Northern Prairie Wildlife Research Center
Habitat and History. The breeding range of the great horned owl includes nearly all of North America (Robbins et al. 1983), and the species is a common resident throughout the prairie pothole region (Houston 1949; Stewart 1975; Salt and Salt 1976; South Dakota Ornithologists' Union 1991). Unlike the other studied raptors, this species is nonmigratory unless severe environmental conditions force it to leave (Craighead and Craighead 1969). The great horned owl was present throughout the prairie pothole region before settlement (Callin 1980) and has remained fairly common in many areas (Mitchell 1924; N. Criddle 1929; Farley 1932). It is primarily a woodland species (Bent 1961) and has benefitted greatly from agricultural development. Planting of trees and expansion of the aspen parkland provided great horned owls with nesting and roosting sites in many prairie areas where wooded habitat was previously lacking.
Population Structure. The great horned owl is the earliest breeding large raptor and is strongly territorial (Craighead and Craighead 1969). Not all pairs nest, however, and the percentage that nest varies greatly by availability of food (Adamcik et al. 1978; Houston 1987). Thus, the number of occupied nests is not a reliable indicator of the abundance of great horned owls.
Densities of great horned owls of up to 0.77 pairs/km2 have been reported in Kansas (Baumgartner 1939), but densities in most areas are less than 0.15 pairs/km2 (Murphy et al. 1969; Adamcik et al. 1978; Petersen 1979). Adamcik et al. (1978) reported an average density of 0.04 pairs/km2 in aspen parkland in the northwest fringe of the prairie pothole region of Alberta during 1966-76. However, densities varied annually from 0.00 to 0.10 pairs/km2. Bird (1929) and Houston (1960) estimated densities of about 0.19-0.39 occupied nests/km2 for sites in southern Manitoba and central Saskatchewan. Gilmer et al. (1983) reported an average density of 0.01 occupied nests/km2 in prairie habitat in south-central North Dakota during 1977-79.
Distribution and Abundance. Great horned owls were seen each year in all except three (Morgan, Plentywood, Parkston) study areas (Appendix Table 13) where their presence may have been undetected because no searches for raptor nests were conducted and the number of observer-hours was low. The percentage of study areas with common or more abundant great horned owls was greater in the United States than in Canada (Appendix Table 6). However, that difference may have been influenced by the dates of nest searches, which were about 1 month later in Canada and after some young probably had left nests. In Canada, great horned owls were common in five study areas (31%), uncommon in nine (66%), and scarce in one (6%; Fig. 21); density was undetermined in one (69%) study area (Holden) where the species was at least common because two nests were found (Appendix Table 16). In the nine study areas where data on nest density were collected in the United States, great horned owls were numerous in one (11%), common in seven (78%), and uncommon in one (11%). Populations in the Central Flyway study areas presumably were similar to populations elsewhere in the prairie. The percentage of study areas with common or more abundant great horned owls did not differ between the prairie and the aspen parkland (Appendix Table 7). Nests of great horned owls were found in 18 of 25 study areas and in 25 of 39 study area-years in which systematic searches for nests were conducted (Appendix Table 15). Except in one study area (Hawley) where four occupied nests were found in 1988, no more than two occupied nests were found in any area in 1 year.
Fig. 21. Study areas in the prairie pothole region in which great horned owls were seen (Appendix Table 13), average density of occupied nests (nests/10km2) found in each area (Appendix Table 15), and ratings of abundance of great horned owls in each area. Results from each area are for >=1 year, 1983-88; results from areas studied in > 1 year were averaged.