly found on forested streams and on clear, deep, large lakes bordered by mature forests (Figure 5). Abundant fish populations and suitable nest cavities are essential. Forest stands are generally coniferous or mixed coniferous-deciduous. Rivers and streams can be particularly important travel corridors for the young to move from the nesting site downstream to larger lakes and rivers that provide them with sufficient cover and feeding opportunities (Baldassarre 2014; Pearce et al. 2015). Recent breeding records south of the Great Lakes northern forest landscape suggest, however, a broader habitat tolerance than previously described.

As for many forest-dependent waterfowl species, reliable, long-term population data on the Common Merganser are sparse. The international Waterfowl Breeding Population and Habitat Survey conducted by the U.S. Fish and Wildlife Service and the Canadian Wildlife Service does not cover all of the merganser’s breeding range, nor does it distinguish between the three merganser species: Common, Red-breasted, and Hooded. Instead, all statistics are presented for the “generic” merganser group, combining the data for all three species because of the difficulty of distinguishing the birds, especially the females, from one another during the aerial surveys.

Nevertheless, Baldassarre (2014) used a variety of estimates from different sources to generate an estimate of 943,000 birds in North America between approximately 1994 and 2007. In 2007, the Sea Duck Joint Venture provided another, slightly higher estimate of 1.2 million birds (Sea Duck Joint Venture 2007).

The annual Breeding Population Waterfowl Survey for Minnesota also does not census the core of the species’ breeding distribution, nor does it distinguish between the Red-breasted and Common Merganser (it does report separate numbers for the Hooded Merganser). The combined survey numbers have ranged from a low of 0 individuals to a high of only 1,032 birds; the average from 1987 to 2015 is only 335 birds (Cordts 2015).

An accurate assessment of population trends is equally difficult to obtain. The combined merganser count collected by the international Waterfowl Breeding Population and Habitat Survey shows relatively stable populations for all 3 mergansers combined from 1990 to 2015 (Figure 6). The federal Breeding Bird Survey (BBS), on the other hand, tracks the 3 merganser species separately. Although it is not a statistically robust monitoring tool for many waterfowl species, including the Common Merganser, it demonstrates a significant population decline averaging 1.98% per year from 1966 to 2015 (Sauer et al. 2017). The most dramatic decline occurred during the first 10 to 15 years of the survey, from approximately 1966 to 1980. The BBS data do suggest a more positive trend in the most recent 10-year reporting period, from 2005 to 2015, showing a 0.82 annual increase (Sauer et al. 2017).

A declining trend from the mid-1960s through the late 1970s was corroborated by Christmas Bird Count data in the United States and Canada (National Audubon Society 2016a). Although the data demonstrate some wide fluctuations, they suggest a long-term increase, particularly since the winter of 1976 (Figure 7).

Factors responsible for the decline in the late 20th century are not entirely known. Overall, one of the most important factors impacting populations may be the availability of suitable nest cavities (Pearce et al. 2015). Contaminants also pose a threat to this fish consumer. In particular, two studies documented significant levels of organochlorines, mercury, and other heavy metals in Common Merganser eggs collected in Great Lakes waters in the 1960s and 1970s (Faber and Hickey 1973; Haseltine et al. 1981).

In Minnesota, BBS data show a slow, recent increase in Common Merganser numbers, but the data are too sparse to be statistically reliable. Given the increasing number of observations of the birds during the summer months in central Minnesota, a small population increase seems reasonable.

In light of its relatively stable population trend and its low status as a favored game species, the North American Waterfowl Management Plan (2004) has categorized the Common Merganser a species of Moderately Low Continental Priority. It has also been assigned a relatively low Continental Concern Score of 7/20 by Partners in Flight (2017).

Because of their heavy consumption of fish, Common Mergansers are not a species targeted by waterfowl hunters. Nevertheless, they are a game species, and from 1999 to 2008 the U.S. harvest averaged 19,797 birds per year, while the Canadian harvest averaged 7,127 birds per year. Nearly 44% of the harvest occurs in the Atlantic Flyway (Baldassarre 2014).

Specific conservation measures targeted for the Common Merganser are lacking. Indeed, much remains to be learned regarding the species reproductive biology and population status (Pearce et al 2015). The Sea Duck Initiative and Sea Duck Joint Venture were established to improve our knowledge and understanding about an entire suite of species, including the Common Merganser, that are collectively known as sea ducks, species that spend a significant portion of their life in coastal waters. More attention has actually been directed at addressing concerns that local merganser populations may be impacting local fisheries. When more liberal hunting regulations were announced by the U.S. Fish and Wildlife Service in 1944, one of many factors prompting the change was the merganser’s impact on local fish hatcheries. “American [Common] and red-breasted mergansers have become so numerous as to constitute a menace to fish propagation, consequently the new regulations will permit a bag of 25 of these ducks” (U.S. Fish and Wildlife Service memorandum, August 9, 1944). Although there have been some targeted efforts to control merganser populations in order to increase local fish populations, the effectiveness of such measures requires more careful study (Baldassarre 2014).

Today the primary threats to the species appear to be pesticide contamination, due to its position high within the food web, and habitat degradation, resulting from poor water quality and acid precipitation (Pearce et al 2015; Baldassarre 2014). Climate change models have provided variable predictions. The “State of the Birds 2010 Report on Climate Change” considered the Common Merganser to have a relatively low vulnerability to a warming climate. A more recent analysis conducted by the National Audubon Society classified the Common Merganser as “climate threatened” and predicted that only 28% of the species’ current breeding range would remain intact by the year 2080 (Langham et al. 2015; National Audubon Society 2016b).

In Minnesota, maintaining good water quality in the northern forest lakes and rivers and implementing good forest management practices that focus on retaining mature trees and snags are both critical to the future of the Common Merganser.