Northern Prairie Wildlife Research Center
To place our estimates of duck mortality in perspective we examined the mortality from the standpoint of the contribution of ducks to the diet of foxes. Adult ducks comprised 24% of 5,402 individual food items identified at dens in the intensive study area during 1969-73. Other major foods were passerine birds (15%), hares and rabbits (11%), voles and mice (llama), pocket gophers and ground squirrels (10%), and domestic livestock and poultry (7%). The remaining food items (22%) consisted of a wide variety of birds, mammals, amphibians, fish, and refuse. Although these figures support our conclusion that large numbers of ducks were taken by foxes in that area, the percentage of ducks in den food remains does not accurately portray the contribution of ducks to fox diets. Foxes totally consume many small prey but usually leave remains from ducks and other large prey (Errington 1935, 1937; North Dakota Game and Fish Department 1949:23; Sargeant 1978).
Assessing the percentage contribution of ducks taken by fox families to total fox family diets requires knowing both the biomass of the ducks and the average amount of prey biomass consumed by individual fox families during the denning season. Sargeant (1978) found that a typical size fox family of 2 adults and 5 pups on a restricted but adequate diet required 144.1 kg prey biomass during the first 12 weeks of the denning season and 18.9 kg/ week thereafter. Of that total, about 90% was consumed.
The fox denning season in the 3-county intensive study area lasted about 13 weeks (1 Apr-30 Jun) during which time 163 kg prey biomass is required by the average fox family feeding at the rates described by Sargeant (1978). This estimate does not include waste of prey associated with surplus killing (Kruuk 1972) and cache robbing by other species (Murie 1936:26-28).
Sargeant (1978) provided average spring weights of 0.39, 0.71, and 1.15 kg for hen blue-winged teals, hen northern pintails, and hen mallards, respectively. From these weights and from proportions of these species found at dens (here considered to reflect all ducks brought to dens) in our intensive study area, we calculated that the average Drift Plain and Missouri Coteau ducks taken by foxes weighed 0.78 and 0.67 kg, respectively. The greater average weight of Drift Plain ducks was a result of a greater proportion of mallards at Drift Plain dens. We assumed that all duck remains found at dens were from ducks consumed by foxes and not from scavenged carcasses with little food value.
To estimate duck biomass consumed per fox family, we multiplied the average number of ducks estimated taken by fox families in the Drift Plain and Missouri Coteau portions of the intensive study area by the calculated average weight of the ducks taken by foxes in each area (Table 12). The resulting estimate divided by the total prey biomass required by the average fox family during the denning season was the percentage contribution of duck to fox diets. Based on these calculations we estimated that duck averaged 15.8 and 17.0% of the total prey biomass consumed by foxes in the Drift Plain and Missouri Coteau portions of the intensive study area, respectively (Table 12). Consumption varied most on the Drift Plain where duck averaged 7.7-31.5% of the annual denning season diet during the 5-year period. We attribute the greater variation in amount of duck in fox diets in the Drift Plain to the more intensive cultivation and more variable breeding duck population in that area compared to the Missouri Coteau. Because of these factors, the overall prey base available to foxes in spring in the Drift Plain was probably much less than in the Missouri Coteau and predation on ducks in the Drift Plain was buffered less by predation on other prey. Thus, during the very wet spring (1969) when many ducks were available to foxes in both areas, foxes in the Drift Plain ate more ducks than foxes in the Missouri Coteau. However, during the dry springs (1971, 1973), when few Drift Plain ponds held water, foxes in the Drift Plain had fewer ducks available than foxes in the Missouri Coteau where pond water levels were more stable, and consequently they ate less duck.
From these data we concluded that ducks were an important but relatively small part of the total diet of most foxes in our intensive study area during the denning season. We found no evidence to indicate that foxes in that area were dependent on ducks for survival. In this regard, some fox families ate few ducks, even in 1969 when ducks were especially abundant (Sargeant 1972). Our data also indicated that during the denning season individual fox families have the capacity to consume several times the average number of adult ducks we estimated they took annually in the intensive study area (Table 12).