Northern Prairie Wildlife Research Center
The present study documents red fox predation as a major source of hen-selective mortality affecting ground nesting ducks in the midcontinent area and shows that it is not an important cause of mortality to overwater nesting ducks. Red fox predation has not previously been identified as an important source of anatid mortality except by Sargeant (1972) and Johnson and Sargeant (1977) who made use of certain data reported here. This is because both the predator and prey are distributed over a vast area, the mortality is relatively inconspicuous, and studies of waterfowl mortality have largely ignored the biology of predators. Although most predation probably occurs at nests, little evidence of fox-caused hen mortality has been reported in duck nesting studies (e.g., Moyle 1964, Schranck 1972, Duebbert and Kantrud 1974, Higgins 1977). Stoudt (1971: 54), however, considered predation by red foxes on nesting hens to be "very serious" but he was unable to quantify the mortality. The lack of evidence of predation on hens at nests is due to aspects of both fox and duck behavior. Sargeant and Eberhardt (1975) found that adult ducks become immobile instantly and feign death rather than struggle when captured by foxes. They also found that foxes usually do not feed on ducks at capture sites. Hence, evidence of predation (body parts, excessive feathers, blood, trampled vegetation) is seldom seen at nests where foxes capture hens.
Almost no waterfowl nesting studies have included information from food remains found at fox dens. An exception is Stoudt (1971:35) who reported finding 3 freshly killed hen mallards and 1 hen northern pintail at a fox den in Saskatchewan. We found 2 midcontinent studies of food remains at fox dens that can be compared in a general way with our findings, but neither documents species and sex composition of ducks found. Errington (1937) reported 21 ducks among 3,858 prey items at 313 dens in Iowa (0.1 duck/ den), and North Dakota Game and Fish Department (1949:22) reported 308 ducks among 1,075 prey items at 62 dens in eastern North Dakota (5.0 ducks/den). The incidence of ducks at dens in both studies is higher than we found for the same areas, but we suspect results are inflated relative to our findings because of almost certain inclusion of some subsurface duck remains in the lists of total prey items. Except for Johnson and Sargeant (1977), no attempts have been made to estimate total numbers of ducks taken by foxes from food remains found at dens.