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Distribution and Abundance of Predators that Affect
Duck Production: Prairie Pothole Region

Species Accounts -- Raptors

Ferruginous Hawk

Habitat and History. The breeding range of the ferruginous hawk, one of several species that characterize the avifauna of the mixed-grass prairie (Stewart 1975), includes most of the semi-arid plains of west-central North America (Robbins et al. 1983). The ferruginous hawk was common throughout the prairie pothole region before settlement, but populations declined after settlement; presently it is scarce or absent in much of the region (Wood 1923; Stewart 1975; Callin 1980; Bechard 1981; Houston and Bechard 1984; Schmutz 1984; Smith 1987). Williams (1946) reported that ferruginous hawks were locally common in southern Alberta and southwestern Saskatchewan during 1923-26, and Todd (1947:393) reported that they were still "fairly common (for a bird of prey)" in southern Saskatchewan in 1932. Several factors that harmed this hawk included extensive collecting of eggs (Bechard and Houston 1984), indiscriminate shooting, expansion of the aspen parkland, and plowing of grasslands. The latter destroyed nesting habitat and habitat of principal prey, especially Richardson's ground squirrels (Schmutz et al. 1980; Houston and Bechard 1984; Schmutz 1989). Because ferruginous hawks avoid nesting where over 50% of the land is cultivated (Schmutz 1984), most of the prairie pothole region is presently unsuited to the species.

The ferruginous hawk is presently absent from about 40% of its former breeding range in Alberta (Schmutz 1984) and Saskatchewan (Houston and Bechard 1984). However, Schmutz (1989) reported a 60% increase in the nesting population of ferruginous hawks in study areas in southeastern Alberta from 1982 to 1987 after an increase in abundance of Richardson's ground squirrels. Bechard (1981) thought no breeding ferruginous hawks were left in Manitoba, but Ratcliff and Murray (1984) and Ratcliff (1987) discovered four occupied nests in southwestern Manitoba during 1984-85, the first records since 1927 (Ratcliff and Murray 1984). The number of nesting pairs in Manitoba increased to at least 35 pairs by 1989 (De Smet and Conrad, Manitoba Department of Natural Resources, unpublished report). The ferruginous hawk is still common in portions of the Missouri Coteau of North Dakota and South Dakota (Stewart 1975; Lokemoen and Duebbert 1976; Gilmer and Stewart 1983; South Dakota Ornithologists' Union 1991).

Population Structure. The ferruginous hawk is a strongly territorial species that tends to space its nests widely (Lokemoen and Duebbert 1976). Densities of ferruginous hawk nests were estimated in several locations in the prairie pothole region. Schmutz et al. (1980) found 0.12 occupied nests/km2 in a 335-480-km2 area of grassland of southeastern Alberta during 1975-77, but Schmutz (1984) estimated a density of 0.02 occupied nests/km2 in the entire breeding range in southeastern Alberta. Lokemoen and Duebbert (1976) found an average density of 0.06 occupied nests/km2 in a 269-km2 area of north-central South Dakota during 1973-74. Gilmer and Stewart (1983) also found an average density of 0.06 occupied nests/km2 in a 1,259-km2 area in south-central North Dakota during 1977-79.

Distribution and Abundance. Ferruginous hawks were seen in 16 study areas but were a minor (<5%) portion of the sightings of large hawks in all areas except 5 (Fredonia, Hosmer, Morgan, Plaza, Streeter) in the United States (Appendix Table 14; rounding caused the apparent discrepancy in the Kulm study area). Populations were low in nearly all study areas (Fig. 20). In Canada, ferruginous hawks were scarce in eight study areas (50%) and undetected in eight. The ratings include one area (Holden) in 1983 where searches for nests were incomplete because no ferruginous hawks were seen in that area (we assumed if no individuals were seen that no occupied nests were present). Although no occupied nests were found in any study area in Canada, one was found 0.5 km east of one of them (Shamrock) in 1983. In the nine study areas in the United States from which data on nest density were available, ferruginous hawks were common in two (22%), scarce in two (22%), and undetected in five (56%). Occupied nests were found only in two study areas (Fredonia, Kulm); the maximum number during one study area-year was three in one area (0.13/km2; Appendix Table 15). The species composition data from the eight Central Flyway study areas for which no data on nests were available indicated populations there were similar to populations in other study areas in the prairie pothole region in the United States.

Ferruginous hawks were 0-35% of the sightings of large hawks in individual study area-years (Appendix Table 14) but were the most commonly observed large hawks only in one study area (Hosmer). The two study areas with occupied nests of ferruginous hawks and all six study areas in which sightings of ferruginous hawks were ≥5% of sightings of large hawks were in the prairie along the southwest edge of the region where grassland was most abundant.

Fig. 20. Density of occupied ferruginous hawk nests (nests/10km2) in study areas in the prairie pothole region, and ratings of abundance of ferruginous 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.

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