Northern Prairie Wildlife Research Center
Duck mortality from fox predation varies among areas and years depending primarily on fox density, duck density, and nesting conditions. The combination of a high fox population, high duck population, and good nesting conditions throughout most of the Prairie Pothole Region would result in mortality much higher than our estimated average of about 900,000 adult ducks, annually. For example, during the present study, we estimated the average fox density throughout the Prairie Pothole Region of Canada was low compared to east river North Dakota. If fox density in Canada had been equal to that in east river North Dakota (moderate), our estimate of average number of adult ducks taken annually by foxes in the midcontinent area during the nesting season would have more than doubled. Conversely, during drought years when nesting is curtailed or during periods of low fox density, mortality from foxes would be much less than our estimated average. These factors must be considered when extending our findings to other areas or to other time periods.
Our estimates of total mortality include ducks both killed and scavenged by foxes. We believe that a high percentage of the ducks found at dens were killed by foxes because this is the only explanation that is consistent with the abundance and composition of ducks found at dens. Although the fox is a noted scavenger (Scott 1943), few carrion birds appear to be available to most foxes and many that are available may not be consumed by foxes. Johnson and Sargeant (1977:22-25) identified collisions with overhead wires, vehicles, and farm machinery (primarily mowers); disease; and weather as the most probable sources of carrion ducks for foxes during spring. They concluded that these and other similar causes of nonhunting mortality affected only a small percentage of the duck breeding population. Sargeant (1981) investigated duck casualties on roads in the pothole region of North Dakota during April through July of each year 1969-78 and concluded that duck mortality (primarily dabbling ducks) due to collisions with vehicles averaged <5,000 adults annually (0.2% of the population). The mortality was greatest on the most heavily traveled roads that were accessible to relatively few foxes sampled in the present study. Mowing casualties are a source of duck carcasses for foxes, but hay cutting in North Dakota usually does not start until mid-June, and little hay was harvested in many areas we investigated.
The sex composition of carrion ducks available to foxes in spring likely approximates the composition of the breeding duck population, which has more drakes than hens (Bellrose et al. 1961), and therefore does not match the composition of ducks we found at dens. Johnson and Sargeant (1977:22-25) concluded that although some nonhunting mortality factors such as mowing were hen selective, others such as collisions with wires were drake selective. Dabbling duck collisions with vehicles involve slightly more hens (52%) than drakes (Sergeant 1981), and weather and disease apparently affect both sexes almost equally (Johnson and Sargeant 1977:24).
Even when carrion ducks are present, red foxes must compete with other predators for this food source and many foxes may avoid them because of exposure during the fur harvest season to traps baited with carrion and other lures. According to trappers, adult foxes often become shy of placed baits; we have observed this phenomenon repeatedly both with wild and captive foxes. Except when temperatures are low early in the denning season, uncovered duck carcasses are soon infested with blowfly maggots and are unpalatable to foxes (A. B. Sargeant, unpubl. data). Thus, if foxes avoided carrion ducks even for 1 or 2 days, most carcasses would be taken by other scavengers or would be unfit for consumption.