Northern Prairie Wildlife Research Center
Ducks represent a seasonally available food supply to midcontinent foxes. They become available at the time of year when fox food demands are increased because of care of pups (Sergeant 1978). Returning ducks spread out across the midcontinent area to nest, and in the Prairie Pothole Region most fox families feed on them. Hence, the arrival of ducks in spring is a timely infusion of an abundant, desirable, large prey. Our data establish that large numbers of dabbling ducks are taken by foxes each year and that predation focuses on upland nesting hens. The predation occurs at the time of year when duck populations are at their lowest levels and adjusted to prevailing habitat conditions. Although predation by foxes does not jeopardize the survival of any midcontinent duck species, it does affect annual production, population composition, and may, when coupled with other mortality factors, be sufficient to hold populations at much lower levels than would otherwise occur.
The effect of fox predation on the sex composition of some dabbling duck species is noticeable. The preponderance of drakes in most dabbling duck populations is well known but largely unexplained, although it is generally believed to be caused by factors other than hunting, including predation on nesting hens (Bellrose et al. 1961: 24-25). Johnson and Sargeant (1977) examined the impact of fox predation on mallard sex ratios in the pothole region of North Dakota during 1963-73 and concluded the predation was sufficient to cause a disparate sex ratio of about 1.2 drake/hen, which was comparable to the sex composition of the population. The disparate sex ratio would have been much more pronounced if hunting by man had not been drake selective (Johnson and Sargeant 1977:16-17). Similar relations likely exist among most dabbling ducks because fox predation on all species was hen selective.
Because of little-understood compensatory mechanisms affecting duck populations (Anderson and Burnham 1976), caution must be exercised before assuming that losses of adult ducks to foxes during 1 spring necessarily affect population size the following spring. Nevertheless, fox predation of nesting ducks decreases both adult survival and recruitment of young and thereby acts to suppress density of local populations in subsequent years. The effect of reduced predation (all types) on size of local populations may at times be pronounced. Doty and Lee (1974) noted that homing by adult hens and their progeny resulted in buildup of local breeding populations of mallards using predator proof nesting structures. Deubbert and Lokemoen (1980) also found a buildup of ducks, primarily mallards, in an area with greatly reduced predator densities (including red foxes).