Northern Prairie Wildlife Research Center
There are plausible explanations for the differences in food habits of foxes detected between the two study areas. The greater frequency of plains pocket gophers in scats from rangeland than from cropland reflects the lower availability of pocket gophers in cropland landscape. Plains pocket gophers primarily feed on roots and tubers (Jones et al., 1983), which would be largely unavailable to them in the cropland because of the predominance of dryland crop-fallow rotation practices. Our finding that Ord's kangaroo rats are more common in the diets of swift foxes in rangeland than in cropland in spring also reflects availability. Our spotlighting survey results indicated Ord's kangaroo rats were more abundant in rangeland than in cropland. Kangaroo rats prefer sandy soils and do well in grazed pastures (Jones et al., 1983). The lack of no difference between study areas in the frequency of occurrence of rabbits (all Leporidae) in scats, agrees with the similar observation rates for Leporidae species overall from the spotlighting surveys.
Studies have reported that arthropods are common in the swift fox diet but contribute relatively little to the total biomass (Kilgore, 1969; Zumbaugh et al., 1985). Kitchen et al. (1999) reported swift fox scats in late spring and early fall had a higher volume of insects than any other food group. We found relatively high frequency of occurrence and percent volume of arthropods in all seasons in both areas (Table 1 and Fig.2). We identified insect remains only to family, and without genus or species information it is difficult to determine if these prey items had a propensity to occur in either area. Our data may reflect some yearly bias because grasshoppers were observed in remarkably high abundance in October 1993 (L. B. Fox, Kansas Department of Wildlife and Parks, pers. comm.).
Although some bird remains were definitely passerine, we could not identify these remains to species. Other studies of swift fox food habits identified ground-nesting birds, such as meadowlarks (Sturnella spp.) and lark buntings (Calamospiza melanocorys) as food items (Cutter, 1958b; Kilgore, 1969; Uresk and Sharps, 1986). The difference we found in swift fox use of adult birds between areas in fall reflect the increased passerine populations in the cropland during this season. In the fall large flocks of wintering horned larks (Eremophila alpestris) and longspurs (Calcarius spp.) frequent wheat fields in western Kansas (Best et. al., 1998; L. B. Fox, Kansas Department of Wildlife and Parks, pers. comm.). In winter birds are more available in cropland both as prey and as carrion when sudden drops in temperature or blizzard conditions cause high bird mortality (Dane and Pearson, 1971; Krapu, 1986).
Although our sample size is small, we attribute the greater frequency of eggshell fragments in swift fox scats collected in rangeland than in cropland in summer to the availability of eggs. Best et al. (1997) reported greater abundance of passerines nesting in perennial grassland than in cropland in Kansas. Our finding of bird eggshell fragments in scats collected in fall, following the nesting season, indicates swift foxes may be caching eggs, a behavior well documented in red foxes (Vulpes vulpes; see Sargeant et al., 1998). Swift foxes have been observed caching eggs, rodents and other food items (M. A. Sovada, pers. obs.).
Swift fox studies have typically reported that a high proportion of samples contained native plant seed and grass, but in relatively small amounts per sample (Cutter, 1958b; Kilgore, 1969; Zumbaugh et al., 1985; Uresk and Sharps, 1986; Hines and Case, 1991). Zumbaugh et al. (1985) and Uresk and Sharps (1986) did not consider plants to be significant in the diet of swift foxes. However, Hines and Case (1991) found prickly pear cactus fruit in swift fox stomach samples, and agreed with Kilgore (1969) and Cutter (1958b) who suggested that plants may be a food source for swift foxes. Our finding of wild plum and prickly pear cactus seeds supports the contention that swift foxes use native plants as food.
Carnivore use of sunflower seeds in the Great Plains region has been documented for American badger (Taxidea taxus; Sovada et al., 1999), red fox (Sargeant et al., 1986), raccoon (Greenwood, 1981), striped skunk (Mephitis mephitis, Greenwood et al., 1999) and coyote (Sovada et al., 2000). Direct observation of sunflower seed consumption by swift foxes was made in 1993 by L. B. Fox (Kansas Department of Wildlife and Parks, pers. comm.). After examining a harvested sunflower field used by radio-marked swift foxes, he determined that the foxes had been feeding on waste seed (based on evidence at the site) and had cached a seed head. Our results provide further evidence of swift fox use of commercial sunflower seeds as a food. The typically late harvest of commercial sunflower crops makes waste seed available to wildlife throughout the fall and winter (Sargeant et al., 1986). Sunflower crops supplement the diet of swift foxes during winter when availability of some animal prey is generally lower. The difference in occurrence of plants in swift fox scats between areas can be attributed to the proportionally larger amount of commercial sunflower seeds available in the cropland than the rangeland.
We found little evidence of swift fox use of reptiles as a food resource. Frequently disturbed habitats, such as cropland, typically do not support high or diverse reptile populations (Oldfield and Moriarty, 1994), which explains the higher proportion of scats containing reptile remains in rangeland than in cropland in spring.
Other studies have reported evidence of swift foxes feeding on carrion, specifically cows (Cutter, 1958b; Kilgore, 1969; Zumbaugh et al., 1985; Uresk and Sharps, 1986). Our finding of cows and deer in swift fox scats provides further evidence of scavenging behavior. We categorize raccoons and opossums as carrion, but we can not eliminate the possibility that young animals of these species were preyed upon rather than scavenged.
Despite the extreme differences between the landscapes of the rangeland and cropland study areas, the major components of the swift fox diet were fairly similar when season was ignored (Fig. 2). Numerous differences between landscapes were evident, however, when we examined diet at lower taxonomic levels and within season. Kilgore (1969) proposed that swift fox use of prey was closely linked with availability of prey. Although we found that the diets of swift foxes were similar in the two landscapes at higher taxonomic levels, the differences we did detect between landscapes may be attributed to this link, attesting to the opportunistic foraging behavior of swift foxes.