Great Horned Owl

Permanent, year-round resident; uncommon in abundance during the Minnesota Breeding Bird Atlas (MNBBA).

The Great Horned Owl is widely distributed across North America, from the tree line south to the Gulf Coast and even further south into Central and South America. Its sparse distribution across the southern two-thirds of the continent, from southern Canada across the United States, is illustrated in Figure 1.

Assigned a Continental Concern Score of 8/20 by Partners in Flight.

Primarily non-migratory, but occasionally there are irruptions of northern birds into Minnesota during the winter months when there are food shortages in Canada.

Considered a “perch and pounce” predator, this species has one of the most diverse diets of any raptor but feeds largely on mammals and birds.

Primarily abandoned platform nests of hawks and crows; also nests in tree cavities, the hollows of standing stumps, and on human-built structures.

The versatile Great Horned Owl occurs in open woodlands throughout Minnesota. Roberts (1932) described it as a common, permanent resident throughout the forested regions of the state, even occurring in riparian woodlands scattered across the prairie landscape. At the time of his writing, the Great Horned Owl was “much more common” than the Barred Owl, its population size augmented in part by an influx of northern migrants during the winter months. Confirmed nesting reports (nests with eggs) were available from seven counties stretching from the southeast corner of the state to the far northwest, including Hennepin, Houston, Isanti, Meeker, Roseau, Sherburne, and Wabasha.

This widely distributed owl, considered by some as royalty among birds of prey, is a complex species. Not only does it demonstrate significant geographical variation in its plumage, but its nomadic and “migratory” movements vary as well. Attempts to define subpopulations have confused the best of taxonomists over the years (Dickerman 1993). In the early 1900s, Roberts delineated three subspecies that occurred in Minnesota: 1) the eastern Great Horned Owl (B.v. virginianus), which was found throughout the eastern United States, 2) the western Great Horned Owl (B.v. occidentalis), which was restricted largely to the northern Great Plains, and 3) the Arctic Great Horned Owl (B.v. subarcticus), which occurred in the central Canadian provinces. The eastern subspecies was the most common breeding resident in Minnesota, while the paler, western subspecies was less abundant. The arctic subspecies, a winter visitant to the state, was readily distinguished by its very pale, grayish-white plumage.

Since then, B.v. occidentalis and B.v. subarcticus have been combined into one subspecies, B.v. subarcticus. B.v. virginianus remains the primary breeding resident in the state (Artuso et al. 2013). As Dickerman (1993) points out, however, the regional boundaries of B.v. subarcticus still require closer examination, including in western Minnesota.

Forty years later, Green and Janssen (1975) noted that banding studies suggested that horned owls residing in central and southern Minnesota were more sedentary then those residing further north: some banded in the northern counties were recovered 805 km away. Janssen (1987) would later refer to the northern residents as partially migratory. Both accounts note the influx of the Arctic Horned Owl, B.v. subarcticus, during the winter months. Although most of these birds were seen in northern Minnesota, they occasionally ranged much further south.

Many early accounts use the term migratory to describe the movements of some Great Horned Owl populations, including the Arctic Horned Owl. A more recent review by Artuso and his colleagues (2013) concluded that most birds are permanent residents and not migrants. Dispersals, they note, are more accurately considered irruptions in response to declines in prey populations, particularly in boreal snowshoe hare populations. Once the owls move, it is unclear if the dispersing birds return the following spring. At least one study showed that territorial owls were less likely to respond to food shortages than nonbreeding birds, or floaters (Rohner 1997).

Much remains to be learned about the true nature of Minnesota’s Great Horned Owl population. Nevertheless, by 1987 it had been confirmed nesting in 39 counties across the state since 1970 (Janssen 1987). By 1998, an additional 15 counties were added to the list (Hertzel and Janssen 1998). In a review of the status and distribution of all Minnesota raptors, Johnson (1982) concluded that although the species was uncommon in the northeastern forest region, it was still the most common breeding owl in the state. Since Johnson’s report, field biologists with the Minnesota Biological Survey have documented a total of 73 breeding season locations for the species, widely dispersed across all but the far northeastern counties (Minnesota Department of Natural Resources 2017). They did not, however, conduct surveys that specifically targeted nocturnal raptors.

During the MNBBA, observers tallied a total of 750 Great Horned Owl records in 12.2% (585/4,811) of the atlas blocks that were surveyed and in 12.2% (286/2,337) of the priority blocks. Breeding was documented in 4.2% (202) of the surveyed blocks (Figures 2 and 3; Table 1). Owls were observed in all but 3 of Minnesota’s 87 counties (Mahnomen, Mille Lacs, and Renville) and were confirmed breeding in 61 counties. Included were 18 counties not previously identified by Hertzel and Janssen (1998). Although they are widely dispersed across Minnesota, the Great Horned Owl was least abundant in far north-central Minnesota and was most commonly reported in the larger Twin Cities metropolitan corridor, from Anoka and Washington Counties south through Rice and Goodhue Counties. Its abundance in this region may more accurately reflect a greater number of observers than a greater number of owls. Also, the species is more easily detected in open habitats of residential areas compared to more closed forest habitats in northern Minnesota.

Although considered the most abundant owl in the state nearly 100 years ago, during the MNBBA the Great Horned Owl and Barred Owl were nearly identical in abundance. A total of 750 records were reported for the Great Horned Owl and 756 for the Barred Owl. The Great Horned Owl was reported from 585 atlas blocks, and the Barred Owl from 558. It was easier, however, for observers to confirm nesting for the Great Horned Owl and nesting was confirmed in a total of 202 blocks compared to only 59 for the Barred Owl. The open platform nest of the Great Horned Owl is certainly easier to locate than the nesting cavities of the Barred Owl. Although both species are relatively common in the Twin Cities region, the Barred Owl is clearly more abundant in Minnesota’s northern forest landscape, while the Great Horned Owl is more frequently encountered in the western agricultural landscape.

Elsewhere within its breeding range, the species’ distribution has changed little overall. Iowa, Michigan, Ohio, and Ontario, which have each conducted two atlases, all documented fewer Great Horned Owl reports during their second atlas. The factors responsible for these declines were invariably due to large-scale habitat changes and/or less intensive efforts to survey nocturnal species during the second atlas (Iowa Ornithologists’ Union 2017; Chartier et al. 2013; Rodewald et al. 2016; Cadman et al. 2007).

*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.

Given its broad distribution across North, Central, and South America, it is no wonder that the list of habitats used by the species is extensive. As Johnsgard (1988) commented, there is probably no other owl in North America that “lives in so many habitats and under so many climatic variations as the Great Horned Owl.”

Although it is found in a wide variety of forested cover types, the Great Horned Owl demonstrates a preference for open, second-growth deciduous or mixed deciduous-coniferous woodlands. Fragmented landscapes, with considerable heterogeneity, including numerous forest edges and open habitats, are characteristic of most breeding territories (Figure 4; Artuso et al. 2013).

Johnsgard (1988) delineated three habitat features that were critical regardless of where the birds were breeding: suitable nest sites, roosting trees, and foraging areas. The Great Horned Owl is quite adaptable in utilizing a variety of potential nest sites. It prefers used nesting platforms of hawks and crows, but it also utilizes large hollows in dead tree stumps, cliff ledges, and caves, and it may even nest on the ground. Roosting trees provide concealment and protection during the day and are typically conifer trees or deciduous trees, such as oaks and beeches that retain their leaves year-round. Foraging usually occurs over agricultural fields, wetlands, or grassland habitats, where some scattered trees are available for perching as the owls keep a watchful eye out and ear open for prey. All three of these habitat features (nesting sites, roosting trees and open lands for foraging) can often be found in a diversity of habitats ranging from undisturbed woodlands and wetlands, to intensively cultivated agricultural landscapes, to urban and suburban developments.

The only intensive study conducted on Great Horned Owls in Minnesota was undertaken at the University of Minnesota’s Cedar Creek Ecosystem Reserve in Anoka and Isanti counties. Fuller (1979) documented that the birds utilized open fields and forest edges for foraging but preferred the landscape’s upland oak forests for roosting and nesting.

Although the federal Breeding Bird Survey (BBS) is not a robust monitoring tool for nocturnal species like the Great Horned Owl, it is the only long-term data set available for the species. Using these data, biologists have estimated the species global population at 3.9 million breeding adults in the United States and Canada (Rosenberg et al. 2016). Minnesota is estimated to support 0.75% of the North American population (Partners in Flight Science Committee 2013), which would place the statewide estimate at approximately 29,000 adults. Sparsely distributed throughout its range, the density of the Great Horned Owl is usually quite low, averaging 0.10 to 0.20 nesting pairs/km² in quality habitat (Artuso et al. 2013).

Given its nocturnal habits, its early nesting phenology, and its low population density, the Great Horned Owl is not well suited to most well-established long-term monitoring efforts, making it difficult to assess long-term population trends. Because of its low relative abundance, the BBS trend line is not statistically robust, either across the entire survey area or in Minnesota. With these caveats, the BBS data showed a significant decline across southern Canada and the United States of 0.81% per year since 1966 that appears to stabilize in more recent years. BBS data for Minnesota demonstrated a long-term significant decline of 2.2% per year that continues unabated through 2015 (Sauer et al. 2017). No state or region showed a statistically significant increase in population numbers between 1966 and 2015. The 2016 Landbird Conservation Plan estimated the species has declined 27% across the United States and Canada since 1970 (Rosenberg et al. 2016).

In their recent review of the species, Artuso et al. (2013) examined two other important long-term data sets in an effort to assess population trends. First, they analyzed Christmas Bird Count data from Canada, the United States, and Mexico from 1950 through 2013. Overall the data illustrated an increase in the number of detections per party hour from the 1960s to the 1980s, followed by a decrease through the year 2000, with a relatively stable detection rate since then. They also examined breeding bird atlas data from 13 states and provinces that had each completed two atlases since the 1980s. Of these 13 atlas projects, 11 reported declines in the number of Great Horned Owl observations, even though the amount of observer effort was greater in the second atlas.

More locally, the Western Great Lakes Owl Monitoring program tracked owl populations in both Minnesota and Wisconsin from 2005 to 2014. Surveys were conducted during the first two weeks of April and utilized a network of existing randomized routes established for other surveys but conducted during the evening hours. A total of 169 routes were located in Minnesota and 92 in Wisconsin. Volunteers were recruited each year to run as many routes as possible (Grosshuesch and Brady 2015).

In contrast to the conclusion reached by Artuso and his colleagues, results during the 10 years of the survey showed an overall increasing trend in Minnesota and greater variation in Wisconsin (Figure 5). In 2014, the mean number of Great Horned Owl detections per route was 0.58; it was the second most frequently encountered owl in the state, following the Barred Owl, which was detected at a rate of 0.77 birds per route. Figure 6 illustrates the distribution and abundance of the Great Horned Owl along the survey routes in 2014. Perhaps the species’ long-term decline is beginning to show at least a stabilized if not an increasing population in the state.

The Great Horned Owls’ non-discriminating palate, coupled with powerful talons that can kill animals as large as hares, waterfowl, and grouse, did not endear it to either sportsmen or farmers over the years. Persecuted for decades, losses were particularly great in agricultural regions, where the Great Horned Owl could inflict considerable losses to farmstead poultry. Roberts (1932) used numerous reports from Minnesota game wardens, detailing the number of birds trapped and killed, as a measure of the species’ abundance.

Although some illegal hunting may continue, today the Great Horned Owl is more revered for its prowess then persecuted. A widespread and adaptable species, it has been assigned a Continental Concern Score of 8/20 and is considered a low conservation priority (Rosenberg et al. 2016). A preliminary assessment of its vulnerability to a warming climate was also ranked as low (North America Bird Conservation Initiative, U.S. Committee 2010).