Northern Prairie Wildlife Research Center
Habitat and History. The northern harrier has occupied all of the prairie pothole region since settlement (Robbins et al. 1983). It has probably always been the most abundant large hawk in the region, although populations seemingly have declined substantially during the last 20 years (Judd 1917; Taverner 1926; N. Criddle 1929; Bird 1961; Fyfe 1976; Whitney et al. 1978; Evans 1982). In North Dakota, northern harriers were the most abundant and widely distributed of all raptors in the late 1800's (Coues 1897); in the early 1900's Wood (1923) considered the species common in all parts of the state. Monson (1934:42) referred to northern harriers as a "Common summer resident, nesting two to four pairs to a section of meadowland" in an area about 8 km2 in eastern North Dakota during 1925-32. Mitchell (1924) reported them common in southern Saskatchewan in the early 1900's. In southern Alberta, they were considered abundant and the most common hawk in 1920 and were widely distributed but "hardly common" in 1945 (Rand 1948a:16). Farley (1932) also reported them to be common in south-central Alberta during the 1920's. Bird (1961) considered them the most abundant hawks in the aspen parkland before 1960 but stated they were less abundant than in earlier times. Todd (1947:393) reported northern harriers were "fairly common" in southern Saskatchewan in 1932. Fyfe (1976) categorized their abundance in the prairie provinces as low to medium during the mid-1970's. Stewart and Kantrud (1972) found them to be the most abundant hawks in North Dakota in 1967. However, Whitney et al. (1978) noted in South Dakota that, although northern harriers were one of the most numerous hawks, they were scarce in much of the eastern part of the state after 1966.
Habitat changes from agricultural development have been detrimental to the northern harrier (Evans 1982). Extensive drainage and cultivation and intensive grazing by livestock destroyed much nesting habitat of this species as well as much habitat of its principal prey species.
Population Structure. The population structure of the northern harrier varies from spaced pairs to groups of adults in loose nesting colonies of possibly polygamous individuals (Hecht 1951; Clark 1972; Hamerstrom et al. 1985). Breeding densities of northern harriers vary greatly in the prairie pothole region. Stewart and Kantrud (1972) estimated an average density of 0.19 pairs/km2 throughout North Dakota during 1967. Lokemoen and Duebbert (1976), however, reported annual densities of only 0.02 and 0.01 pairs/km2 in a 269-km2 area in north-central South Dakota during 1973 and 1974. Sutherland (1987) found densities of 1.35 and 1.26 nests/km2 on Mallard Island (11 km2) in Lake Sakakawea, North Dakota, during 1984 and 1985, to which she referred as the highest reported densities in North America.
Densities of northern harriers in local areas often fluctuate greatly from year to year in response to availability of prey, especially meadow voles (Hamerstrom 1979, 1986). For example, Clark (1972) reported a density of about 3 nesting pairs in approximately 8 km2 of the prairie pothole region in Manitoba during a year when meadow voles were scarce and at least 15 occupied nests in the same area the next year when meadow voles were abundant.
Distribution and Abundance. Northern harriers were in all study areas each year (Appendix Table 14), but populations in most study areas were low (Fig. 17). In Canada, they were numerous in two study areas (13%), common in five (31%), and uncommon in nine (56%). In the nine study areas in the United States where abundance was rated, northern harriers were numerous in one (11%) and uncommon in eight (89%). The species composition data indicated that populations in the five Central Flyway study areas in Montana and North Dakota, in study areas in Manitoba and Saskatchewan, and in the small unit management study areas in North Dakota were similar (Appendix Table 14). The species composition data indicated that populations in the three Central Flyway study areas in South Dakota were lower than in most other study areas. The especially low incidence of northern harriers (2% of the sightings of large hawks) in one study area (Parkston) in South Dakota (Appendix Table 14) indicated near absence of the species. However, the percentage of study areas with common or more abundant (Appendix Table 6) northern harriers did not differ between Canada and the United States or between prairie and aspen parkland (Appendix Table 7).
Northern harriers were 2-76% of the sightings of large hawks annually in each study area and were the most commonly observed large hawks in 12 (36%) areas (Appendix Table 14). Populations in some areas studied >1 year varied greatly between years, seemingly in response to changes in availability of prey. The proportion of northern harriers among sightings of large hawks in one study area (Hay Lakes) declined from 45% in 1983 to 8% in 1984 (Appendix Table 14), concomitant with a decline from 4 to 0 nests (Appendix Table 15). The abundance of meadow voles (based on incidental sightings) was much greater in that study area (Hay Lakes) in 1983 than in 1984. Similar changes occurred in three other study areas (Ceylon, Craik, Moore Park) but could not be linked to apparent changes in abundance of meadow voles. In one study area (Fredonia), the number of occupied nests declined from 15 in 1987 to 3 in 1988 (Appendix Table 15), following an apparent decline in abundance of meadow voles; but the proportion of northern harriers in the sightings of large hawks remained the same (Appendix Table 14). A widespread severe drought began in 1988 and perhaps northern harriers returned to the area, but few nested.
Numbers of occupied northern harrier nests in individual study areas in a year ranged from 0 to 15 (Appendix Table 15); the greatest numbers were 14 (Goodwater) and 15 per study area (Fredonia). The meadow vole population seemed to be low in one area (Goodwater) but high in the other (Fredonia). The nests in the former were clustered with 13 of 14 in a 6.4-km segment (40%) of the area. In the latter, 14 of 15 nests (some may have been renests; Bildstein and Gollop 1988) in 1987 were in 3 km2 of a Waterfowl Production Area. Another instance of clustering was near another study area (Hay Lakes) in 1983 where nine occupied nests were over water in cattails (Typha spp.) in a 31-ha wetland.
Fig. 17. Study areas in the prairie pothole region in which northern harriers were seen (Appendix Table 14), average density of occupied nests (nests/10km2) found incidentally in each area (Appendix Table 15), and ratings of the abundance of northern harriers in each study area. Results from each area are for >= 1 year, 1983-88; results from areas studied in >1 year were averaged.