- Minnesota Breeding Bird Distribution
- Breeding Habitat
- Population Abundance
- Literature Cited
A regular breeding resident and migrant; the Least Bittern was an uncommon species during the Minnesota Breeding Bird Atlas (MNBBA).
The species has a unique distribution that stretches from the coast of Canada’s Maritime Provinces, south to the Caribbean and portions of Central America, and west through the eastern deciduous forest region. It is absent from much of the Appalachian region and has scattered populations in the Great Plains and several western states. Because it is a sparsely distributed, secretive marsh bird, its relative abundance across its breeding range is poorly known. The only long-term monitoring data available suggest that breeding densities may be highest along the Gulf coast of Louisiana (Figure 1).
Assigned a Continental Concern Score of 10/20 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.
A short-distance migrant that winters in the southeastern United States, Central America, and the Caribbean Islands.
Small fish and aquatic invertebrates comprise the primary food items, but other wetland-inhabiting vertebrates and invertebrates are occasionally taken.
A loosely woven platform of marsh vegetation placed several centimeters above the water surface.
In the early 1900s, Roberts (1932) wrote that “this nimble little acrobat of the reeds is a common inhabitant of the more extensive sloughs and cane-fringed lakes of southern and western Minnesota.” Although observations were apparently common from Heron Lake in Jackson County, north to the Canadian border, and east along the Minnesota River valley, confirmed nesting records (nests with eggs) were available from just 6 counties: Carver, Hennepin (Minneapolis), Meeker, Marshall, Otter Tail, and Polk. Commenting on the lack of records from northern Minnesota, Roberts wrote, “Our files contain no records for the heavily forested portion of the state although suitable lowlands are not lacking.”
Green and Janssen (1975) described the Least Bittern as a resident throughout much of the state, with the exception of northeastern Minnesota and the adjacent counties of Itasca and Koochiching. Several years later, Janssen (1987) included a distribution map that excluded northeastern Minnesota and all or portions of the adjacent north-central counties, the Red River valley, and all of southeastern Minnesota with the exception of the backwaters of the Mississippi River. He noted that the number of observations in southern Minnesota was increasing, but it was unclear if this represented an increase in Least Bitterns or an increase in the number of observers. Seven counties where nesting had been confirmed since 1970 were delineated: Clearwater, Marshall, Nicollet, Polk, Pope, Stearns, and Wright. Hertzel and Janssen (1998) later deleted Clearwater from the list.
Intensive survey work by the Minnesota Biological Survey, beginning in the late 1980s, documented 68 breeding season locations. Surprisingly, more than one-third of the locational records were from wetlands in southwestern Minnesota (Minnesota Department of Natural Resources 2016).
During the course of the MNBBA, participants reported Least Bitterns in 2.2% (103/4,741) of the surveyed atlas blocks and in 2.1% (49/2,337) of the priority blocks. Breeding was confirmed in only 4 blocks (Figures 2 and 3; Table 1). Bitterns were reported in 57 of Minnesota’s 87 counties and were reported breeding in 5 counties: Cass/Morrison (one block straddled both counties), Hennepin, Scott, and Nicollet. All but Nicollet were additions to the list published by Hertzel and Janssen in 1998.
Although the MNBBA records were few in number, they were widely scattered throughout many regions of the state. The species remains rare throughout northeastern Minnesota, from Pine County north across the Arrowhead region and west to Beltrami, Itasca, and Koochiching Counties. The exception was one Probable record in northern St. Louis County and a Possible breeding record in northern Koochiching County. In southeastern Minnesota, they were restricted primarily to the Mississippi River valley. It is difficult to know if the paucity of Least Bittern records is an accurate representation of its abundance or a consequence of its secretive nature. For instance, the wetland habitats of the Least Bittern are usually inaccessible, and many observers are unfamiliar with its vocal repertoire. These challenges were also noted by Roberts (1932), who commented, “the elusiveness and quiet disposition of the Least Bittern, combined with its slender form and perfect mimicry of its surroundings in color and action, cause it to be overlooked even where it is dwelling commonly.”
Roberts’s (1932) broad assertions regarding the species’ abundance in southern and western Minnesota in the late 1800s and early 1900s suggest that the small number of MNBBA records represents a decline in overall abundance in the past century. Certainly the significant loss of wetlands, especially in southern Minnesota, supports that conclusion. It seems, however, that the extent of the bittern’s distribution in the state has changed very little. It is encouraging that despite the extensive loss of wetlands in the southern and western regions of the state, the species retains a presence in these regions. Although there are sufficient wetlands to support a robust population of Least Bitterns in the northeastern and north-central counties, these habitats appear to be unsuitable to the species, whose Minnesota breeding distribution represents the northern periphery of the species’ breeding range in North America.
A comprehensive review of the species in 2009 makes no mention of historical change to its distribution anywhere within its breeding range (Poole et al. 2009). The loss of 68% of the wetlands in southern Ontario resulted in a significant decline in Least Bitterns in the province prior to the initiation of its first breeding bird atlas in 1981 (Cadman et al. 2007). Similar declines were noted in Ohio between the 1930s and 1960s (Rodewald et al. 2016), and in Michigan between the 1950s and 1980s (Chartier et al. 2013). Declines in both states were due primarily to the loss of critical wetland habitats. Despite these declines in abundance, the species’ overall distribution remained relatively unchanged.
*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.