- Minnesota Breeding Bird Distribution
- Breeding Habitat
- Population Abundance
- Literature Cited
A regular breeding resident and migrant; a few late migrants have been reported as lingering into December and January. The Black-crowned Night-Heron was uncommon during the Minnesota Breeding Bird Atlas (MNBBA).
The Black-crowned Night-Heron breeds on every continent except Antarctica and Australia. In North America, it can be found wherever sufficient wetlands are available for foraging, so it is largely absent from the Appalachian Mountains and the northern Rocky Mountains. A colonial nesting species, the Black-crowned Night-Heron is locally abundant in coastal areas, in the northern Great Plains, and wherever major water impoundments provide suitable habitat (Figure 1).
Assigned a Continental Concern Score of 10/20 by Partners in Flight and designated a species of Moderate Concern by the North American Waterbird Conservation Plan; designated a Species in Greatest Conservation Need by the Minnesota Department of Natural Resources.
The species’ migration status is variable; some populations are year-round residents while others are short- to long-distance migrants, depending on weather and food availability. Birds banded from northern populations have been recovered in the southern United States, the Caribbean Islands, and Central America.
An opportunistic feeder that consumes virtually anything that is available; primary prey items include aquatic invertebrates, fish, amphibians, young birds and eggs, and small mammals. The species can be a significant predator on the young of other colonial nesting waterbirds including Common Terns.
Depending on the colony’s location, the nest might be constructed of small twigs and placed within trees or shrubs or constructed of emergent vegetation and placed just above water level within a large, extensive wetland. The Black-crowned Night-Heron usually nests in mixed-species colonies with other colonial waterbirds.
Roberts (1932) described the Black-crowned Night-Heron as breeding throughout the “prairie and semiprairie” regions of the state, south of the boreal forest. It was most abundant in southern and central Minnesota, with confirmed nesting records from 7 counties: Grant, Hennepin, Jackson, Kandiyohi, McLeod, Murray, and Ramsey. His account also mentions two other colony sites; the first is along the St. Croix River in Pine County, and the second is at King Lake in Meeker County. Confirmed nesting records, however, were not reported for either site. Frequent observations of the Black-crowned Night-Heron in the west-central counties of Otter Tail and Douglas also suggested its presence at nearby nesting colonies. Because the species was known to nest in Manitoba, summer reports of birds seen as far north as Kittson, Marshall, and St. Louis Counties suggested the presence of additional colonies north of the species’ documented breeding range in Minnesota.
Indeed, when Green and Janssen wrote their book on Minnesota birds in 1975, a colony had been discovered in the northwest corner of the state on the Agassiz National Wildlife Refuge in Marshall County in 1963. The authors reiterated Roberts’s assertion, however, that the species was primarily a resident of the southern half of the state yet absent from much of the southeastern region.
Beginning in the late 1970s, the Minnesota Nongame Wildlife Program in the Minnesota Department of Natural Resources (MNDNR) issued several reports that were an effort to compile a comprehensive inventory of all colonial nesting waterbirds in the state (Guertin and Pfannmuller 1985; Henderson 1977, 1978, 1984; Henderson and Hirsch 1980). The reports were not based on a systematic inventory of the state but rather a compilation of reports from MNDNR field staff and other sources. With the exception of a few rare species, the results were considered to be a fair representation of the distribution of many species, particularly herons and egrets.
From the time the first report was issued in 1977 to the last publication in 1985, the number of reported Black-crowned Night-Heron nesting colonies remained fairly stable, ranging from 14 to 17. Some smaller colonies became inactive while others were newly established; but the largest mixed-species colonies remained active in all years and supported the greatest number of nesting pairs. These sites usually supported Great Blue Herons, Great Egrets, and Double-crested Cormorants, as well as Black-crowned Night-Herons. With the exception of 2 colonies located in Marshall County (Agassiz National Wildlife Refuge and Thief Lake State Wildlife Management Area), all remaining colonies were located in the central and southwestern regions of the state.
Janssen (1987) identified the eight largest colonies that were active in the late 1970s and early 1980s as Big Stone National Wildlife Refuge (Big Stone County), Egret Island on Pelican Lake (Grant County), Howard Lake (Anoka County), Lake Johanna (Pope County), Lake of the Isles (Hennepin County), Long Lake (Kandiyohi County), Pig’s Eye Lake (Ramsey County), and Swan Lake (Nicollet County). Although the birds were known to be nesting at Agassiz National Wildlife Refuge, the size of the colony was unknown. At least two of these sites are no longer active: Howard Lake (Anoka County) and Lake of the Isles (Hennepin County). Overall, Janssen identified 18 counties where nesting had been confirmed since 1970.
Beginning in the late 1980s, a total of 78 breeding season observations reported by the Minnesota Biological Survey (MBS) showed a similar pattern of distribution, including multiple reports in the northwestern counties of Marshall and Roseau (MNDNR 2016). There also were multiple reports from southwestern Minnesota including from Jackson, Lyon, Murray, and Nobles Counties. By 1998, Hertzel and Janssen had delineated a total of 20 counties where breeding had been confirmed since 1970. There were no nesting records in northeastern, north-central, or southeastern Minnesota.
During the MNBBA, participants reported only 66 records of the Black-crowned Night-Heron in 1% (49/4,734) of the atlas blocks that were surveyed and 1.2% (29/2,337) of the priority blocks. Breeding evidence was limited to 10 atlas blocks (Figures 2 and 3; Table 1). The species was reported in 24 of Minnesota’s 87 counties and was confirmed breeding in 11 counties. One of the blocks where nesting was confirmed straddled 3 counties (Ramsey/Dakota/Washington), but the colony (Pig’s Eye) was located within Ramsey County. Two of the counties, Carver and Dakota, were additions to the list Hertzel and Janssen published in 1998. Unfortunately, nest counts from several sites are not available. Table 2 lists the 10 nesting sites identified during the MNBBA.
Despite the variety of data that has been gathered over the years, including the MNBBA, an accurate assessment of the status of Minnesota’s Black-crowned Night-Heron population is difficult. Although the species is approximately half the size of the common Great Blue Heron, it is still a relatively large, stocky bird that is easy to identify in flight or while foraging. Nevertheless, several factors can challenge the ability of field biologists and researchers to accurately track its population status. First, because it is most active at dusk and during the night, observations during the daytime hours can be uncommon. This is particularly true during the non-breeding season. During the height of the breeding season, however, the demands of feeding young often cause it to actively forage during the daylight hours. Second, Black-crowned Night-Herons nest in habitats that are difficult to survey. Great Blue Herons, Great Egrets, and Double-crested Cormorants frequently nest in the canopies of tall trees, where they are often visible from afar or during aerial surveys. When Black-crowned Night-Herons nest in these large, mixed-species colonies, they are often in smaller trees or shrubs in the subcanopy where they are less visible. They also nest in emergent vegetation within large wetland complexes that are difficult to access. As Hothem and his colleagues (2010) noted in their comprehensive review of the species: “Nesting individuals tend to be inconspicuous during aerial surveys, so aerial counts often grossly underestimate true numbers.” Even surveys conducted from boats, they note, may underestimate the number of pairs present. Finally, anyone who has participated in ground surveys of large-mixed species colonies during the height of the breeding season can appreciate the challenges of accurately assessing the number of nesting pairs present.
Nevertheless, although the exact number of colonies present in the early 1900s is unknown, Roberts’s (1932) account implies the Black-crowned Night-Heron was a far more abundant species than was reported by the MNBBA. It also appears that numbers may have declined since the late 1970s and 1980s, when at least 17 colonies were documented. To the east, in Wisconsin, observers during the state’s first atlas also documented a decline in the number and size of Black-crowned Night-Heron colonies from earlier reports in the 1980s and 1990s (Cutright et al. 2006). But further east, in Michigan and Ontario, the number of records actually increased and expanded north between the first and second atlases (Chartier et al 2013; Cadman et al. 2007). Ohio also documented an increase in the number of records between its first and second atlases (Rodewald et al. 2016).
*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.