Northern Prairie Wildlife Research Center
Habitat and History. The range of the American crow in North America includes all of the prairie pothole region Stewart 1975; South Dakota Ornithologists' Union 1991). American crows were distributed throughout the region before settlement but were scarce or absent in most areas (Pittman 1928; Farley 1932; Bird 1961; Houston 1977). Populations expanded greatly after settlement (Taverner 1926; N. Criddle 1929; Johnston 1961; Houston 1977) and the species was abundant in much of the region by the 1930's (Hochbaum 1944; Sowls 1955; Bird 1961; Kiel et al.1972; Houston 1977). Populations seemingly peaked in the 1940's but then began to decline (Bird 1961; Kiel et al.1972). In North Dakota, the decline continued to the early 1980's (Rolandelli 1986).
Although the American crow benefitted greatly from agricultural development that provided trees for nesting and food (Schorger 1941), the species has been persecuted by humans. American crows have long been considered a major predator of clutches of ducks and other game birds (Taverner 1926), and efforts to reduce their numbers were widespread in many parts of the region during the 1920's to early 1950's (Todd 1947; Sowls 1955; Johnson 1964; Bihrle 1990). An estimated 100,000 American crows were killed in North Dakota during 1924 (Johnson 1964). In 1931 in Alberta, the Municipal District of Lloyd George paid bounties for 14,908 crow eggs and 924 pairs of crow feet (Farley 1932). The North Dakota Game and Fish Department sponsored annual statewide contests to control American crows in spring during the 1930's and 1940's and awarded prizes for the greatest numbers of destroyed birds and eggs (Anonymous 1941; Bihrle 1990). Ducks Unlimited sponsored extensive campaigns in the prairie pothole region of Canada and in 1944 reported that 760,238 adult birds and 982,697 eggs and nestlings had been destroyed during the previous 6 years (Anonymous 1944).
In 1972, the Migratory Bird Treaty Convention of 1936 between the United States and the United Mexican States was amended to include corvids (U.S. Fish and Wildlife Service 1975), thereby providing American crows with considerable protection in the United States. However, American crows remained unprotected in Canada (Godfrey 1986). Although organized campaigns to reduce the abundance of American crows were largely discontinued, hunting of the species in spring continues in many areas (Bihrle 1990).
Population Structure. American crow populations in spring consist of adult breeding pairs, usually dispersed throughout an area but sometimes clumped, and yearling birds. Territorial behavior has been reported among neighboring groups (Kilham 1985). American crows seemingly first breed when 2 years old (Good 1952). Many yearlings seemingly return to their hatching locality (Good 1952) where they serve as helpers to breeding birds (Chamberlain-Auger et al. 1990). Flocks of nonbreeding adults may also be present throughout spring (Good 1952).
American crows can become very numerous. A population of breeding pairs, the density of which exceeded 0.62/km2, was reported for an area of approximately 31 km2 in Saskatchewan in 1934 (Kalmbach 1937). Smith (1971:33) reported in Alberta, "Crow populations were occasionally censused throughout the parklands, and they were often larger than the local duck populations. It was not unusual to find 60 to 80 crows per square mile in prime duck habitat." Stoudt (1971:35) reported in the Redvers area in aspen parkland of southeastern Saskatchewan "The crow population was very high from 1952 to 1957 and was estimated to equal that of the mallard, 45 pairs per square mile." Ignatiuk and Clark (1991) reported annual densities of 0.34-0.80 occupied nests/km2 in three areas in the aspen parkland of Manitoba (near Moore Park) and Saskatchewan (between Hanley and Leask) during the late 1980's.
Distribution and Abundance. American crows were observed (Fig.15; Appendix Table 13) but did not nest (Fig. 15; Appendix Table 12) in all study areas. We observed mostly scattered breeding pairs but found a few clumped nests, several nesting sites where more than two adults were present, scattered individuals, and several groups of nonbreeders. American crows were common or more abundant in a significantly greater percentage of study areas in Canada than in the United States (Appendix Table 6). In Canada, they were numerous in 2 areas (13%), common in 13 (81%), and uncommon in 1 (6%). Occupied nests were in every study area in Canada each study area-year when thorough searches for nests were conducted; nest densities ranged from O.1/km2 in one study area (Ceylon) in 1984 to l.9/km2 in another (Hanley) in 1985 (Appendix Table 12). In the United States, American crows were uncommon in 14 study areas (82%) and scarce in 3 (18%; Fig. 15); occupied nests were found in five (29%) of the areas, including the nest found incidentally in one study area (Plentywood; Appendix Table 12). The highest density of nests in study areas in the United States was 0.09/km2 (Hawley) in 1988. Thus, the lowest density of nests in study areas in Canada approximated the highest density in the United States.
In Canada, the largest populations were in study areas in the aspen parkland, especially two (Hanley, Holden) with 1.7 occupied nests/km2 (Fig. 15). Counts of American crows along road transects also were highest in study areas in the aspen parkland. The average percentage of quarter sections in which American crows were detected per road-transect count in Canada was significantly greater in the aspen parkland (19.8% [SD = 12.23]) than in the prairie (11.8% [SD = 6.50]; t = 1.56, 14 df, P = 0.140). Both the lowest average densities of nests (Fig. 15) and the lowest average count along road transects (Appendix Table 4) were in the three southernmost study areas in Saskatchewan (in prairie). The average percentage of quarter sections in which American crows were detected per road-transect count in Canada did not differ between study areas in eastern (Manitoba and Saskatchewan) and western (Alberta) portions of the aspen parkland (17.5% [SD = 11.03] vs. 24.5% [SD = 15.65]; t = 0.79, 7 df, P = 0.453).
In the United States, the highest densities of American crows were in the aspen parkland of western Minnesota where four of six occupied nests were found or estimated present in study areas (Appendix Table 12). Behavior of American crows suggested four additional nests were in the study areas in Minnesota in 1987. The observation rates of American crows in small unit management study areas in Minnesota aspen parkland averaged 13 times the observation rates in small unit management study areas in North Dakota prairie (Appendix Table 13).
Fig. 15. Observed or estimated density of occupied nests of American crows (nests/10km2) in study areas in the prairie pothole region and ratings of the abundance of American crows in each area (rounding accounts for apparent discrepancies in ratings of the Gayford, Leask, Denzil , Craik study areas). Results for areas are for >= year,1983-88 (Appendix Table 13); results from areas surveyed >1 year were averaged.