Northern Prairie Wildlife Research Center
Red foxes are opportunistic when foraging and use a wide variety of prey species (Errington 1937; Scott 1943,1947). In general, hunting intensity on particular prey depends on the desirability, abundance, and vulnerability of that prey relative to other prey, and possibly on behavioral factors associated with hunting efficiency. For example, it may be efficient for foxes to use large prey (i.e., ducks) in feeding pups (Murie 1944:213). We assumed that all duck species and sexes are equally desirable to foxes; therefore differences in the occurrence of individual species and sexes at dens should reflect differences in abundance and vulnerability.
We found that predation on dabbling ducks was much more severe than predation on diving ducks and that it was strongly hen selective. Both relationships were related to duck nesting behavior. From arrival on the breeding grounds to initiation of nesting, all ducks spend much time on water or in upland areas where chances of fox predation are slight. During this prenesting period, vulnerability of both sexes to foxes is probably similar. Once egg laying commences, the activities of hens and drakes begin to differ markedly. Drakes gradually become less attentive to hens and in June many begin to congregate on large marshes to molt (Johnsgard 1975:7-8, Bellrose 1976:25). Hens become progressively more attentive to nests (Caldwell and Cornwell 1975 Johnsgard 1975:6-7, Afton 1979), and their physical condition deteriorates (Harris 1970, Krapu 1981). Because red foxes tend to avoid entering water (A. B. Sargeant, unpubl. data), vulnerability increases greatly when ducks leave water to nest. Nearly all dabbling duck hens nest in uplands whereas most diving duck hens nest over water. Hence, as the nesting season progresses, vulnerability of drakes to foxes decreases whereas vulnerability of hens, especially of dabbling ducks, increases.
It is apparent from our data that the approximate 35-day nesting period (Bellrose 1976) is hazardous for hens of upland nesting ducks living among red foxes. Agricultural crop production practices, hail or heavy rain, and especially predators terminate most duck nests prematurely in the midcontinent area (Moyle 1964, Miller 1971, Smith 1971:33-35, Duebbert and Kantrud 1974, Higgins 1977, Pichl 1979). Cowardin and Johnson (1979:24) summarized nesting success for 9 midcontinent data sets involving different areas, years, and conditions and found that except for 1 data set with predator control (87% success), nesting success averaged 22% (range 14-39%). Nest destruction prolongs the high-risk period of individual hens to foxes because hens losing clutches generally renest if habitat conditions are favorable (Sowls 1955:129-142, Bellrose 1976). Renesting effort is strongest during wet years and, as we have shown, dabbling ducks were most vulnerable to foxes during those years. Because of these factors, fox predation on dabbling ducks and no doubt on upland nesting diving ducks is greatest when conditions for nesting are best. The opposite may be true for overwater nesting ducks where vulnerability of nesting hens to foxes may increase during drought because receding water levels provide foxes with increased access to nests (Stoudt 1971:45).
Data from our 3-county intensive study area indicated that the number of ducks taken by foxes was exponentially related to the duck availability index. If the index accurately reflected duck availability, then vulnerability of individual ducks (primarily hen dabbling ducks) to foxes increased as the duck population increased. Such a relationship is plausible and could be caused by reward-stimulated hunting in which capture success enhanced by higher duck densities stimulated greater hunting for ducks. Another explanation is that the predation reflected a strong positive relationship between total time hens spent on nests and duck abundance. This likely occurred because both duck nesting effort and duck abundance are a function of wetland conditions (Bellrose 1979, Swanson et al. 1979). Because ducks are most vulnerable to foxes while on nests, it follows that foxes took progressively more ducks when nesting effort was strongest, which was also when duck populations were highest.
Some factors may act to decrease vulnerability of nesting hens to foxes as the nesting season progresses. Vegetation grows rapidly after mid-May and many prey, especially insects, young rodents, and other ground nesting birds, become more abundant. Growing vegetation may result in greater dispersion of nests and may alter fox movements thereby decreasing the probability of detection of nests. Increasing food supplies likely reduce fox predation on ducks. Precisely how these or other factors such as age and health of birds interact to affect vulnerability of individual hens is unknown and beyond the scope of the present study.