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Fig. 1. Numbers of red fox rearing dens visited in individual counties or within states and provinces of midcontinent North America. Four additional dens without county designation were visited in Iowa (2), Nebraska (1), and east river South Dakota (1).

Since the mid-1800's, most of the midcontinent area has changed from a largely pristine wilderness to an intensively farmed area. Although much wetland drainage has occurred (Aus 1969, Whitesell 1970), the area still contains enough wetlands to attract millions of breeding ducks annually (Bellrose 1976:46). The Prairie Pothole Region is especially noted for high breeding duck populations; densities averaged 17 pairs/km² in North Dakota during 1967-69 when the present study began (Stewart and Kantrud 1974). Duck densities in the pothole region of Canada are generally higher than in the United States (Trauger and Stoudt 1978, Bellrose 1979) and up to 116 pairs/km have been recorded in certain areas (Smith 1971:26). Under present conditions, ducks nesting in the midcontinent area face environmental hazards very different from those encountered in pristine times. In addition to land use modifications, the changes include alterations in the composition, distribution, and density of mammalian predators (Bailey 1926, Kiel et al. 1972:13, Banfield 1974, Johnson and Sargeant 1977:35-40, Sargeant 1982). Some predator species such as the gray wolf (Canis lupus) and kit fox (Vulpes velox) have been extirpated from nearly all of the area, whereas others such as the raccoon (Procyon lotor) and red fox have greatly increased in numbers.

Predation is one of the most important factors affecting annual recruitment of ducks in the midcontinent area (Cowardin and Johnson 1979). However, since the studies of duck nesting success (percentage of clutches with hatched eggs) in that area were initiated by Kalmbach (1937, 1938), nearly all investigations of predation on breeding duck populations have addressed only 1 aspect of predator impact, namely egg destruction (e.g., Sowls 1955, Moyle 1964, Schranck 1972, Duebbert and Kantrud 1974). Predation on adult and juvenile ducks is difficult to measure because evidence of predation is usually limited or lacking, especially at site of capture. Predation on nesting hens is especially important because death of each hen also eliminates a potential fledged brood from fall populations.

Mammals are the principal predators affecting duck production in the midcontinent area (Keith 1961:59-60, Stout and Cornwell 1976, Johnson and Sargeant 1977) although some birds, especially the common crow (Corvus brachyrhynchos) may have considerable impact in certain areas. Species implicated in significant predation on adult ducks are the coyote (Canis latrans), long-tailed weasel (Mustela frenata), mink (M. visor), and red fox (Kalmbach 1937, Keith 1961, Sargeant 1972, Eberhardt and Sargeant 1977, Johnson and Sargeant 1977). Except for determination of the impact of red fox predation on the sex ratio of mallards in North Dakota (Johnson and Sargeant 1977) and an estimate of the impact of long-tailed weasel predation on a local population of dabbling ducks in Alberta (Keith 1961:60), there have been no quantitative assessments of impact of predator species on adult duck populations in the midcontinent area.

The purpose of this paper is to describe and assess the magnitude of red fox predation on breeding duck populations in the midcontinent area, especially in the Prairie Pothole Region of North Dakota. The findings are based on evaluations of duck remains found at red fox rearing dens.

Acknowledgments

This study was made possible by cooperation of many individuals. For participation in a questionnaire survey and for submitting waterfowl remains for examination, we thank cooperating administrators and field personnel of the U.S. Fish and Wildlife Service (USFWS) Animal Damage Control in Nebraska, North Dakota, and South Dakota; USFWS Wildlife Resources in Minnesota, North Dakota, South Dakota, and Wisconsin; USFWS Northern Prairie Wildlife Research Center (NPWRC); Iowa Conservation Commission; Minnesota Department of Natural Resources; North Dakota Game and Fish Department; South Dakota Department of Game, Fish, and Parks; and individual students at North Dakota State University, University of North Dakota and University of Minnesota. For special assistance in field work, we thank D. T. Allen, L. E. Eberhardt, E. K. Fritzell, W. H. Howell, G. L. Rohde, and J. F. Wolf. For aerial survey assistance we thank T. C. Hendrickson, R. E. Nelson, C. M. Pfeifer, and W. K. Pfeifer. We are grateful to A. T. Klett for unpublished data on duck nesting chronologies, to W. Runge and R. R. P. Stardom for unpublished data on red fox harvests in Saskatchewan and Manitoba, to A. D. Davenport and D. H. Johnson for computer programming services, to D. H. Johnson for statistical advice, to E. K. Bartels for library services, and to C. W. Shaiffer for preparation of figures. For review of manuscript drafts, we sincerely thank H. F. Duebbert, R. J. Greenwood, R. L. Jessen, D. H. Johnson, M. A. Johnson, A. T. Klett, G. L. Krapu, S. J. Maxson, H. W. Miller, M. D. Sargeant, C. H. Schroeder J. R. Serie, and G. A. Swanson.