Black-crowned Night-Heron

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.

Throughout its range, the Black-crowned Night-Heron can be found in a variety of wetland habitats, including both freshwater and saline. In the Great Lakes region, it nests in woodlots, riparian forests, or shrub thickets, either on the mainland or on islands (Figure 4). Further west, including in Minnesota’s Prairie Parkland and Tallgrass Aspen Parklands Provinces, the species not only nests in small trees and shrubs but is frequently found nesting within dense stands of emergent vegetation in large wetland complexes.

Large wetlands with minimal access, particularly islands, are preferred. Mammalian predators and human disturbances are usually considerably reduced in such settings. One study suggested the Black-crowned Night-Heron may be area-sensitive, occurring only in wetlands that are greater than 20 ha in size (Soulliere et al. 2007). Although the species may be willing to travel some distance to suitable foraging areas, sites that are large enough to provide a variety of shallow bodies of water with abundant prey are more suitable.

As a colonial nester, the Black-crowned Night-Heron’s clumped distribution, often restricted to inaccessible wetlands, coupled with its crepuscular and nocturnal habits, present challenges for effectively monitoring its status and population trend. In 2002, the North American Waterbird Conservation Plan estimated the population north of Mexico at greater than 50,000 breeding adults (Kushlan et al. 2002). Several years later, within the Upper Mississippi River & Great Lakes Region Joint Venture (which includes much of eastern Minnesota), the regional population was estimated at more than 17,000 breeding adults (Wires et al. 2010).

No systematic statewide census of Black-crowned Night-Heron nesting colonies has been conducted within Minnesota. Adams and his colleagues (1973) estimated that in the 1940s the statewide population was a minimum of 6,060 nesting pairs based on the number of pairs reported at two known colonies at Heron Lake in Jackson County and in Ramsey County, north of St. Paul. Forty years later, based on their non-systematic compilation of nesting reports across the state, Henderson and Hirsch (1980) estimated the statewide population at 2,000 nesting pairs in 1980; four years later Henderson (1984) estimated the population numbered between 1,858-3,850 nesting pairs. Since that time only those Black-crowned Night-Heron colonies that are also occupied by Double-crested Cormorants and/or American White Pelicans have been systematically surveyed by Cuthbert and her colleagues from 2004 to 2015. The primary purpose of their work was to document the status of Minnesota’s Double-crested Cormorant and American White Pelican breeding populations. Data on Black-crowned Night-Herons were only collected incidentally at breeding colonies of the former two species. Among the subset of colonial waterbird nesting sites visited, at least 5 mixed-species colonies supported a minimum of 305 Black-crowned nesting pairs in 2004; at least 4 colonies supported a minimum of 77 nesting pairs in 2010, and 3 colonies supported a minimum of 9 nesting pairs in 2015 (Hamilton and Cuthbert 2016).

Although Hamilton and Cuthbert’s work censused many large, mixed-species colonies in central Minnesota where Black-crowned Night-Herons have historically nested, they did not include the Agassiz National Wildlife Refuge, at one time an important site for the Black-crowned Night-Heron. Annual surveys of all waterbirds nesting on the refuge were conducted for many years. The last published report, however, was in 1991, when a total of 360 Black-crowned Night-Heron pairs were present (U.S. Fish and Wildlife Service 1991). Unfortunately, systematic surveys are no longer conducted. During the 2016 breeding season, refuge staff estimated that fewer than 50 nesting pairs were present (Jordan Young, refuge biologist, pers. comm., 2017). Smaller Black-crowned Night-Heron colonies, including several that were reported during the MNBBA, also were not surveyed by Hamilton and Cuthbert if pelicans and cormorants were not present.

Unfortunately, the efforts by Cuthbert and her colleagues are the only systematic data available since 2004 that provide even a glimpse of the status of Black-crowned Night-Herons in Minnesota. Despite all the caveats associated with the limited scope of their work and with the non-systematic surveys conducted by the Minnesota Department of Natural Resources in the 1970s and 1980s, it would appear that Minnesota’s Black-crowned Night-Heron population has declined, especially at the historically important, large mixed-species colonies in central Minnesota. Only a more systematic assessment that focuses specifically on Black-crowned Night-Heron nesting colonies could provide a more definitive status assessment.

The apparent decline in the Black-crowned Night-Heron population at one of the largest colonies in the state, the Pig’s Eye heron rookery just south of downtown St. Paul, may serve as a bellwether for the species. From 1973 to 1978 estimates of the number of nesting pairs present ranged from 560 to 815; in the mid 1980s  estimates ranged from 236 to 646 nesting pairs (Table 3). There are no records of the site being visited between 1985 and 2003 before Cuthbert and her colleagues began monitoring the status of Double-crested Cormorants nesting at the rookery. Black-crowned Night-Herons were not counted during the site visit in 2004 but in 2010 only 13 nests were tallied and in 2015 only 2 nests were tallied. Although the 2010 and 2015 census data were compiled from ground counts, these numbers represent a minimum number of nests. The canopy at the Pig’s Eye heron rookery is very high and nests were difficult to detect (Cuthbert, pers. comm.). Nonetheless, the scale of the number of nests reported is considerably smaller than 30 to 40 years ago.

In his 1985 report on the status of the Pig’s Eye colony, Warner (1985) commented on major changes that were impacting the forest composition and the subsequent species composition of the colony. First was the demise of American elm trees, a preferred nesting tree of the Great Blue Heron, due to Dutch elm disease. The loss of elms resulted in a decline in the rookery’s Great Blue Heron population. As younger trees or shrubs replaced the elms, their small branches were favored nesting sites for the Black-crowned Night-Heron. Over time, however, the shrubs were shaded out and the young trees grew much taller, becoming more suitable nesting sites for the Great Blue Heron and less suitable for the Black-crowned Night-Heron. These natural changes in the floodplain forest impacted the species composition of the Pig’s Eye rookery and the number of Black-crowned Night-Heron nesting pairs.

Historically, another important nesting site in Minnesota was Heron Lake in Jackson County. This large wetland complex supported thousands of nesting pairs in emergent vegetation in the early 1900s (Roberts 1932; Adams et al. 1973). Unfortunately, the site is rarely used today. Intensive cultivation in the lake’s small watershed means that storms have an immediate impact on the lake’s water levels, often drowning nests, eggs, and young. Conservation efforts in the watershed have improved, but the site still is used only intermittently. There was a time in the mid-1990s when anywhere from 5 to 1,000 birds were observed on the lake, but the species’ nesting status was unknown (Koster 1995).

Major changes elsewhere in the state include the apparent loss of a colony on Long Lake in Kandiyohi County. From 1979 to 1981, Janssen (1987) identified Black-crowned Night-Heron nest counts in the colony ranged from a low of 279 (1981) to a high of 424 nests (1979).  In 2004, the site was estimated to have supported a minimum of 207 Black-crowned Night-Heron nesting pairs. By 2010 only 7 nesting pairs were observed, and by 2015 this once large, mixed-species colony only supported 9 Great Egret nesting pairs and 81 Great Blue Heron nesting pairs; no Black-crowned Night-Heron nests were observed (Hamilton and Cuthbert 2016). Again, a more comprehensive ground search of the colony is warranted to clearly assess the Night-Heron’s breeding status.

Although the federal Breeding Bird Survey (BBS) is not considered a robust survey method for colonial nesting species, it provides the only continent-wide monitoring data available for the Black-crowned Night-Heron. The data are imprecise at the national scale as well as in Minnesota because of the species’ low relative abundance. Nevertheless, since 1966, the data have demonstrated a relatively stable population nationally but a population that is declining significantly in Minnesota (7.14% per year) (Sauer et al. 2017).

Lacking a systematic statewide program to monitor colonial waterbirds, it is difficult to independently assess the downward population trend shown by the Minnesota BBS data. The most statistically rigorous assessment of the Black-crowned Night-Heron’s population is conducted in the Great Lakes by the U.S. Fish and Wildlife Service and the Canadian Wildlife Service. Conducted every 10 years since the 1970s, the Black-crowned Night-Heron is among the 11 species surveyed. In the most recent survey, conducted between 2007 and 2010, a total of 1,833 nesting pairs were documented at 37 different sites in the United States and Canada. This represented a 12% increase above the number reported in the previous survey (1,640 pairs in 1997–1999) but was still considerably lower than numbers reported in the late 1970s (4,220 pairs) and late 1980s (2,800 pairs). The most hopeful sign was that the species had increased at 28 new or preexisting colonies while it had declined in or vacated just 17 colonies (Cuthbert and Wires 2013).

Factors responsible for the species’ decline are not well understood. Studies in the 1970s and 1980s documented high pesticide contaminant levels in some populations as well as a decline in eggshell thickness, but the impact on overall reproductive success appeared minimal (Hothem et al. 2010). Habitat loss and degradation, coupled with human disturbance, are considered to be the major problems in those regions where populations continue to decline.

Given the species’ decline in many regions, including Minnesota and many inland areas of the Upper Midwest (Hothem et al. 2010), the North American Waterbird Conservation Plan ranked the Black-crowned Night-Heron as a Species of Moderate Concern (Kushlan et al. 2002). It was assigned a Continental Concern Score of 10/20 by Partners in Flight (2017). The Northern Prairie and Parkland Waterbird Conservation Plan also considers it a Species of Moderate Concern (Beyersbergen et al. 2004), and the Upper Mississippi River/Great Lakes Waterbird Conservation Plan considers it a conservation priority for region-wide monitoring (Wires et al. 2010). In Minnesota the species has been designated as a Species in Greatest Conservation Need (MNDNR 2015).

Few specific management recommendations are available for the species. One study (Rodgers and Smith 1995) recommended that buffers of at least 100 m should be established around nesting colonies to minimize disturbance.

The State of the Birds 2010 Report on Climate Change ranked the Black-crowned Night-Heron as having a low vulnerability to climate change (North American Bird Conservation Initiative 2010), but a recent analysis by the National Audubon Society came to a more startling conclusion. Classifying the species as “climate endangered,” the society predicted that the species could lose 88% of its current summer breeding habitat by the year 2080 (Langham et al. 2015; National Audubon Society 2016). Certainly, increasing water levels in coastal habitats and the Great Lakes, as well as more intense rainstorms in the Great Plains, could directly impact both tree-nesting and marsh-nesting colonies.

Overall, the species’ restriction to wetland habitats in the southern and western regions of the state, which are heavily urbanized and cultivated, means that the species’ future in Minnesota depends on the protection of existing nesting islands and large wetland complexes. Given the species’ precarious status in the state, a more accurate population assessment at the handful of major nesting sites that remain is critical. To increase the accuracy of population estimates, Cuthbert and Wires (2013) recommend the development of a methodology specific for locating and censusing Black-crowned Night-Herons.