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Northern Harrier

Circus hudsonius
Overview
Minnesota Seasonal Status:

A regular summer breeding resident and migrant; regularly observed in the winter months, especially in southern Minnesota. The Northern Harrier was a common species during the Minnesota Breeding Bird Atlas (MNBBA).

North American Breeding Distribution and Relative Abundance:

Distributed across Canada, the northern United States, and the western United States south to southern California, northern New Mexico, and northern Texas. The species is most abundant in the northern Great Plains of the United States and the central Canadian provinces (Figure 1).

Conservation Concern:
Conservation Status Score 11

Assigned a Continental Concern Score of 11/20 by Partners in Flight; designated a Species in Greatest Conservation Need in Minnesota.

Life History
Migration:

Partial to long-distance migrant; northern breeding populations are believed to winter farther south than populations that breed farther south. Some birds are present year-round in Minnesota, but it is unknown if these are northern migrants or overwintering summer residents.

Food:

Plunges from low flight to capture prey on the ground; food consists primarily of small mammals and small birds.

Nest:

A small depression or platform constructed of grasses and forbs; nests are located on the ground in an upland field or over very shallow water at the edge of a wetland.

Northern Harrier Northern Harrier. Circus hudsonius
© Smi Shonja
Figure 1.

Breeding distribution and relative abundance of the Northern Harrier in North America based on the federal Breeding Bird Survey, 2011–2015 (Sauer et al. 2017).

Minnesota Breeding Bird Distribution*

In the late 1800s, Hatch (1892) considered the Northern Harrier, or Marsh Hawk as it was then known, as “undeniably the most abundant of the hawks that visit the state.” Forty years later, Roberts (1932) commented that although numbers were reduced, it remained abundant. It was often the only hawk seen during extensive forays across the state, particularly in the open country of western and central Minnesota. At the time, confirmed nesting records (nests with eggs or young) were available from 12 counties stretching from Jackson and Lincoln Counties in the southwest, east to the Twin Cities, and northwest to Marshall County.

As descriptions did of so many hawks, early accounts of the “Marsh Hawk” focused on its feeding habits and whether the species was considered beneficial to the farmer. Most naturalists and ornithologists of the late 19th and early 20th century classified it as a beneficial species; its diet consisted principally of field rodents and small birds (Bent 1937; Roberts 1932). Although the species was not entirely immune to the wrath that many farmers had for all hawks, Roberts wrote that “it survived the warfare better than any other species.”

When Green and Janssen (1975) prepared an updated account of the species’ status, the Northern Harrier was considered to be one of the three most common raptors in the state; the other two were the Red-tailed Hawk and the American Kestrel. Farther north the Northern Harrier was found in small marshes, wet meadows, and peatlands that dotted the forest landscape but was outnumbered by many forest-dependent species. Despite its being a relatively abundant species, there was evidence of recent declines due to the loss of wetland habitats.

In 1982, Johnson prepared a comprehensive summary of the status of all raptors nesting in Minnesota. He noted a total of 61 nest records for the species from 1880 to 1981; only 16 of these records were since 1960. In addition, from 1970 to 1981 there were no observations of Northern Harriers from 16 counties where they had been observed or reported nesting prior to 1970. Johnson acknowledged the lack of observers actively searching for nests might contribute to the smaller number of records. Nonetheless, he believed the decline in records was more likely an accurate reflection of a true population decline due to the “tremendous loss of habitat.”

In 1987, Janssen commented that the Northern Harrier had virtually disappeared from southern Minnesota but remained common in the northwest and north-central regions. He delineated 13 counties where nesting had been confirmed since 1970; all were located north of the Minnesota River valley (Janssen 1987). By 1998, it appeared that the species might be increasing in numbers and reoccupying portions of its former range, as Hertzel and Janssen (1998) identified 17 counties where nesting had been confirmed since 1970. One county (Mahnomen) identified by Janssen (1987) was no longer listed, but the 5 new counties added were all south of the Minnesota River (Lac qui Parle, Jackson, Nobles, Rice, and Wabasha).

Beginning in the late 1980s, field biologists with the Minnesota Biological Survey tallied a total of 473 breeding season locations for the Northern Harrier. The reports were well distributed across the state, from Rock County east to Winona County, and north to Kittson and Cook Counties. Included were numerous observations in southwestern Minnesota (Minnesota Department of Natural Resources 2016).

During the MNBBA, observers reported 1,417 Northern Harrier records in 21.9% (1,048/4,780) of the surveyed atlas blocks and in 27.3% (639/2,337) of the priority blocks. Breeding was confirmed in 1.1% (54) of the surveyed blocks (Figures 2 and 3; Table 1). The bird was observed in all but 1 (Watonwan) of Minnesota’s 87 counties, and breeding evidence was gathered in 35 counties, which were well dispersed across all regions of the state. At least 26 of the 35 counties were additions to the list published by Hertzel and Janssen in 1998.

Incorporating MNBBA data, the landcover suitability map for the Northern Harrier predicts that suitable habitat is found in nearly all regions of the state with the exception of the Twin Cities and much of northeastern and far north-central Minnesota (Figure 4). Northwestern Minnesota provides the most suitable habitat, and there are scattered pockets throughout the central and southwestern regions as well. The block of highly suitable habitat in Anoka County is the Carlos Avery Wildlife Management Area.

Clearly the Northern Harrier is faring better than records suggested in the mid- to late 1900s and has reoccupied its former range south of the Minnesota River valley. It remains most abundant, however, in the Prairie Parkland and Tallgrass Aspen Parklands Provinces of western Minnesota, and in central Minnesota south and west of Duluth. Although records are sparse in the north-central, northeastern, and southeastern regions of the state, Northern Harriers occur in these regions as well.

Despite its relatively broad distribution in the state, it is doubtful that the Northern Harrier has recovered to the former levels of abundance implied by Roberts in the early 1900s. And who would have imagined 40 years ago that the species would eventually become less abundant than the Bald Eagle! Not only is the harrier no longer one of the three most abundant raptors in Minnesota, but there were 50% more MNBBA records tallied for the eagle (2,113) than for the harrier (1,417). Among all raptors, the Northern Harrier fell to fifth place behind the Red-tailed Hawk, the Bald Eagle, the Turkey Vulture, and the American Kestrel.

To the east, in Wisconsin, the Northern Harrier was found to be common throughout most of the state during its first atlas. Its distribution had changed little since the late 1980s (Cutright et al. 2006). To the south, in Iowa, Northern Harriers were found in twice as many blocks during its second atlas (2008–2012) as they were during its first atlas approximately 20 years earlier (1985–1990) (Iowa Ornithologists’ Union 2017).

*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.

Figure 2.

Breeding distribution of the Northern Harrier in Minnesota based on the Breeding Bird Atlas (2009 – 2013).

Print Map
Figure 3.

Summary statistics of observations by breeding status category for the Northern Harrier in Minnesota based on all blocks (each 5 km x 5 km) surveyed during the Breeding Bird Atlas (2009-2013).

Breeding statusBlocks (%)Priority Blocks (%)
Confirmed54 (1.1%)32 (1.4%)
Probable180 (3.8%)101 (4.3%)
Possible814 (17.0%)506 (21.7%)
Observed0 (0.0%)0 (0.0%)
Total1,048 (21.9%)639 (27.3%)
Table 1.

Summary statistics for the Northern Harrier observations by breeding status category for all blocks and priority blocks (each 5 km x 5 km) surveyed during the Minnesota Breeding Bird Atlas (2009-2013).

Figure 4.

Landcover suitability of the Northern Harrier in Minnesota based on habitat, landscape context, and climate data gathered during the Minnesota Breeding Bird Atlas (2009-2013) using the MaxEnt modeling approach.

Breeding Habitat

Throughout its breeding range, the Northern Harrier occupies a wide variety of open upland and wetland habitats (Figure 5). It may utilize disturbed sites, such as croplands, lightly grazed pastures, and old fields, as well as native habitats, such as freshwater and alkali wetlands and native prairies (Smith et al. 2011; Dechant et al. [1998] 2002; Slater and Rock 2005). Primary habitat features that unite all suitable habitats include relatively open landscapes dominated by tall, dense vegetation and abundant residual vegetation (Dechant et al. [1998] 2002).

In the Great Plains, Northern Harriers often select seasonal or semipermanent wetlands as well as wet meadows and mesic grasslands; farther west drier uplands are utilized more frequently. Across their range, they display a preference for large, undisturbed landscapes and generally are considered to be an area-sensitive species. In North Dakota, they were uncommon in Conservation Reserve Program (CRP) fields that were less than 100 ha in size (Dechant et al. [1998] 2002). Although they nested on tracts smaller than 45 ha in Illinois, these were not small, isolated fragments. Biologists believed the birds were actually responding positively to the presence of large blocks of grassland habitat located nearby (Herkert et al. 1999). Woody vegetation becomes a deterrent when it covers more than 30% of a site (Smith et al. 2011).

Even when a site appears entirely suitable from a vegetative perspective, the ultimate predictor of Northern Harrier occupancy may be the abundance of prey, particularly voles and other small rodents. Indeed the species is considered nomadic; local population densities and reproductive success are strongly tied to the abundance of prey, especially microtine voles, in the spring (Smith et al. 2011).

Figure 5.

Typical breeding habitat of the Northern Harrier in Minnesota (© Lee A. Pfannmuller).

Population Abundance

Although the federal Breeding Bird Survey (BBS) is not particularly well suited for monitoring diurnal raptors, it is one of several long-term data sets that provide some indication of population size and long-term trends. Using BBS data, biologists estimated the North American population of Northern Harriers to be 790,000 breeding adults in 2016 (Rosenberg et al. 2016). Several years earlier, Minnesota was estimated to support 2.9% of the North American population (Partners in Flight Science Committee 2013). When applied to the most recent population estimate, the statewide estimate for Minnesota is approximately 23,000 breeding adults.

The Northern Harrier is sparsely distributed throughout its range; the core of its breeding range is in the northern Great Plains and southern Canadian Prairie provinces (Figure 1). BBS routes in the Prairie Pothole Region report an average of nearly 2 birds per route each year. This compares to less than 1 per route in the Boreal Hardwood Transition Region, in the Prairie Hardwood Transition Region, and in Minnesota (Sauer et al. 2017). At the site level, reported densities are quite variable and range from a low of 0.02 nests per 10 km2 in Idaho’s desert shrublands (Howard et al. 1976) to a high of 19.5 nests per 10 km2 in central Minnesota’s wet meadows (Breckenridge 1935).

Given the Northern Harrier’s low abundance, BBS trends at the national level, as well as in Minnesota, are considered imprecise due to the small number of data points. Nevertheless, populations have not demonstrated a statistically significant increase anywhere across the BBS survey area since 1966. The most robust statistical data come from two regions in the northern plains: the Prairie Pothole Region (which includes western Minnesota) and the Badlands. In the former, Northern Harriers show a significant annual decline of 1.30% per year (Figure 6); in the latter, populations have been relatively stable. Survey-wide populations are estimated to have declined 37% since 1970 (Rosenberg et al. 2016).

Another important source of long-term data is the fall migration counts conducted at sites throughout the United States. These data are now compiled by a partnership of hawk watch and migration research organizations, known as the Raptor Population Index Project (http://rpi-project.org/index.php). Migration counts conducted at Hawk Ridge in Duluth, Minnesota, from 1970 through 2012 show a slow decline in numbers of Northern Harriers, although it is not statistically significant (Figure 7). The overall trend is variable, however, depending on which migration count site is examined; even sites located quite close to one another can show dramatically different trend lines (Brandes et al. 2017Crewe et al. 2017).

Overall, the consensus is that Northern Harrier numbers declined during the 20th century. The principal factor was the loss of habitat due to wetland drainage, intensive agricultural practices, and the reforestation of many farmlands in the northeastern United States. During the mid-1900s, the impacts of habitat loss were compounded by the ingestion of organochloride pesticides, particularly DDT and DDE. Lower reproductive success, due to a reduction in egg shell thickness and egg mass, was directly linked to declining numbers of Northern Harriers (Smith et al. 2011).

In light of these factors, fall migration counts before the late 1990s generally indicate declining population trends. More recently there are a few regions where populations are at least stable or slowly increasing, including along the Gulf coast of Texas and in Montana (Smith et al. 2011). Summer observations and nesting records in Minnesota that were gathered by the MNBBA certainly suggest that the Northern Harrier’s status as a breeding resident in Minnesota has improved in recent years.

Figure 6.

Breeding population trend for the Northern Harrier for the Prairie Pothole Region for 1966–2015 based on the federal Breeding Bird Survey (Sauer et al. 2017).

Figure 7.

Raptor Population Index at Hawk Ridge, Duluth, Minnesota, 1970–2012 (Crewe et al. 2013; Brandes et al. 2013).

Conservation

Largely dependent on grasslands and wetlands, two of the habitats most impacted by habitat loss and degradation in North America, the Northern Harrier has been assigned a Continental Conservation Score of 11/20 (Rosenberg et al. 2016). It has also been designated a Species in Greatest Conservation Need in Minnesota (Minnesota Department of Natural Resources 2015). Although it is not state listed in Minnesota, many states in the eastern United States have added the Northern Harrier to their lists of endangered or threatened species (Smith et al. 2010).

Specific management recommendations for the species focus on the need to protect and restore large tracts of grassland and wetland habitats with moderate to dense herbaceous cover. Because residual ground cover is important for nesting, large tracts should always be managed in small units that allow some of the vegetation to be mowed or burned regularly while still leaving some sites idle for three to five years (Dechant et al. [1998] 2002; Slater and Rock 2005).

Aggressive grassland and wetlands protection and restoration programs are critically important for the Northern Harrier as well as for many other species dependent on such open country. Such efforts are especially important in light of the potential consequences of warming temperatures, particularly in the Great Plains. A recent analysis by the National Audubon Society predicted that the Northern Harrier could lose up to 85% of its current summer breeding range, forcing it to extend its range even farther north (Langham et al. 2015National Audubon Society 2016). As a result of the analysis, the species was classified as “climate endangered”. The ability of the harrier to recover from past assaults is noteworthy and provides hope for its long-term adaptation to further challenges.

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