- Minnesota Breeding Bird Distribution
- Breeding Habitat
- Population Abundance
- Literature Cited
A regular breeding resident and migrant; the American Bittern was an uncommon species during the Minnesota Breeding Bird Atlas (MNBBA).
Broadly distributed across Canada from central British Columbia east to the Maritime Provinces and south through the northern half of the United States. The American Bittern reaches its highest breeding densities in the Prairie Pothole Region of North and South Dakota and the south-central Prairie Provinces of Canada (Figure 1).
Assigned a Continental Concern Score of 12/20 by Partners in Flight and ranked a High Concern by the North American Waterbird Conservation Plan; designated a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources.
In the northern regions of its breeding range, the American Bittern is a medium-distance migrant, wintering in the southern United States and Mexico; southern breeding populations may be year-round residents.
Feeds on a wide variety of invertebrates, amphibians, small fish, and small mammals.
A simple platform constructed with emergent wetland vegetation and placed above standing water.
Both Hatch (1892) and Roberts (1932) devoted far more of their written accounts on this secretive marsh bird to its unique behavior and “pump-handle” call than to a description of its distribution. They simply noted that the American Bittern occurred statewide wherever suitable habitat was available. As Roberts noted, “The concealing coloration and the solitary, stealthy habits of the Bittern lead to its being comparatively little known, though it is an abundant bird throughout the state wherever suitable lowlands exist.” He identified confirmed nesting records (nests with eggs) from 9 counties, all located north of a line between Lac qui Parle County in the west and Hennepin County in the east: Aitkin, Grant, Hennepin, Isanti, Lac qui Parle, Marshall, Meeker, Polk, and Sherburne. A confirmed nesting report was also available from Leech Lake.
Green and Janssen (1975) also made brief note of the bittern’s statewide distribution, adding only that it was most abundant in central Minnesota. A few years later, however, Janssen (1987) depicted its distribution as largely confined to counties north of the Minnesota River but absent from the intensively cultivated region of the Red River valley. Although it might still occur sparingly south of the river, he noted that it had declined “drastically” in the southern and western regions of the state in recent years. Indeed, just a few years before Janssen’s updated account was published, the bittern was officially listed as a Special Concern Species in Minnesota in 1984 (Coffin and Pfannmuller 1988). Janssen (1987) identified 8 counties where nesting had been confirmed since 1970: Aitkin, Beltrami, Clearwater, Lake, Lake of the Woods, Marshall, Mille Lacs, and St. Louis. Hertzel and Janssen (1998) later deleted Clearwater but added 3 more northern counties to the list: Becker, Morrison, and Polk.
Since the Minnesota Biological Survey (MBS) began intensive survey work on rare species in the late 1980s, field staff have documented 353 breeding season locations for the American Bittern. At least 14 sites were located south of the Minnesota River, most in the south-central and southwestern regions. Farther north, the birds were especially common in the northwest Tallgrass Aspen Parklands Province (Minnesota Department of Natural Resources 2016).
MNBBA participants reported American Bitterns in 10.2% (484/4,746) of the surveyed atlas blocks and in 10.9% (255/2,337) of the priority blocks. Breeding evidence was documented in just 7 surveyed blocks, while probable breeding was reported in 120 blocks (Figures 2 and 3; Table 1). The birds were reported in 52 of Minnesota’s 87 counties (1 block straddled Yellow Medicine and Chippewa Counties) . Breeding was confirmed in 5 counties; 2 were additions to the list compiled by Hertzel and Janssen in 1998 (Koochiching and Lac qui Parle).
The overall distribution of MNBBA records for the bittern was very similar to that reported by the MBS. The birds were most abundant in the north-central and northeastern counties of the Laurentian Mixed Forest Province and in the Tallgrass Aspen Parklands Province. Few were reported in the Eastern Broadleaf Forest Province, particularly south of the Twin Cities metropolitan region. Records south of the Minnesota River were limited and confined to the south-central and southwestern regions.
When MNBBA data are combined with data on climate, habitat, landscape potential, and detectability, the core of the American Bittern’s distribution is predicted to be in northern Minnesota (Figure 4). Throughout this region, breeding densities are predicted to be quite low. Although it is predicted to occur sparingly elsewhere in the state, moderate centers of abundance are predicted in the broad, expansive wetlands of the Tallgrass Aspen Parklands Province that border Canada and in the Agassiz Lowlands Subsection just west of Red Lake. The extent of wetlands in these areas suggests that both the size and landscape context of wetlands may be important criteria in determining habitat suitability. The moderately high breeding densities predicted in these regions are substantiated by a map of the species’ relative abundance generated by field data from the federal Breeding Bird Survey (Figure 1). However, the MNBBA probability model may underpredict the species’ abundance in west-central and southwestern Minnesota.
As the MNBBA data demonstrated, not only is the American Bittern no longer statewide in its distribution, as Roberts (1932) once described, but its center of abundance may have shifted north since 1975, when it was considered most abundant in central Minnesota. A nearly identical shift has occurred in Wisconsin. There the species was considered most abundant in the southern regions of the state in the early 1900s, receding north to central Wisconsin in the mid-1900s. By the time the state’s first breeding atlas was completed in 2000, the center of abundance had shifted farther north yet again (Cutright et al. 2006). A similar decline in bittern numbers has been documented in the southern regions of Michigan’s Lower Peninsula (Chartier et al. 2013).
Certainly the loss of wetlands in southern Minnesota, Wisconsin, and Michigan in the face of intensive agriculture and urban development was a primary factor for the observed changes. But the northward shift may also be part of a much longer, historical change in the species’ distribution. Lowther and his colleagues (2009) speculated that the species’ range shifted north as the glaciers retreated. Even in the more recent past, there is strong evidence that the species has gradually retreated northward from areas where it was formerly more abundant along the south-central Atlantic coast (Post 2004).
*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.
|Breeding status||Blocks (%)||Priority Blocks (%)|
|Confirmed||7 (0.1%)||4 (0.2%)|
|Probable||120 (2.5%)||66 (2.8%)|
|Possible||352 (7.4%)||181 (7.7%)|
|Observed||5 (0.1%)||4 (0.2%)|
|Total||484 (10.2%)||255 (10.9%)|
The American Bittern is often classified as an obligate wetland species. Although it uses a variety of grassland and wetland habitats during the breeding season, in late summer it confines itself largely to the dense cover and protection of wetlands when it undergoes a nearly complete molt that leaves it flightless (Figure 5; Azure 1998). During this period, the birds require deep, stable water levels that provide sufficient food resources and protection from predators (Dechant et al. 2004; Lowther et al. 2009). Although many aspects of the species’ life history are poorly known, numerous field studies in Minnesota have helped to delineate the range of habitats selected (e.g., Azure 1998; Brininger 1996; Faanes 1981; Hanowski and Niemi 1986, 1990; Lor 2007; Svedarsky 1992).
Wetlands utilized include permanent and semipermanent basins dominated by dense stands of emergent vegetation, willow and alder shrublands, and wet meadows. When standing water is present, depths range from 8 to 65 cm (Dechant et al. 2004). Wetlands may range in size from 1 to at least 100 ha (Hanowski and Niemi 1990).
Grasslands utilized included native and restored grasslands, hayfields, idle pastures, and old fields. Sites that have been recently mowed, heavily grazed, tilled, or burned are rarely used. In one study, most uplands selected were along the edge of wetlands (Svedarsky 1992); other studies have demonstrated that the average distance to the nearest wetland is approximately 100 m (Dechant et al. 2004). Vegetation height ranges from 60 to 75 cm (Svedarsky 1992; Brininger 1996).
Habitat profiles within 200 m of MNBBA point counts where bitterns were detected emphasize the species’ association with marshes, wet meadows, and shrub wetlands, as well as its prevalence in wetlands embedded within upland coniferous forests (Figure 6).
As for most solitary nesting marsh birds, data on population size are difficult to obtain and wrought with challenges due to small sample sizes. The federal Breeding Bird Survey (BBS) monitors American Bitterns, but because it is a roadside survey, it generally underrepresents wetland habitats. The sparse distribution of some species, such as the American Bittern, further confounds the ability to gather reliable data.
Regardless, BBS data are the only population data available across much of the American Bittern’s North American breeding range. Biologists have used that information to generate an estimate of the North American population at 3 million breeding adults (Delany and Scott 2002). A reliable statewide estimate is not available.
The American Bittern is uncommon throughout its breeding range; the highest concentration of breeding adults is found in the Prairie Pothole Region of the United States and south-central Canada. An average of 1 to 2 bitterns is encountered on BBS routes in this region, the highest for any landscape region in North America. By contrast, in Minnesota, an average of less than 1 bird is encountered (Sauer et al. 2017).
Because of the species’ low abundance throughout North America, statistical analyses of population trends lack precision. Nevertheless, throughout the BBS survey area, American Bitterns have demonstrated a relatively stable population since monitoring began in 1966, with a long-term, nonsignificant decline of 0.52% per year from 1966 to 2015 (Sauer et al. 2017). Regionally, however, population trends are quite variable. In the Prairie Pothole Region, populations are faring quite well, but to the east they are declining throughout much of the Great Lakes region (Figure 7).
The more intensive Great Lakes Marsh Monitoring Program coordinated by Bird Studies Canada corroborates this declining trend in the Great Lakes region. Begun in 1995, the binational program surveys marsh birds and frogs at more than 5,000 locations in the Great Lakes Basin. From 1995 through 2012, American Bitterns demonstrated a statistically significant decline of 2.4% per year (Tozer 2013). Recently several Midwest states, including Minnesota, have initiated similar, volunteer-based, marsh bird monitoring programs. If these efforts continue, they will eventually provide more robust data on the status of these secretive species.
Although the data lack statistical precision, in Minnesota, BBS data demonstrate a significant, long-term population decline, averaging 3.22% per year since 1967, and declining further at the rate of 4.21% per year from 2005 through 2015 (Sauer et al. 2017). The bittern’s downward trend line is not unlike that observed for many waterfowl species in the state. Following years of drought conditions on the prairies, populations have rebounded in the center of the Prairie Pothole Region but have remained low or have continued to decline in Minnesota. This may explain some of the population decline in the western regions of the state. However, other factors must be at play in the north-central and northeastern counties, where the bittern is an uncommon but regular species and where wetland habitats are still well represented across the landscape. Nevertheless, habitat loss and degradation are consistently mentioned as factors affecting local populations throughout the species’ range.
In light of the relatively poor understanding of the species’ status and its dependence on two vulnerable habitats, wetlands and grasslands, the American Bittern has been assigned a Continental Concern Score of 12/20 (Partners in Flight 2017). The North American Waterbird Conservation Plan ranked it a species of High Concern (2006),the U.S. Fish and Wildlife Service designated it a Species of Conservation Concern in the Upper Midwest Region (U.S. Fish and Wildlife Service 2008), and it is recognized as a priority conservation species by numerous other resource planning initiatives (e.g., Soulliere et al. 2007; Wires et al. 2010).
The species was officially listed as a state Special Concern Species in 1984. In preparation for a review of the list in the mid-1990s, Minnesota Department of Natural Resources (DNR) personnel requested field staff in northeastern Minnesota to submit observations in an effort to reassess its status. A total of 200 reports were submitted between 1991 and 1994. In addition, in the spring of 1994, DNR field biologists conducting Ruffed Grouse drumming surveys were asked to record American Bitterns along these routes for one season. A total of 119 10-mile routes were run in 40 counties in northern Minnesota; 215 bitterns were recorded on 51 of the routes. Although field observations from the southeastern region of the state were lacking, the abundance of records in the northern regions indicated that a relatively large, well-distributed population still occurred in suitable habitat throughout northern Minnesota. As a result, the species was delisted in 1996 (Minnesota Department of Natural Resources 1995). It remains, however, a Species in Greatest Conservation Need (Minnesota Department of Natural Resources 2015).
Habitat loss and degradation remain the most significant threats to this species. In the western regions of the state, aggressive programs designed to protect and restore wetland and grassland habitats should provide critical benefits. Both the Minnesota Duck Recovery Plan (Minnesota Department of Natural Resources 2006) and the Minnesota Prairie Conservation Plan (Minnesota Prairie Plan Working Group 2011) have established goals for protecting, restoring, and actively managing wetland habitats throughout the western region of the state and should provide multiple benefits to all wetland-nesting species. Wetlands are less threatened in the northern regions, but degradation from encroaching development remains a concern.
Climate change is another concern. Although the “State of the Birds 2010 Report on Climate Change” classified the species as having low vulnerability to climate change, a more recent analysis by the National Audubon Society predicted that by the year 2080 the bittern would lose 84% of its current breeding habitat (National Audubon Society 2016; Langham et al. 2015). As a result of their analysis, the species was classified as “climate endangered”.
General management recommendations for the American Bittern include protecting and managing large wetland and grassland complexes. Within each complex, individual wetlands should range in size from 20 to 30 ha up to 180 ha. Water levels should be managed at less than 65 cm and complete drawdowns should be postponed until after mid-August when the adults have completed their molt (Dechant et al. 2004). In uplands, management practices to maintain grasslands (e.g., burning and mowing) should occur only every 2 to 5 years, ensuring that suitable adjacent habitats are undisturbed (Dechant et al. 2004).
Like so many wetland inhabitants that are relatively unknown to the public, the American Bittern faces challenging threats to its future. The birds warrant close monitoring in the years ahead to ensure they remain an integral part of Minnesota’s avifaunal community.
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