Northern Prairie Wildlife Research Center
Habitat and History. The red-tailed hawk is distributed throughout most of North America and its present breeding range includes the entire prairie pothole region (Robbins et al. 1983; Godfrey 1986). This hawk is a woodland species that was scarce or absent throughout much of the region before agricultural development; it is the only studied large hawk that seems to have benefitted from agricultural development. The expansion of the aspen parkland zone and widespread planting of trees for windbreaks have favored this species (Houston and Bechard 1983). In Saskatchewan, Houston and Bechard (1983:105) concluded that "all evidence points to the red-tailed hawk being rare on the open plains, even in migration, until about 1920." As other hawks, the red-tailed hawk was subjected to indiscriminate shooting until the 1950's (N. Criddle 1929; Bird 1961; Salt and Salt 1976; Callin 1980). Presently, red-tailed hawks are considered at least common throughout the aspen parkland and are known to occasionally nest in the prairie where tall trees are present (Stewart 1975; Fyfe 1976; Lokemoen and Duebbert 1976; Salt and Salt 1976; Houston and Bechard 1983).
Population Structure. Red-tailed hawks are strongly territorial and tend to space their nests widely (McInvaille and Keith 1974; Rothfels and Lein 1983). Populations may include nonbreeding pairs and single birds (McInvaille and Keith 1974). Abundance tends to change little from year to year, and reuse of nests during successive years is high (McInvaille and Keith 1974; Adamcik et al. 1979).
Densities of red-tailed hawks in the prairie pothole region have been estimated. Adamcik et al. (1979) reported an average density of 0.12 breeding pair/km2 during 1966-75 in a 162-km2 area in the extreme northwestern portion of the aspen parkland in Alberta. In mixed aspen parkland prairie, Schmutz et al. (1980) found 0.002 nesting pair/km2 in a 335-480-km2 area in south-central Alberta during 1975-77 (the pairs were in the most heavily wooded areas), and Rothfels and Lein (1983) reported a density of 0.42-0.47 nesting pair/km2 in a 150-km2 area in southwestern Alberta during 1979-80. In prairie habitat, Lokemoen and Duebbert (1976) found an average density of 0.004 occupied nest/km2 in a 269-km2 area in north-central South Dakota during 1973-74 and Gilmer et al. (1983) found an average density of 0.006 occupied nest/km2 in a 1,259-km2 area in south-central North Dakota during 1977-79.
Distribution and Abundance. Red-tailed hawks were seen in all except six (Ceylon, Goodwater, Morgan, Plentywood, Shamrock, Streeter) study areas (Fig. 19) and were seen only once each study area-year in two (Fredonia, Plaza; Appendix Table 14). The eight study areas where none or only one hawk was seen annually formed a contiguous block along the west-central edge of the prairie pothole region (Fig. 19) and had the fewest trees (Table 1). In Canada, red-tailed hawks were numerous in two study areas (13%), common in six (38%), and uncommon or less abundant in seven (44%). Abundance was undetermined in one (6%) study area (Holden) but was at least common (four occupied nests were found [Appendix Table 15]). In the nine study areas in the United States where data on density of nests were collected, red-tailed hawks were common in two (22%), uncommon in six (67%), and scarce in one (11%). The species composition data from the eight Central Flyway study areas where data on nests were not collected, indicated absence or low populations of red-tailed hawks in nearly all areas (Appendix Table 14). The percentage of study areas with common or more abundant red-tailed hawks did not differ between Canada and the United States (Appendix Table 6).
Red-tailed hawks were common or more abundant in more study areas in the aspen parkland (includes Holden) than in the prairie (Appendix Table 7). Nest densities in study areas that were completely searched (excludes 1983 data) averaged 0.18 occupied nest/km2 (SD = 0.19) in the aspen parkland and 0.02 occupied nest/km2 (SD = 0.04) in the prairie (calculated from data in Appendix Table 15). The highest density of occupied nests in 1 year was 0.73/km2 (19 nests) in one study area (Hay Lakes) in 1984 (Appendix Table 15) and was one of the highest recorded densities of the species in North America (Palmer 1988a). Eleven occupied nests were found in one study area (Hay Lakes) in 1983 during incomplete searches for nests. Only one study area (Hanley) in the aspen parkland had no nests. However, three occupied red-tailed hawk nests were found within 0.8 km2 of that study area in both 1983 and 1984; no data from outside the study area were recorded in 1985. The near absence of red-tailed hawks from study areas in the south-central portion of the prairie pothole region (Fig. 19) is not representative of this entire portion of the region. For example, red-tailed hawks commonly nest in the Lostwood National Wildlife Refuge and in its vicinity (about 70 km north of the Plaza study area), where aspen parkland habitat invaded the prairie (R. K. Murphy, Lostwood National Wildlife Refuge, personal communication).
Red-tailed hawks were 0-91% of the sightings of large hawks in individual study area-years and the most commonly observed large hawks in nine (27%) study areas, all of which were in the aspen parkland (Appendix Table 14). Populations in areas studied >1 year seemed to be stable except in one study area (Penhold) where the number of occupied nests declined from seven in 1984 to two in 1985 (Appendix Table 15). Two nests there in 1984 were in quarter sections that could not be searched in 1985 (access denied). Remains of two dead red-tailed hawks were found in this study area in 1985 near nests that were occupied in 1984 but not in 1986. The mortality, possibly human-inflicted, reduced the number of breeding pairs.
Fig. 19. Density of occupied red-tailed hawk nests (nests/10km2) in study areas in the prairie pothole region, and ratings of abundance of red-tailed hawks in each area. Results from each area are for >=1 year, 1983-88 (Appendix Table 15); results from areas studied in > 1 year were averaged.