- 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.