Aegolius funereus (Linnaeus, 1758)
Basis for Listing
The Boreal Owl (Aegolius funereus) is a forest-dwelling owl that breeds in boreal and subalpine forests in North America and Eurasia. The first published report indicating that Boreal Owls may breed in Minnesota was from northern Cook County in 1926 (Schorger 1926). However, it was not until 1978 that the first Boreal Owl nest was discovered in the state, which was also the first documented nesting of this species in the lower 48 states (Eckert and Savaloja 1979). Since then, it has been found to be a resident breeding species in several states in the Rocky Mountains. Since 1978, Boreal Owl nests have continued to be documented, though sporadically, in Lake, Cook, and St. Louis counties (primarily in the Border Lakes and Laurentian Uplands subsections of the Laurentian Mixed Forest Province). Auditory survey efforts have regularly detected singing males during the breeding season in these counties with very infrequent reports from other northern counties (e.g. Itasca, Lake of the Woods, and Roseau). However, reports of Boreal Owl nests and singing males have declined dramatically in Minnesota over the past decade. While the species exhibits a somewhat cyclic population fluctuation that may reflect changes in prey populations and snow depth, the degree to which this dynamic may influence the observed trend in the state’s Boreal Owl population is unknown. In light of the rarity, very limited distribution, and highly specific habitat requirements (and continuing loss of this habitat) of this species in Minnesota, in 2013 the Boreal Owl was designated a species of special concern.
Boreal Owls are a relatively small owl, approximately 25 cm (10 in.) in length, with a wingspan of about 56 cm (22 in.). They have brown upperparts with numerous white spots and light underparts heavily streaked with chestnut-colored markings. They are similar in appearance to the closely-related Northern Saw-whet Owl (Aegolius acadicus) or NSWO, which is much more common in Minnesota. Comparing these two species, the Boreal Owl has a paler yellowish bill (black in NSWO); the crown and forehead is grayish with numerous fine light spots (fine streaks rather than spots in NSWO); and the pale facial disk is framed with black (brown, without obvious dark frame in NSWO). The Boreal Owl is slightly larger than the NSWO. Both species have yellow eyes and lack ear tufts. As with other owls, female boreals are significantly larger than males.
In Minnesota, Boreal Owls’ preferred breeding habitat is usually described as upland mixed coniferous-deciduous forest (Lane 1997, Belmonte 2005). However, suitable Boreal Owl habitat must have forests with a key set of specific characteristics. During its breeding cycle, this owl requires three distinct types of microhabitat within upland (fire-dependent forest and mesic hardwood forest) and lowland (forested rich peatland) forest types.
1) Song perches
Male Boreal Owls call from song perches near potential nest cavities in an attempt to attract a female, preferring to do so from relatively large coniferous trees. Nearly 93% of trees with song perches were conifers, with 20% white spruce (Picea glauca), 18% balsam fir (Abies balsamea), 16% black spruce (Picea mariana), 16% jack pine (Pinus banksiana), and 13% white pine (Pinus strobus) being the most commonly selected species (Belmonte 2005). Coniferous trees containing song perches averaged 35.6 cm (14.0 in.) diameter at breast height (dbh) and 18.5 m (60.7 ft.) in height. It is thought that conifers provide necessary cover for singing males at a time when deciduous trees have not yet leafed out.
2) Nest cavities
Within their territory, males select one to several potential nest cavities, which are usually in relatively large deciduous trees. Typically these cavity trees are located within older mixed coniferous - deciduous forest stands rather than purely deciduous forest (since song perch trees, typically conifers, must be nearby). Of known Boreal Owl cavities, nearly 90% were in deciduous trees, primarily trembling aspen (Populus tremuloides) and paper birch (Betula papyrifera). Deciduous cavity trees averaged 41 cm (16 in.) dbh. Heights of cavity trees varied, largely dependent on whether they were alive or dead. Live deciduous cavity trees averaged 22.5 m (73.8 ft.) in height, while dead deciduous cavity trees averaged 9.8 m (32 ft.) tall. Boreal Owls have been documented using artificial nest boxes, which in some settings may be surrogates for larger diameter cavity trees. However, significant use of nest boxes by Boreal Owls has not been documented in Minnesota.
3) Roosting and foraging
Boreal Owls roost in upland (78%) or lowland (22%) mixed or coniferous forest stands. Roost trees, identified through radio telemetry, were almost exclusively (97%) conifers, 73% of which were black spruce (Belmonte 2005). Similarly, Lane (1997) found that nearly 99% of roost trees were conifers, with 87% in black spruce (reportedly 92+% of Cook County roost sites were in lowland conifer stands). Belmonte (2005) found that 85% of roost trees were within 100 m (109 yd.) of a lowland conifer stand and 94% were within 200 m (218 yd.).
Telemetry studies have indicated that the majority of foraging was in mixed conifer and mixed coniferous-deciduous habitats (Belmonte 2005). About two-thirds of nocturnal foraging locations were in upland forests (35% mixed forest and 32% coniferous) with 21% in lowland conifers.
Biology / Life History
The Boreal Owl is a nocturnal species that roosts during the day and forages at night. Boreal Owls establish their nests in tree cavities, typically originally excavated by woodpeckers. Prior to nesting, males sing from perches, usually in taller conifers located near potential cavity trees. Nest cavities are usually in older mixed coniferous-deciduous forest stands. When a female investigates, the male will move to and sing from one to several song perches near a potential nest cavity. Once a nest cavity is selected, the female lays and incubates 3-6+ (usually 6) eggs. Eggs hatch asynchronously (i.e., in the order laid), after approximately four weeks. The young are fed by both adults and fledge in about one month. However, females may desert their first brood around fledging, particularly in good food years, presumably in order to pair with another male and raise a second brood.
In Minnesota, Boreal Owls are known for periodic irruptive behavior. This is thought to be related to population fluctuations in prey in which much higher than normal numbers of owls move beyond their typical winter range. The largest irruption of Boreal Owls documented in the state in the winter of 2004-05 may have numbered nearly 600 individuals (Eckert 2005). Although the number of breeding Boreal Owls in Minnesota may be influenced by winter irruptions, the relationship is unclear.
Conservation / Management
Conservation and management for Boreal Owls should provide forest types with trees of suitable size, age, species, proximity, and acreage such that the needs of this owl are met.
Cavity nesting birds like the Boreal Owl require trees old enough to support the development of heart rot or other processes that allow suitably sized cavities to be formed. For Boreal Owls in Minnesota, large trees with suitable cavities are most often aspen or birch. Nest boxes may be used by Boreal Owls in areas where older large-diameter cavity trees are lacking. In fact, in portions of their global range, Boreal Owls are largely dependent on nest boxes (see Hayward & Hayward 1993). However, simply providing artificial nest boxes for Boreal Owls is not enough, as nest boxes in Minnesota have shown low occupancy rates; therefore, other habitat parameters are also required.
Another key habitat component for Boreal Owls is relatively large conifers, most often white or black spruce, balsam fir, jack pine, or white pine, which are used as song perches. Large conifers for song perches and large deciduous trees with cavities must be near each other, typically in the same forest stand, and must in turn be within 100-200 m (108-218 yd.) of lowland conifers. Adequate lowland coniferous forest must be maintained in areas adjacent to key upland habitats.
One of the big questions remaining regarding Boreal Owls in Minnesota is their presence, abundance, and distribution in the one million acre (404,694 ha) Boundary Waters Canoe Area Wilderness (BWCAW). The roadless nature of the BWCAW makes status surveys logistically difficult due to challenging access during the optimal survey period (early spring), when terrain is often snow-covered (sometimes to significant depths) and waterways are not ice-free. While multiple Boreal Owls were detected on routes outside the BWCAW, only one was found (1988 - 1989) along the entire length of the Fernberg Trail, which penetrates into the heart of the BWCAW.
Potential Boreal Owl habitat in the BWCAW is susceptible to large-scale natural disturbances such as fires and wind-throw events. For example, in 2011 an area with a significant cluster of Boreal Owl cavity trees was consumed in the 93,000 acre (37,637 ha) Pagami Creek Wildfire.
Winter habitat for this species is undoubtedly important particularly during years with major population irruptions. However, specific habitat components important to winter survival of Boreal Owls have not been well-studied in Minnesota.
Conservation Efforts in Minnesota
Several research projects investigated Boreal Owl distribution and habitat in Minnesota during the 20+ years following the discovery of this species breeding in Minnesota (Lane 1997; Belmonte 2005; Wilson, personal communication).
Initiated in 2005, the Western Great Lakes Region Owl Survey, led by the University of Minnesota Duluth's Natural Resources Research Institute (NRRI) and Hawk Ridge Bird Observatory (HRBO), has provided data on owl abundance and distribution in Minnesota and Wisconsin (Grosshuesch and Brady 2014). Few Boreal Owls were detected in the early years of this effort. In 2015, HRBO partnered with the U.S. Forest Service (USFS) to conduct surveys targeting the Great Grey (Strix nebulosa) and Boreal Owls, and this focused effort was successful, detecting 10 Boreal Owls in northeastern and north-central Minnesota (Grosshuesch 2015). Still, this is fewer Boreal Owls than were found during earlier surveys and research projects in the late 1980s through the 1990s.
Steven P. Stucker (MNDNR), 2018
(Note: all content ©MNDNR)