Northern Prairie Wildlife Research Center
Because of differences in their behavior, food habits, and abundance, individual predator species affect duck production differently. Some species prey almost exclusively on eggs, others prey almost exclusively on adults and ducklings, and a few prey on all three. Moreover, different predator species affect ducks differently, depending on factors such as date of nesting by ducks and location of nests. For example, red foxes prey much more extensively on hens that nest in uplands than on hens that nest in wetlands (Sargeant et al. 1984), but minks do the reverse (Eberhardt and Sargeant 1977).
Among the predators we surveyed that prey on adult ducks, three species of mammals (coyote, red fox, mink), one mammal group (weasels), and four species of birds (northern harrier, Swainson's hawk, red-tailed hawk, great horned owl) were known or thought (weasels) to be common or numerous in one or more study areas (Tables 4 and 5) and thus sufficiently abundant to reduce survival of nesting female ducks. The number common or numerous of these species averaged 3.0 (SD = 1.26) per individual study area and always included the red fox, coyote, or both. Predation on adult ducks was evidenced by partially consumed carcasses found each year in all study areas (A. B. Sargeant and R. J. Greenwood, unpublished data). No data on mortality of ducks were collected in the Central Flyway study areas. Remains of 573 adult ducks were found in the stabilized regulations study areas and remains of 224 adult ducks in the small unit management study areas and adjoining lands. The remains represented an undetermined but probably small portion of the depredated adult ducks. Sargeant et al. (1984) estimated that red foxes took over 800,000 adult ducks, mostly female dabblers, annually from the prairie pothole region during spring and early summer 1969-73. Twenty percent of the dead adult ducks we recovered were from red fox dens (above ground and in den entrances only). Ducks recovered in this manner during a single visit to one den of a red fox family represented on average 4.5% of the ducks taken by the family (Sargeant et al. 1984).
The abundance of red foxes has a profound effect on the survival of adult ducks in the prairie pothole region and should be monitored. However, other predators, especially coyotes and certain raptors, probably also prey extensively on adult ducks. Of 87 dead adult ducks found in study areas in Alberta, 83% were from study areas (Gayford, Hay Lakes, Holden) that had little to no evidence of minks, red foxes, and weasels; 67% were from one area (Hay Lakes) where coyotes (Appendix Tables 5 and 8) and red-tailed hawks (Appendix Table 15) were especially numerous.
In the prairie pothole region, annual duck production is most affected by destruction of nests (Cowardin and Johnson 1979; Klett et al. 1988). Mayfield-corrected estimates of duck nest success (Johnson 1979) in the five principal species of dabbling ducks (mallard, gadwall [Arias strepera], blue-winged teal [A. discord], northern shoveler [A. clypeata], northern pintail [A. acuta]) ranged from 7 to 31% in Central Flyway study area-years and from 3 to 29% in stabilized regulations study area-years (Johnson et al. 1987; R. J. Greenwood, unpublished data). Comparable estimates were not available on ducks in the small unit management study areas, but other data for that part of the region show similarly low nest success (Klett et al. 1988). More than 85% of the destroyed nests were attributed to predation (Greenwood et al. 1987; Johnson et al. 1987; Klett et al. 1988).
All predator species we surveyed except raptors prey extensively on duck eggs, although predation by minks is primarily in wetlands and predation by gulls is primarily on islands (Sargeant and Arnold 1984; A. B. Sargeant, unpublished data). The number of common or numerous egg-eating species (excludes large gulls and weasels, which were not rated) averaged 4.6 (SD = 0.90) per study area (calculated from data in Tables 4 and 5). Only two study areas (Gayford, Courtenay) had less than four (three) common or numerous egg-eating species, and one of those (Gayford) was adjacent to a large nesting colony of ring-billed gulls and California gulls. Because of their great abundance, the gulls had potential for extensive predation on duck eggs in that study area. Thus, duck nests in all but one study area were at risk of predation by at least four predator species.
The average numbers of common or numerous egg-eating predator species per study area did not differ among provinces and states (4.0 in Alberta [SD = 0.82], 4.0 in Minnesota [SD = 0.00], 4.3 in South Dakota [SD = 0.58], 4.3 in North Dakota [SD = 0.87], 4.5 in Montana [SD = 0.71], 5.1 in Saskatchewan [SD = 0.88], and 6.0 in Manitoba [SD = 0.00]ócalculated from data in Tables 4 and 5). However, species composition of common or numerous egg-eating predators differed among study areas in various parts of the prairie pothole region and reflected a gradual replacement of mammals by birds from southeast to northwest across the region. For example, all common and numerous egg-eating species in the United States were mammals, whereas birds comprised nearly half of such species in Alberta ( Tables 4 and 5). The portion of the region with the greatest number of common or numerous egg-eating species was southwestern Manitoba and southeastern Saskatchewan where as many as seven such species were in individual study areas (Tables 4 and 5). Coyotes and red foxes were common or numerous in most study areas there, but often their presence in study areas was strongly segregated.
Among egg-eating predators, the striped skunk, American crow, and red fox probably have greatest effect on nest success of ducks in uplands, whereas the raccoon probably has greatest effect on nest success of ducks that nest over water. At least two of these predators were common or numerous in all study areas except one (Gayford). Although the red fox was largely absent from study areas in Alberta, it was replaced by high populations of coyotes (Figs. 5 and 7). Raccoons were common or numerous in all study areas in the eastern half of the prairie pothole region but were uncommon or less numerous in nearly all other study areas (Fig. 8). Unlike red foxes (A. B. Sargeant, unpublished data), coyotes are prone to entering water (Young and Jackson 1951; Hanson and Eberhardt 1971). In shallow wetlands in some study areas where coyotes were particularly abundant, they may have been major predators of clutches in over-water nests. The harm of avian egg-eating predators on clutches in over-water nests is largely undetermined. Sullivan and Dinsmore (1990), who studied artificial duck nests, suggested that clutches in over-water nests are less vulnerable than clutches in upland nests to predation by American crows. With the diverse community of egg-eating predators in all of our study areas, we concluded that duck eggs in nests throughout the prairie pothole region are at high risk of predation but that localized differences in predation rates should be expected.
The magnitude of predation on ducklings and the principal predator species of ducklings in the prairie pothole region are unclear. However, the principal predator species and species-groups affecting duckling survival probably should include all species that prey on adult ducks as well as the American crow, ferruginous hawk, and large gulls. Also, all predators of duck eggs probably prey on newly hatched ducklings discovered in nest bowls.
Considerably more data than are currently available are required to clarify interactions between predator populations and duck production in the prairie pothole region. However, because predation is a major factor of duck production throughout the region, the composition and abundance of predators will probably emerge as a major cause of variability in duck production. For this reason, we urge investigators of waterfowl recruitment to assess the composition of predator populations and abundance of predator species in their study areas. Such assessments should be as important as gathering information on habitat condition and composition. The information need not be detailed to be useful, and descriptive narratives with quantification of observations may suffice. The absence of a predator species can be as important as its presence. Where possible, however, we recommend use of systematic monitoring that permits comparison of findings with those of other studies. Our surveys were not labor-intensive and provided baseline data that can be used to gauge future changes in composition, abundance, and distribution of predators in the prairie pothole region.