The swift fox is a severely endangered species. It has faced habitat losses due to agricultural, industrial and urban development. Hundreds of swift foxes were killed accidentally during the early 1930s from predator control programs aimed at removing wolves, coyotes, and ground squirrels from prairies. In 1978, the species was declared extirpated in Canada. There are currently populations of swift foxes in the U.S. ranging from South Dakota to Texas. However, the population is stable only in the central part of the range. Reintroduction programs in Western Canada have established small populations in southeast Alberta and southwest Saskatchewan, totaling 350 foxes. The current goal of reintroduction programs in Canada is to establish a viable, self-sustaining population distributed across the prairies and to remove the species from the endangered category by the year 2000.
Several things can be done to try to prevent further loss of and to encourage repopulation of swift foxes. Preserving the habitats of the foxes is crucial. Also, captive breeding could help increase the number of swift foxes. Reintroduction programs, like the ones in western Canada, may also be successful in restoring the swift fox to its natural habitat. However, a large number of reintroduced individuals do not survive their first year in the wild for one reason or another. Therefore, populations must be monitored and protected from human harm. Even though it is illegal to kill swift foxes, they are sometimes mistaken as coyotes and killed.
US Federal List: endangered
CITES: no special status
IUCN Red List of Threatened Species: least concern
Perception Channels: tactile ; chemical
There is ongoing controversy over whether swift foxes should be divided into two separate species, swift foxes (Vulpes velox) and kit foxes (Vulpes macrotis). Protein electrophoresis data and morphometric analyses have suggested that the species Vulpes velox contains both swift foxes (Vulpes velox velox) and kit foxes (Vulpes macrotis) (Ewer 1973; Clutton-Brock et al. 1976; Dragoo et al. 1990; Wozencraft 1993). However, other morphometric analyses (Stromberg and Boyce 1986) and analyses based on mitochondrial DNA (Mercure et al. 1993) have suggested that swift foxes and kit foxes are indeed distinct species. In fact, the study by Mercure et al. (1993) demostrated that swift foxes and kit foxes are as closely related to Arctic foxes (Vulpes lagopus) as they are to one another. In any case, swift foxes and kit foxes are ecologically and morphologically similar taxonomic groups. The former is distributed to the east of the Rocky Mountains, and the latter is distributed to the west. There is a narrow hybrid zone between the two populations in eastern New Mexico and western Texas (Mercure et al. 1993).
The cost of captive breeding programs and monitoring after reintroduction can often be high.
In the past, the fur of the swift fox was a valued commodity.
The diet of the swift fox varies seasonally, depending on what is available. It typically eats whatever live prey it can catch. Its diet includes small mammals, birds, reptiles, amphibians, fish, insects, but also includes berries and grasses.
Animal Foods: birds; mammals; amphibians; reptiles; fish; eggs; carrion ; insects; terrestrial non-insect arthropods
Plant Foods: seeds, grains, and nuts; fruit
Primary Diet: omnivore
Swift foxes originally ranged from the plains of western Canada and across the Great Plains of North America to Texas. Swift foxes disappeared entirely from Canada in the 1930s, but have been reintroduced there. At present there are a few scattered populations of swift foxes in the Great Plains of the U.S. and in western Canada. The largest population is in Colorado, Kansas, New Mexico, and Wyoming, where the species is stable. There are currently approximately only 350 individuals located in Southeast Alberta and Southwest Saskatchewan.
Biogeographic Regions: nearctic (Native )
Swift foxes live primarily in shortgrass prairies and deserts. They often form their dens in sandy soils on open prairies, along fences or in plowed fields.
Habitat Regions: temperate ; terrestrial
Terrestrial Biomes: desert or dune ; savanna or grassland
Swift foxes usually live between 3 and 6 years in the wild, but may live up to 14 years in captivity.
Range lifespan
Status: wild: 3 to 6 years.
Range lifespan
Status: captivity: 14 (high) years.
Average lifespan
Status: captivity: 12.0 years.
Average lifespan
Status: captivity: 12.8 years.
The swift fox is the smallest of the wild dogs in North America. Adults weigh between 2 and 3 kilograms and are approximately 30 cm tall and 80 cm long. They are about the size of a domestic cat. Males and females look similar except that males are slightly larger. The fur of V. velox is light grey with orange-tan coloring on the sides and legs. The throat, chest, underside and inside of the ears are creamy white. The tail is bushy and marked with black at the tip. There are also black patches on either side of the snout.
Range mass: 2 to 3 kg.
Other Physical Features: endothermic ; homoiothermic; bilateral symmetry
Sexual Dimorphism: male larger
Average basal metabolic rate: 4.948 W.
Individuals sometimes pair for life, but may not necessarily mate with the same partner each year.
Mating System: monogamous
Male swift foxes mature and mate at one year, while females may wait until their second year before breeding. The breeding season for individuals in Canada begins in March. The gestation period is 50-60 days and pups are born in mid-May. The breeding season for individuals farther south in the United States begins in late December, early January, with pups born in March and early April. Swift foxes have only one litter annually, with a litter size ranging anywhere from 2 to 6.
Breeding interval: Breeding occurs yearly.
Breeding season: The breeding season begins from December through March.
Range number of offspring: 2 to 6.
Average number of offspring: 4.25.
Range gestation period: 50 to 60 days.
Range weaning age: 42 to 49 days.
Average time to independence: 6 months.
Average age at sexual or reproductive maturity (female): 2 years.
Average age at sexual or reproductive maturity (male): 1 years.
Key Reproductive Features: iteroparous ; seasonal breeding ; gonochoric/gonochoristic/dioecious (sexes separate); sexual ; viviparous
Average birth mass: 39.65 g.
Average number of offspring: 4.5.
Average age at sexual or reproductive maturity (male)
Sex: male: 365 days.
Average age at sexual or reproductive maturity (female)
Sex: female: 365 days.
Pups are born in the underground den and typically remain there for about one month. After birth, the eyes and ears of the pups remain closed for 10 to 15 days, thus leaving them dependent on the mother for food and protection. Pups are weaned when 6 to 7 weeks old but usually remain with the mother and father until the fall.
Parental Investment: altricial ; pre-fertilization (Provisioning, Protecting: Female); pre-hatching/birth (Provisioning: Female, Protecting: Female); pre-weaning/fledging (Provisioning: Male, Female, Protecting: Male, Female); pre-independence (Provisioning: Male, Female, Protecting: Male, Female); extended period of juvenile learning
Activity patterns: Swift foxes are most active in the evenings and at night. On a southeastern Colorado study site comprised of shortgrass prairie and Colorado pinyon-oneseed juniper (Pinus edulis-Juniperus monosperma) communities, swift foxes moved less than or about 0.25 km/hour from 6:00 AM to 4:00 PM [49]. In prairie dominated by sedges (Carex spp.), needle-and-thread, and blue grama in Nebraska, activity began from 6:00 to 8:00 PM and continued until 3:00 to 6:00 AM. The only individuals in this study that moved during the day were females with pups [36]. In northwestern Texas, females and males generally spent days at their dens and traveled during the evening, night, and early morning hours [57]. A similar pattern of movement at night and lower activity levels focused around dens during the day was observed in southeastern Colorado [49]. In Nebraska swift foxes moved an average of 0.7 mile (1.2 km) per hour and 8.1 miles (13.1 km) per night [36]. Swift foxes have been reported to run at speeds of 25 to 30 mph [103]. More detail on movements and activity periods is provided by Hines and Case [36]. Energy requirements of swift fox movements are addressed by Covell and others [12]
Swift fox activity levels and social interactions vary seasonally. In southeastern Colorado, daytime activity near dens occurred more often in summer, nocturnal movement periods were longer in winter, and estimates of distance moved per hour were lower in summer than winter. Slower movement in summer may be due to greater food availability [12]. A review suggests that greater movement during the winter could be due to breeding and may increase predation risk during this period [1]. In restored and moderately-grazed shortgrass prairie in northwestern Texas, movement away from dens increased slightly for males and substantially for females following the emergence of pups from dens [57]. On a southeastern Colorado study site comprised of shortgrass prairie and Colorado pinyon-oneseed juniper communities, swift fox pairs spent more time together during the breeding season than at other times of year [47].
Associated species: Swift foxes may den outside of coyote core use areas. In an area of high coyote density of northwestern Texas, only 3 of 36 dens were located within coyote core use areas. The swift foxes occupying those 3 dens were killed by coyotes from 4 to 9 weeks after establishing the den [41]. However, in study areas in southeastern Colorado [49] and southern Alberta and Saskatchewan, and northern Montana [76] it was common for coyote home ranges to entirely overlap swift fox home ranges. For more information on the impacts of coyotes, see Predators.
There is uncertainty regarding the importance of prairie dog towns in den site selection. In a second-growth prairie in New Mexico, den sites were significantly (P<0.01) closer to prairie dog towns than random sites [46]. In contrast, swift fox dens in northwestern Texas were located in prairie dog towns as expected based on availability [78]. For information on swift fox use of prairie dog towns, see Preferred Habitat.
Den use: Swift foxes use multiple dens and use the same dens multiple times throughout the year [1,10,21,66]. In northwestern Texas swift foxes used an average of 7.5 dens annually, and dens were reused an average of 3 times during a minimum of 8 months [66]. Pups emerge from the natal den in May or June and subsequently use nearby temporary dens [63]. In northwestern Texas swift fox dens were 7,600 feet (2,317 m) apart, on average [66].
Although dens are used for shelter and to raise young [10], dens appear especially important for escaping predators [1,49,67]. In southeastern Colorado, the average distance from locations where swift foxes were killed by coyotes to the nearest den was 0.8 mile (1.34 km). This was significantly (P<0.005) further than the 0.2 mile (0.32 km) average distance of swift fox locations to their current den [49]. In western Kansas, all predation-caused mortalities occurred over 0.6 mile (1 km) from the nearest den site [100]. According to a review, den availability may influence predation rates and consequentially impact population trends [1]. Lower exposure to predators due to greater den and food availability was suggested as an explanation for lower predation rates in a kit fox population in northwestern Chihuahua, Mexico than a swift fox population in southern Alberta and Saskatchewan and northern Montana [76]. Increased swift fox survival, abundance, and distribution following installation of artificial dens imply that den availability can have impacts on swift fox population trends. However, lack of effects on recruitment and home range size suggests that more research is necessary to determine the long-term benefits of artificial dens (see Population management) [67]. A review suggests that more research is needed on the impacts of predation pressure, food availability (see Food Habits), and den quality on den use [109].
Dens are typically shared by closely related individuals [66,95]. Dens in northwestern Texas were shared only with mates [66]. In shortgrass prairie of southeastern Colorado, den sharing occurred only within family groups [95]. Data from the same site over a longer time demonstrated occasional den sharing by neighboring swift foxes. However, the positive association between home range overlap and relatedness implies that swift foxes on adjacent territories that shared dens may be related [50].
Roads: Swift foxes often den near roads. Swift fox dens in New Mexico occurred significantly (P=0.005) closer to roads and in areas with significantly (P<0.001) greater road density than random sites [31]. In a second-growth prairie in New Mexico, road densities within 0.6 (1 km) and 1.2 miles (2 km) of den sites were significantly (P<0.001) greater than random sites [46]. In Nebraska, 68% of swift fox dens were located within 755 feet (230 m) of a road. Sixty-six percent of swift fox radio locations were within 0.6 mile (1 km) of a road at this site [36], and radio locations of swift foxes in South Dakota were closer to roads than expected at random [90]. In the transition between shortgrass prairie and big sagebrush steppe cover types in Wyoming, the distance from dens to roads or fences was significantly (P<0.001) shorter than from random points to roads or fences [81]. In southern Alberta and Saskatchewan, both analyses found that occupied natal dens occurred closer to roads than unoccupied dens [87].
Possible explanations for the association with roads include avoidance of coyotes, food availability, and use as travel corridors. Swift fox use of areas near roads in South Dakota [90] and southern Alberta and Saskatchewan [87] was potentially due to avoidance of these areas by coyotes. The availability of carrion along roads may also explain swift fox selection of areas near roads [34,36,87]. Reviews suggest that areas near roads generally have higher vegetation cover than neighboring grazed fields and therefore may have greater abundance of small mammals [10], as was the case in a southern Canada study area in late winter (Klausz 1997, cited in [34]). Swift foxes may also use roads as travel corridors [34,87]. A synthesis of swift fox den selection notes that the influence of topography and soil on road placement may confound the relationship between swift fox dens and roads [34]. Although roads do provide advantages, substantial vehicle-caused mortality ([38,100], see survivorship) and fragmentation of habitat [10] suggest that overall impact on swift fox populations could be detrimental in at least some areas.
Although swift foxes tend to use roaded areas more than expected, limited evidence suggests they do not show a similar preference for human residences. Density of residential property within 0.6 (1 km) and 1.2 miles (2 km) of dens was significantly (P<0.05) lower than density of residential property near random sites in a second-growth prairie in New Mexico [46].
Soil: Although swift fox dens occur in a variety of soil textures, they are often found in loams. Dens in loam [31,37,81,116], sandy loam [31,36,46], and clay loam [46,116] are commonly reported. In the transition between shortgrass prairie and big sagebrush steppe in Wyoming, loamy soil was used significantly (P<0.001) more than expected for denning [81]. In Nebraska 79% of den sites occurred in sandy loam soil [36]. In second-growth prairie in New Mexico, most swift fox dens occurred in clay loam, fine sandy loam, and silty clay loam [46]. In rangelands and cropland of Kansas, soils at all dens were either loam or silt loam [37]. In grazed shortgrass and midgrass prairie in South Dakota, soil texture at den sites were loam, clay loam, and sandy clay loam [116]. In Wyoming impervious clay, sandy soils, and saline loamy soils were used for denning significantly (P≤0.003) less than expected based on availability [81]. Despite the trend of swift fox dens in loams, swift fox dens in northeastern New Mexico occurred in clay soil significantly (P<0.05) more than expected based on availability [46]. Dens in clay also occurred in South Dakota [116], and 21% of dens on a Nebraska site occurred in loamy sand [36].
In southeastern Colorado, swift foxes selected (P=0.03) areas with moderate amounts of rock cover (5-20%) more than expected based on availability, possibly due to increased structural stability of dens [48].
Topography: Swift fox dens often occur in elevated areas with gentle slopes. In shortgrass prairie of New Mexico den sites had significantly (P=0.047) lower slopes and occurred on ridge tops significantly (P=0.004) more than random sites [31]. In big sagebrush-shortgrass transition habitat in Wyoming, swift fox dens occurred in areas with 0% to 3% slopes significantly (P<0.001) more and areas with 3% to 6% slopes significantly (P<0.001) less than expected based on availability. Slopes of 6% to 9% were rare and used in accordance with availability [81]. Den sites occurred significantly (P<0.01) higher than the surrounding landscape in a second-growth prairie in New Mexico dominated by blue grama, sideoats grama, buffalo grass and sand sagebrush (Artemisia filifolia) [46]. Natal dens in grazed shortgrass and midgrass prairie in South Dakota occurred on hilltops [116]. In shortgrass prairie of southern Alberta and Saskatchewan, 1 of 2 analyses found that occupied natal dens had more gradual slopes than unoccupied dens and both analyses found that occupied dens occurred on the tops of hills more than unoccupied dens. Unoccupied dens were mostly badger dens [87] and therefore not likely to represent random distributions [34]. Swift foxes occupying homogenous prairie in southeastern Colorado did not select for hillside position [48].
The influence of aspect on den sites and den entrances is uncertain. In New Mexico the aspects of den sites did not differ from a random distribution. However, few den entrances faced southeast or northwest and many faced east and west [31]. In western Kansas den entrances faced all directions and no trend was discernable [37]. According to a review, the aspect of entrance openings could be influenced by prevailing winds [34].
Vegetation composition and height: Dens often occur in short- and mixed-grass prairies. Dominant plant species at den sites in 2 study areas in South Dakota included buffalograss, needleleaf sedge (Carex duriuscula), blue grama, and western wheatgrass. One of these sites had a substantial component of needle-and-thread, while forbs such as curlycup gumweed (Grindelia squarrosa) and Virginia pepperweed (Lepidium virginicum) occurred frequently at den sites in the other study area [116]. On denning sites in Nebraska live vegetation had an average coverage of 17%. Average coverage of live vegetation was 41% blue grama, 29% needle-and-thread, 22% sedges, and 8% other plants. Bare soil and litter had average cover of 14% and 69%, respectively [36]. Denning and foraging sites in South Dakota were dominated by western wheatgrass, blue grama, and buffalograss, with coverage of these species ranging from 15.2% to 38.8%. Composition at denning and foraging sites differed significantly (P=0.055), with den sites having less western wheatgrass [115].
Although vegetation in areas occupied by swift foxes is generally short [31,49,64,81,90,112] (see Preferred Habitat), there is limited evidence suggesting that vegetation at den sites may be taller than generally available vegetation. In South Dakota, the average height of dense vegetation that provided complete cover (100% visual obstruction) at den sites was 4.6 inches (11.7 cm). This was significantly taller (P=0.08, α = 0.1) than the 3.8 inch (9.6 cm) average height of dense vegetation on random sites [115]. Grass from the previous growing season was significantly (P<0.05) taller on den sites than unoccupied, predominantly badger-constructed dens in southern Alberta and Saskatchewan. The association of swift foxes with taller grass may be due to greater abundance of insect prey [87]. In southeastern Colorado habitat characteristics at den sites did not differ from available habitat within home ranges, possibly due to the homogeneity of the study area [48]. A review notes that differences in vegetation height at den and random locations are likely small and the biological impact of these differences is uncertain [34].
Water: Although swift foxes can survive without freestanding water [24], several factors suggest that rainfall may influence swift fox populations and/or behavior, especially on comparatively arid sites. These factors include the importance of water for swift fox prey (review [63]) and predators (Golightly and Ohmart 1984, cited in [87]), the need for supplemental feeding in a reintroduced swift fox population during a drought in southern Alberta and Saskatchewan [10], and the association between kit fox population density and rainfall (Cypher and others 2000, cited in [34]). A review suggests that drought in combination with other limiting factors could threaten swift fox populations [10].
Potential association with prairie dogs: More research is needed to determine the geographic extent and degree of influence prairie dog towns have on swift fox habitat use [105]. In and around the Badlands of South Dakota, a location where prairie dogs were an important component of the swift fox's diet in some periods, swift foxes used areas closer to prairie dog towns than expected at random [90]. Swift foxes were included in a list of species associated with prairie dogs in western South Dakota [96]. In the panhandle of Oklahoma, swift fox detection in prairie dog towns and paired sites without prairie dogs were statistically similar, with more detections in the sites without prairie dog towns than in prairie dog towns. This may have been at least partially due to the trend for more coyote detections near prairie dog towns in this region [97]. In northwestern Texas swift foxes occurred in prairie dog towns significantly (P<0.001) less than expected based on availability in 5 of 6 comparisons made on 2 sites over 3 years [78]. A review suggests that prairie dog towns can be detrimental to swift foxes due to a likely increase in exposure to poisons in these areas [106].
COVER REQUIREMENTS:
Swift foxes are den dependent, relying on dens year-round for shelter, escape from predators, and rearing of young. Swift fox dens typically occur in flat areas of shortgrass prairies. Swift foxes occasionally den in rangelands and croplands [21,37]. A review notes the occurrence of swift fox dens in cemeteries [21]. Swift foxes build their own dens in suitable soil or modify those of species such as American badgers (Taxidea taxus) and ground squirrels (Spermophilus spp.) [10]. The relation of dens to water is uncertain. Dens are often near roads. Den location may also be influenced by the use of an area by coyotes and prairie dogs. For a thorough review of swift fox den selection see Harrison and Whitaker-Hoagland [34].
Den use: Swift foxes use multiple dens and use the same dens multiple times throughout the year [1,10,21,66]. In northwestern Texas swift foxes used an average of 7.5 dens annually, and dens were reused an average of 3 times during a minimum of 8 months [66]. Pups emerge from the natal den in May or June and subsequently use nearby temporary dens [63]. In northwestern Texas swift fox dens were 7,600 feet (2,317 m) apart, on average [66].
Although dens are used for shelter and to raise young [10], dens appear especially important for escaping predators [1,49,67]. In southeastern Colorado, the average distance from locations where swift foxes were killed by coyotes to the nearest den was 0.8 mile (1.34 km). This was significantly (P<0.005) further than the 0.2 mile (0.32 km) average distance of swift fox locations to their current den [49]. In western Kansas, all predation-caused mortalities occurred over 0.6 mile (1 km) from the nearest den site [100]. According to a review, den availability may influence predation rates and consequentially impact population trends [1]. Lower exposure to predators due to greater den and food availability was suggested as an explanation for lower predation rates in a kit fox population in northwestern Chihuahua, Mexico than a swift fox population in southern Alberta and Saskatchewan and northern Montana [76]. Increased swift fox survival, abundance, and distribution following installation of artificial dens imply that den availability can have impacts on swift fox population trends. However, lack of effects on recruitment and home range size suggests that more research is necessary to determine the long-term benefits of artificial dens (see Population management) [67]. A review suggests that more research is needed on the impacts of predation pressure, food availability (see Food Habits), and den quality on den use [109].
Dens are typically shared by closely related individuals [66,95]. Dens in northwestern Texas were shared only with mates [66]. In shortgrass prairie of southeastern Colorado, den sharing occurred only within family groups [95]. Data from the same site over a longer time demonstrated occasional den sharing by neighboring swift foxes. However, the positive association between home range overlap and relatedness implies that swift foxes on adjacent territories that shared dens may be related [50].
Den structure: Swift foxes construct relatively shallow dens with 1 to 4 small openings. In shortgrass prairie of New Mexico the average number of entrances per swift fox den site was 1.54 [31]. A review notes that natal dens generally have more entrances than escape dens [10]. In rangelands and croplands of Kansas, dens had 1 to 4 oval or key-shaped openings from 6 to 15 inches (15-39 cm) tall and 5 to 18 inches (12-46 cm) wide. Openings were typically within 11.5 feet (3.5 m) of each other, but ranged from 3.3 to 82 feet (1.0-25.0 m) apart [37]. In South Dakota the average width of den entrances was 7.5 inches (19 cm) and the average height was 8.7 inches (22 cm) (Hillman and Sharps 1978, cited in [10]). A review notes that swift fox dens are generally about 3 feet (1 m) deep and 7 to 16 feet (2-5 m) long [21].
Topography: Swift fox dens often occur in elevated areas with gentle slopes. In shortgrass prairie of New Mexico den sites had significantly (P=0.047) lower slopes and occurred on ridge tops significantly (P=0.004) more than random sites [31]. In big sagebrush-shortgrass transition habitat in Wyoming, swift fox dens occurred in areas with 0% to 3% slopes significantly (P<0.001) more and areas with 3% to 6% slopes significantly (P<0.001) less than expected based on availability. Slopes of 6% to 9% were rare and used in accordance with availability [81]. Den sites occurred significantly (P<0.01) higher than the surrounding landscape in a second-growth prairie in New Mexico dominated by blue grama, sideoats grama, buffalo grass and sand sagebrush (Artemisia filifolia) [46]. Natal dens in grazed shortgrass and midgrass prairie in South Dakota occurred on hilltops [116]. In shortgrass prairie of southern Alberta and Saskatchewan, 1 of 2 analyses found that occupied natal dens had more gradual slopes than unoccupied dens and both analyses found that occupied dens occurred on the tops of hills more than unoccupied dens. Unoccupied dens were mostly badger dens [87] and therefore not likely to represent random distributions [34]. Swift foxes occupying homogenous prairie in southeastern Colorado did not select for hillside position [48].
The influence of aspect on den sites and den entrances is uncertain. In New Mexico the aspects of den sites did not differ from a random distribution. However, few den entrances faced southeast or northwest and many faced east and west [31]. In western Kansas den entrances faced all directions and no trend was discernable [37]. According to a review, the aspect of entrance openings could be influenced by prevailing winds [34].
Vegetation composition and height: Dens often occur in short- and mixed-grass prairies. Dominant plant species at den sites in 2 study areas in South Dakota included buffalograss, needleleaf sedge (Carex duriuscula), blue grama, and western wheatgrass. One of these sites had a substantial component of needle-and-thread, while forbs such as curlycup gumweed (Grindelia squarrosa) and Virginia pepperweed (Lepidium virginicum) occurred frequently at den sites in the other study area [116]. On denning sites in Nebraska live vegetation had an average coverage of 17%. Average coverage of live vegetation was 41% blue grama, 29% needle-and-thread, 22% sedges, and 8% other plants. Bare soil and litter had average cover of 14% and 69%, respectively [36]. Denning and foraging sites in South Dakota were dominated by western wheatgrass, blue grama, and buffalograss, with coverage of these species ranging from 15.2% to 38.8%. Composition at denning and foraging sites differed significantly (P=0.055), with den sites having less western wheatgrass [115].
Although vegetation in areas occupied by swift foxes is generally short [31,49,64,81,90,112] (see Preferred Habitat), there is limited evidence suggesting that vegetation at den sites may be taller than generally available vegetation. In South Dakota, the average height of dense vegetation that provided complete cover (100% visual obstruction) at den sites was 4.6 inches (11.7 cm). This was significantly taller (P=0.08, α = 0.1) than the 3.8 inch (9.6 cm) average height of dense vegetation on random sites [115]. Grass from the previous growing season was significantly (P<0.05) taller on den sites than unoccupied, predominantly badger-constructed dens in southern Alberta and Saskatchewan. The association of swift foxes with taller grass may be due to greater abundance of insect prey [87]. In southeastern Colorado habitat characteristics at den sites did not differ from available habitat within home ranges, possibly due to the homogeneity of the study area [48]. A review notes that differences in vegetation height at den and random locations are likely small and the biological impact of these differences is uncertain [34].
Soil: Although swift fox dens occur in a variety of soil textures, they are often found in loams. Dens in loam [31,37,81,116], sandy loam [31,36,46], and clay loam [46,116] are commonly reported. In the transition between shortgrass prairie and big sagebrush steppe in Wyoming, loamy soil was used significantly (P<0.001) more than expected for denning [81]. In Nebraska 79% of den sites occurred in sandy loam soil [36]. In second-growth prairie in New Mexico, most swift fox dens occurred in clay loam, fine sandy loam, and silty clay loam [46]. In rangelands and cropland of Kansas, soils at all dens were either loam or silt loam [37]. In grazed shortgrass and midgrass prairie in South Dakota, soil texture at den sites were loam, clay loam, and sandy clay loam [116]. In Wyoming impervious clay, sandy soils, and saline loamy soils were used for denning significantly (P≤0.003) less than expected based on availability [81]. Despite the trend of swift fox dens in loams, swift fox dens in northeastern New Mexico occurred in clay soil significantly (P<0.05) more than expected based on availability [46]. Dens in clay also occurred in South Dakota [116], and 21% of dens on a Nebraska site occurred in loamy sand [36].
In southeastern Colorado, swift foxes selected (P=0.03) areas with moderate amounts of rock cover (5-20%) more than expected based on availability, possibly due to increased structural stability of dens [48].
The importance of distance from swift fox dens to water is unclear. In the transition between shortgrass prairie and big sagebrush steppe habitat in Wyoming, swift fox locations (P≤0.041) and dens (P=0.001) were significantly closer to water sources than random sites [81]. A review notes that permanent water may improve the quality of swift fox habitat [10]. However, in southern Alberta and Saskatchewan, 1 of 2 analyses found occupied natal dens were further from water than unoccupied dens, predominantly constructed by badgers. Explanations for this trend included avoidance of coyotes, which have greater water requirements than swift foxes, short distances to several water sources [87], and the tendency for swift foxes to den on well-drained sites [10,37,87].
Density: Reported swift fox densities range from 0.179 to 1.1 fox/km². In southeastern Colorado, swift fox density ranged from 0.179 to 0.301 fox/km² across seasons and years [95]. According to a review, poor quality habitat typically supports swift fox densities of 0.2 to 0.4 fox/km², and densities were 0.8 to 1.1 fox/km² in high quality habitat in Colorado [21].
Swift fox density may be related to coyote abundance and prey densities. Swift fox density tended to be higher on sites with low coyote abundance in northwestern Texas [40] and in southeastern Colorado (P=0.006) [95]. In northwestern Texas swift fox density ranged from 0.24 to 0.31 fox/km² on a site with high coyote abundance and from 0.68 to 0.77 fox/km² on a site with low coyote abundance [40] (also see Predators). In southeastern Colorado, swift fox density was negatively associated (P≤0.017) with the coyote's predominant prey, lagomorphs [95,112], and positively associated (P=0.069) with their own primary prey, rodents [95].
Despite a lack of data on the direct effects of fire on the swift fox, their mobility (see Activity Patterns, Dispersal, and/or Home Range) and use of dens suggests that mortality from fire is rare. According to reviews, medium to large mammals typically have the mobility to avoid fire, although large, fast-moving fires can result in mortality [27,62,71,86].
Swift fox dens likely provide adequate protection from the heat from fire. Burrow depths of a few inches often provide adequate insulation from the heat of fire ([80,110], reviews by [6,15]). Temperatures reached in an experimental burrow during a chaparral fire in the Sierra Nevada foothills suggest that burrows at depths of 12 inches (30 cm) remain well below lethal temperatures [56]. Below about 2 inches soil temperatures rarely increase substantially during fires on sites with dry soils and without heavy fuel loads [15]. Therefore, temperatures in swift fox dens, typically about 3 feet deep (review by [21]), are unlikely to change during fire on semiarid shortgrass prairie sites.
Swift foxes in dens with more than one opening likely have less risk of asphyxiation [56,110]. Information on asphyxiation of burrow-dwelling mammals focuses on rodents. For instance, voles (Microtus spp.) in experimental burrows with 2 openings survived fire in a chaparral community in the Sierra Nevada foothills while piñon mice (Peromyscus truei) in experimental burrows with 1 opening did not [56]. In a backfire in tallgrass prairie of Nebraska with surface temperatures ranging from 530 to 1,044 °F (276-562 °C), 3 meadow voles (Microtus pennsylvanicus) survived in burrows 2 to 4.7 inches (5-12 cm) deep. The deepest of these burrows only had 1 opening, while the others had 3 or more openings [28]. Young swift foxes are likely well protected in natal dens, given their depth and tendency to have more than 1 opening.
Dispersal: Juvenile swift foxes disperse in fall or winter. Juvenile dispersal occurred in 2 periods in studies in Colorado, New Mexico, and Texas, one in September and October and another in January and February [39]. In a reintroduced population in north-central Montana, 77% of juveniles dispersed in September and October, while the remainder dispersed in January and March [3]. In western Kansas, dispersal occurred from 1 October to 27 December, and the average dispersal date was 5 November [102]. Four juveniles in New Mexico dispersed in September, October, November, and February [31].
Juvenile swift foxes typically disperse an average of 6 to 9 miles (10 to 15 km). Average dispersal distance of juveniles was 8 miles (13.1 km) in northwestern Texas [79] and 9 miles (14.7 km) in western Kansas [102]. Juveniles in a reintroduced population in north-central Montana dispersed an average of 6.5 miles (10.4 km). Several juveniles dispersed twice. The initial dispersal event was over 1.2 miles (2 km) and was followed by movements that averaged 14.5 miles (23.4 km) [3]. In New Mexico one female swift fox juvenile dispersed 1.2 miles (2 km) and another dispersed 6.2 miles (10 km) [31]. Some juveniles in southeastern Colorado remained in their natal home range while others dispersed into neighboring territories or further (≤9.9 miles (15.9 km)) [95]. Studies in Colorado, New Mexico, and Texas found that males were significantly (P=0.002) more likely to disperse from their natal home range than females [39]. However, a similar number of male and female swift foxes remained on natal home ranges in a reintroduced population in north-central Montana [3].
Adult swift fox of both genders disperse, although at lower rates than juveniles. Dispersal of male and female adult swift foxes has been observed in Texas, New Mexico, and Colorado [31,95]. Pooling of swift fox dispersal data from these areas indicates significantly (P<0.001) more adult males dispersing than adult females. Although adult male dispersal comprised 25% of all dispersal in these areas, adults dispersed significantly (P<0.001) less than juvenile swift foxes. Dispersing adults were significantly (P=0.028) more likely to settle in a new territory than dispersing juveniles [39]. There are several reports of adults of both genders dispersing after the loss of their mate [31,39,95]. However, in the transition between shortgrass prairie and Colorado pinyon-oneseed juniper communities in southeastern Colorado, none of the female swift foxes that lost mates dispersed from their home ranges while half of males that lost a mate dispersed [47]. In northwest Texas, dispersal distance of resident adults with mates averaged 6.2 miles (10 km), and dispersal distance of adults without mates or continuous ranges averaged 15.8 miles (25.4 km) [79]. Adult dispersal in New Mexico, northwestern Texas, and Colorado occurred year round [39].
More research is needed on the effects of site and fire characteristics on swift fox response to fire. Despite the uncertainty, the neutral or positive swift fox responses to a prescribed fire in shrubsteppe of southeastern Colorado [111], the need for fire to maintain shortgrass prairies [34,63,73,107,122], and the potential for improved habitat quality through reduced shrub cover [9] and/or vegetation height [75] suggest that fire is likely useful for maintaining swift fox habitat [63] and populations [111]. Specific recommendations for prescribed fires, such as frequency, size, severity, and timing, based on swift fox response to fire are not available [122]. However, recommendations for maintaining shortgrass prairie include burning every 5 to 8 years [63] during the dormant season. The use of growing season burns may be limited, especially in dry conditions and on sites where fire has been excluded for prolonged periods [9,63]. Although swift foxes are likely to survive fire that occurs around their den sites (see Direct Effects), a review recommends excluding all activity within 660 feet (220 m) of dens during the kit-rearing period (15 February- 31 July) [10].
Optimal size of burns for managing swift foxes is unknown, but the more home ranges within the fire boundary the greater the potential impacts. Planning prescribed fires so that no more than 25% of prairie habitat is burned each year, and unburned areas are at least 0.5 miles (0.8 km) wide has been recommended for areas occupied by swift fox [63]. This would provide diverse conditions and large refuge areas. Development of guidelines for establishing historic patterns of vegetation composition and structure in northern National Grasslands included prescribed burning targets that ranged from 500 acres (202 ha) per decade on the Ft. Pierre National Grassland in South Dakota to 24,000 acres (9,700 ha) per decade on the Little Missouri National Grassland in North Dakota [92]. The average prescribed burning target of about 2,400 acres (970 ha) per year on the Little Missouri National Grassland is less than 0.2% of its total area and is similar to one relatively small swift fox home range. Detailed prescriptions for maintaining mixed-grass prairie largely dominated by western wheatgrass in the Badlands of South Dakota are provided by Dingman and Paintner [17].
For a summary of patch burning, which uses fire in combination with grazing to create a characteristic prairie mosaic, see Marks [63]. Conservative use of this Habitat management technique is recommended until more data on the effects of grazing and fire on swift foxes and their habitat are available.
Diet Composition: Although swift foxes have a varied diet, small mammals and insects constitute the majority of prey items. They were the predominant components of swift fox diets on sites in New Mexico [31], Texas [43], Kansas [101], Colorado [2,49], Nebraska [36], Wyoming [81], and South Dakota [90,116]. Mammals often occur in the diet of swift foxes at frequencies of 80% to 100%, and insects often occur in the diet at frequencies greater than 50% [36,81,90,101]. Although mammals typically comprise the majority of prey volume [2,43,121], insects can constitute a substantial proportion [2,43]. Lagomorphs (Lepus spp. and Sylvilagus spp.) and rodents are the most frequent mammalian components, and grasshoppers (Caelifera) and beetles (Coleoptera) are the most frequent insect components in swift fox diets [31,36,49,81,101,116]. Typical rodent genera consumed by swift foxes include voles (Microtus spp.) [36,81,101,121], white-footed mice (Peromyscus spp.) [49,101,121], kangaroo rats (Dipodomys spp.) [31,49], pocket mice (Perognathus spp.) [49,81], harvest mice (Reithrodontomys spp.) [36,121], ground squirrels (Spermophilus spp.) [49,81], and woodrats (Neotoma spp.) [49,121]. Black-tail prairie dogs (Cynomys ludovicianus) are also eaten by swift foxes in some locations [49,90,116].
Other food items in swift fox diets include carrion, birds, plants and reptiles. Frequency of birds in swift fox diets varies from less than 10% [2] to 40% or more [36,90]. Bird species eaten are typically ground nesters, such as ring-necked pheasants (Phasianus colchicus), horned larks (Eremophila alpestris) [121], western meadowlarks (Sturnella neglecta), and chestnut-collared longspurs (Calcarius ornatus) [116]. Carrion can also occur at high frequencies in swift fox diets. In southeastern Wyoming, antelope (Antilocapra americana) carrion occurred in 20% of swift fox scats during winter [81], and in Nebraska cattle remains occurred in an average of 38% of scats [36]. Several studies report livestock carrion in swift fox diets [31,36,101,116,121]. Other prey species potentially consumed as carrion include skunks, raccoons [101], and jackrabbits (Lepus sp.) [36]. Vegetation was frequently consumed in Nebraska [36] and in big sagebrush-shortgrass prairie transition habitat in Wyoming [81]. Plant species reported in the swift fox diet include pricklypear cactus (Opuntia spp.) and grasses (Poaceae) [90,101,116]. Reptiles are consumed infrequently [31,101,121]. A review [10] includes amphibians as swift fox prey items.
Effects of low prey abundance: Low prey abundance may increase vulnerability of swift fox to coyote predation. One interpretation of data on swift fox activity patterns in southeastern Colorado suggests that lower food availability during winter resulted in swift foxes traveling further to obtain food [12]. Given the increased risk of predation away from dens [49,100] and in unfamiliar portions of the home range [49,76,83,100], increased travel distances may increase the risk of predation on swift fox. A comparison between a kit fox population in northern Mexico with low rates of predation-caused mortality and a swift fox population in southern Alberta and Saskatchewan, and northern Montana with high rates of predation-caused mortality led to the suggestion that smaller home ranges due to greater food availability in northern Mexico led to fewer encounters with predators [76]. A review suggests that increased exposure to predators due to low food availability could limit swift fox population size [1].
Limited research suggests swift fox densities are correlated with abundance of some prey species. In grassland steppe of southeastern Colorado, swift fox density was negatively correlated (P=0.01) with black-tailed jackrabbit abundance. This was likely due to the positive association between black-tailed jackrabbit abundance and coyote density. Although not statistically significant, swift fox density was positively associated with desert cottontail and Ord's kangaroo rat abundances [112]. A positive relationship between kit fox density and rabbit abundance, and the density dependent nature of kit fox reproductive rates at low prey densities led researchers to suggest that prey abundance was 1 of 2 major factors influencing population densities, the other being behavioral spacing [118]. Russell [90] suggests more research is needed on the influence of prey species on swift fox densities.
Influence of prey availability: Seasonal changes in diet generally reflect changes in food availability ([43,81], review by [10]). For instance, on sites in South Dakota [90] and Nebraska [36] birds were most frequently encountered in summer; and on a site in Wyoming, birds were least frequently encountered in winter [81]. Carrion occurred most frequently in swift fox diets in fall and winter in Wyoming [81] and in fall in South Dakota [90]. Several studies note a peak in the consumption of insects in summer and/or fall [12,43,49,90,101]. On a site in Kansas, frequency of beetles in the diet peaked in summer and grasshopper frequency peaked in fall [101]. Grasshopper frequency peaked in September and November on a Wyoming study site [81]. In Kansas, insects frequently occurred in swift fox stomach contents during winter, but at low volumes [121]. On a site in New Mexico frequency of grasshoppers in the swift fox diet peaked during winter, and frequency of vegetation in the diet peaked in spring [31]. On a site in Kansas, the frequency of mice, rats, and voles (Muridae) in the swift fox diet peaked in summer and fall and the frequency of rabbits peaked in spring [101], possibly due to the availability of young, small rabbits.
Differences in diets of swift foxes in rangeland and cropland in Kansas [101] and northwestern Texas [44] also suggest that swift fox diets are influenced by prey availability. In spring in Kansas, plains pocket gophers (Geomys bursarius, P=0.026) and rodents in the Heteromyidae family (P=0.067) were significantly more frequent in the diets of swift foxes in rangeland than of cropland. These rodents were probably more abundant in rangeland than cropland. In fall, swift foxes in cropland ate significantly (P=0.008) more birds than swift foxes in rangeland, likely due to the presence of overwintering horned larks and longspurs in wheat fields. Swift foxes in cropland ate more vegetation than those in rangeland in both spring (P=0.004) and fall (P=0.001) [101]. Taxonomic and size differences between the diets of swift foxes in contiguous and fragmented prairie in northwestern Texas was significant (P<0.001). Swift foxes occupying prairie habitat fragmented by cropland and planted tall grasslands had more diverse diets, and ate more black-tailed jackrabbits (Lepus californicus), cottontails (Sylvilagus spp.), birds, and vegetation than swift foxes in contiguous prairie. Foxes in contiguous prairie ate smaller prey including more insects. In this study area rabbits were more abundant in fragmented habitat than in contiguous prairie [44]. Despite fragmented habitat apparently meeting the dietary requirements of swift foxes, cropland and planted tall grassland habitats were avoided ([38], see Preferred habitat).
Home range: Swift fox home range size varies with site, year, season, and gender. Calculation methods also influence home range estimates. Home range overlap between neighboring foxes is uncommon compared to overlap of mates' home ranges. Coyotes may influence swift fox home range placement.
Descriptions of swift fox home range size vary from 1,900 acres (760 ha) to about 7,900 acres (3,200 ha); the variability is likely due to yearly variation, differences in estimation methods, and variable site characteristics. The average home range size of swift foxes on a site in Colorado was 1,900 acres (760 ha) based on data from January 1997 to August 1998 [49] and 2,320 acres (940 ha) based on data from January 1997 to December 1998 [95]. In 1986 and 1987 the average home range size of 5 swift foxes in the same Colorado study area was 5,630 acres (2,278 ha) [2]. From 1998 to 1999 the average home range in big sagebrush-shortgrass prairie transition habitat of Wyoming was 4,600 acres (1,860 ha) [82]. In 1996 and 1997 the average swift fox home range in this area was estimated as 860 acres (350 ha) using one method and 2,890 acres (1,170 ha) using another [84]. In New Mexico, 2 methods of estimating annual swift fox home range size resulted in average home range estimates of 5,420 acres (2,192 ha) and 3,700 acres (1,495 ha) [31]. In northwestern Texas, swift fox home range was estimated as 2,890 acres (1170 ha) [38]. The estimated home range size of adult swift foxes in Kansas during the pup-rearing period was 3,930 acres (1,590 ha) [102]. In southeastern Alberta and southwestern Saskatchewan, average swift fox home range size was 7,880 acres (3,190 ha) [76].
Season and gender can influence home range size. In shortgrass prairie of southeastern Colorado [95] and big sagebrush-shortgrass prairie transition habitat of Wyoming, male and female home ranges were significantly (P≤0.04) smaller during the pup-rearing period than other times of year. Females in the Wyoming study area had significantly smaller home ranges than male swift foxes during the pup-rearing (P≤0.04) and breeding (P=0.05) seasons [82].
Home ranges of pair members overlap more extensively than the home ranges of neighboring pairs. In big sagebrush-shortgrass prairie transition habitat of Wyoming, the home ranges of mates overlapped an average of 70.8%, while the average home range overlap of adjacent pairs was 11.9%. Core areas of adjacent pairs rarely overlapped [82]. In a previous study on the same site, the home ranges of mates overlapped significantly (P=0.0001) more than home ranges of swift foxes that were not paired. Core use areas of unpaired swift foxes did not overlap [84]. In southeastern Colorado [2] and western Kansas [102] the home ranges of neighboring pairs overlapped to some extent, but core use areas of neighboring pairs rarely overlapped. In another study in southeastern Colorado, concurrent sharing of dens by neighboring swift foxes occurred occasionally. The degree of overlap of neighboring pairs and occurrence of swift foxes inheriting neighboring home ranges was influenced by the relatedness of neighboring swift foxes [50].
There is conflicting evidence regarding the influence of coyotes on swift fox distribution and home range placement. In northwest Texas, swift fox home ranges were outside or on the margins of coyote home ranges and rarely overlapped coyote core use areas [41]. In the Oklahoma panhandle, swift fox occurred at comparatively low abundance in areas where rates of coyote detection were high [58]. In southeastern Colorado, the opposite relationships of swift foxes and coyotes with habitat characteristics suggests that swift foxes would avoid sites often used by coyotes [112]. However, coyote home ranges completely overlapped most, if not all, swift fox home ranges in southeastern Colorado [49] and "a large proportion" of swift fox home ranges in Canada [76].
Postfire response: In a southeastern Colorado shortgrass steppe site, the only swift foxes influenced by a 643 acre (260 ha), low-severity, spring burn were those whose home ranges overlapped the burned area. These individuals appeared to spend more time in the area of the burn foraging after the fire than before the fire. The percentage of radio-locations occurring within the burned area increased by an average of 14.5% following the fire. However, this result was not significant (P=0.1). The 2 individuals with core use areas that overlapped the burn increased their use of the burn area for denning. Before the prescribed fire these 2 individuals denned within the burn area 75% and 60% of the time. After the fire both denned exclusively within the burned area. Swift foxes with core use areas outside the burn area did not change use patterns in response to the prescribed fire. It is suggested that territoriality restricted response of swift foxes. Survival rates of swift foxes in the study decreased from 88% before the fire to 60% after the fire. The investigators describe the prefire survival rates as abnormally high and imply that the difference is related more to small sample sizes and annual variation than to the effects of the prescribed fire [111].
HABITAT RELATED FIRE EFFECTS: Habitat responses to fire such as reduced shrub cover and vegetation height suggest that burning improves swift fox habitat quality [63,111]. Both dormant and growing season burns reduced cover of shrubs in shortgrass prairie of northeastern New Mexico [9]. The average height of dominant species the 1st growing season after a wildfire in a shortgrass prairie in Kansas was less than the average height of unburned vegetation [55]. Several reviews note that burning prairies within the swift fox's range can prevent conversion to shrubland [34,63,73,107,122].
Thomson and others [111] suggest that fire severity and prefire habitat structure will influence the degree to which fire improves swift fox habitat. Low-severity fire could improve visibility and habitat quality in vegetation that is generally open, while in shrubby and/or dense vegetation severe fire would be most likely to increase swift fox habitat suitability.
Vegetation recovery in the semiarid grasslands occupied by swift foxes is typically slower than in more mesic prairies [63,117]. Recovery takes up to 3 years ([55,117], reviews by [25,63]). Following a March wildfire in central Kansas, herbaceous production in burned shortgrass prairie was significantly (P≤0.01) less than production in unburned prairie in the 1st and 2nd growing seasons following the fire. In the 3rd growing season after fire, herbaceous production was similar in burned and unburned prairie [55].
Average to above-average amounts of precipitation following a fire appear to facilitate recovery of short and mixed-grass prairies [25,117,120]. Following October prescribed burns in mixed-grass prairie of South Dakota, the amount of precipitation influenced whether the biomass of blue grama increased or decreased. Western wheatgrass production increased following October prescribed burns in years with average precipitation [117]. A review of studies that compared unburned and burned shortgrass vegetation following single fires noted an association between greater precipitation and shorter recovery times [25]. Several other reviews note the influence of postfire precipitation on the response of shortgrass prairie vegetation to fire [60,63,73,107].
Vegetation in shortgrass and mixed-grass prairies may take longer to recover following growing-season burns compared to dormant-season burns. Although both growing and dormant season burns in shortgrass prairie of New Mexico resulted in reduced production compared to unburned sites (1,800 kg/ha), production on sites burned in the growing season (600 kg/ha) was lower than production on sites burned in the dormant season (1,200 kg/ha). Increases in grass cover, forb cover, species richness, and foliar concentrations of key nutrients by the fall following the dormant-season burn also suggest that these burns have fewer negative impacts than growing season burns on shortgrass prairie vegetation, at least in the short term [9]. In a mixed-grass prairie in South Dakota, production of western wheatgrass recovered faster following dormant season burns than growing season burns [117]. Reviews also note the impact of fire timing on the postfire response of shortgrass prairies [25,63,107].
Associated species: The response of the swift fox to fire likely depends on the response of its prey and its primary predator, the coyote.
Prey species: Although prey response varies due to differences in species ecology and fire characteristics, several swift fox prey species are either not substantially impacted by fire or benefit from fire. Fire can result in increases in grasshopper and beetle populations [26]. Several swift fox mammal prey species either increase or remain stable after fire including deer mice (Peromyscus maniculatus) [25,61,70,104], eastern cottontail (Sylvilagus floridanus), and thirteen-lined ground squirrel (Spermophilus tridecemlineatus). Several of the birds that occur in swift fox diets can benefit from fire, including western meadowlarks and horned larks [7,8,25]. However, fire can negatively impact swift fox prey species; for instance fires occurring during the breeding season of ground-nesting birds or the nymph stage of grasshopper development can have substantial negative impacts [25]. For information on the impacts of fire on other swift fox prey, see the FEIS reviews on Ord's kangaroo rat (Dipodomys ordii) black-tailed prairie dog, and meadow vole (Microtus pennsylvanicus).
Coyote: Available information suggests that fire has little negative impact on coyotes and may improve the quality of their foraging habitat. Coyote pack and litter size were not significantly impacted by the percentage of territory burned in the 1988 Yellowstone fires [13], and coyote abundance indices did not differ in burned and unburned areas following a 586-acre (237 ha) severe wildfire in central Arizona [14]. Coyote use of a chaparral area following a fire [56], increased coyote abundance concurrent with increases in deer mice and Columbia ground squirrels following stand-replacement fire in red cedar-western hemlock forest [20], and improvements to foraging habitat (see FEIS coyote review) suggest that fire can benefit coyotes.
FIRE REGIMES:
According to a review, fires occur less frequently in short and mixed-grass prairies than in tallgrass prairies [63]. Replacement fires are most common (see table below), and fires can occur in both growing and dormant seasons [9]. Merola-Zwartjes [74] reviews information on the FIRE REGIMES of southwestern grasslands. The number and size of fires in northern National Grasslands from 1978 to 1999 is provided by Samson and others [92].
The following table provides fire regime information that may be relevant to swift fox habitats. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find FIRE REGIMES".
Fire regime information on vegetation communities that may provide habitat for swift fox. For each community, fire regime characteristics are taken from the LANDFIRE Rapid Assessment Vegetation Models [54]. These vegetation models were developed by local experts using available literature, local data, and/or expert opinion as documented in the PDF file linked from the name of each Potential Natural Vegetation Group listed below. Cells are blank where information is not available in the Rapid Assessment Vegetation Model. Southwest Northern and Central Rockies Northern Great Plains South-central US Southwest Vegetation Community (Potential Natural Vegetation Group) Fire severity* Fire regime characteristics Percent of fires Mean intervalHabitat management: Grazing [11,122] and agriculture [38] may reduce the quality of swift fox habitat. Maintenance of shortgrass prairie requires regular disturbances, such as grazing, mechanical thinning of shrubs, or fire [63,75].
Grazing likely has both positive and negative impacts on swift fox populations. Potential reduction in shrub cover and vegetation height following grazing would likely improve swift fox habitat quality [34,122]. Because of this, grazing has been recommended as a way to maintain disturbance regimes [63]. Predator control associated with ranching could be beneficial or detrimental depending on the methods used and associated collateral damage. Other potentially harmful aspects of grazing are reduction in prey abundance [11,122] and accidental mortality [11].
More information is needed to determine habitat suitability of croplands. Swift fox avoidance of agricultural lands is summarized in Preferred habitat. Swift foxes in rangeland weighed significantly (P=0.01) more than those in cropland in western Kansas [65]. However, mortality rates in rangeland and cropland were similar in western Kansas studies [65,102]. Denning sites were apparently not limiting in agricultural land in Kansas, although investigators cautioned that demography of swift foxes in cropland had not been sufficiently studied to make conclusions regarding cropland habitat quality [37]. The effects of various agricultural practices such as irrigation [37], herbicide application, and plowing [34,37] also likely impact the suitability of agricultural lands as swift fox habitat.
Given the avoidance of many croplands by swift fox and the impacts of tall vegetation on predation-caused mortalities (see Preferred Habitat), conservation of shortgrass prairie is likely important for swift foxes [38].
Grazing, mowing, mechanical thinning of shrubs, and prescribed burning have been recommended for maintaining shortgrass prairie. A Fish and Wildlife Habitat Management Leaflet recommends rotational and deferred grazing at varying intensities in individual pastures at least 125 acres (51 ha) in size. Mowing is recommended on a rotational schedule of 3 to 5 years in areas where burning is impractical. Details of site preparation for restoration of grasslands, such as clearing vegetation from open, elevated sites before planting seed are described by Marks [63]. Grazing [63] and mechanical thinning treatments [75] in combination with prescribed burning have been recommended in shortgrass prairie habitat. For more details on shortgrass prairie management using prescribed fire, see Fire Management Considerations. In nonnative grasslands, light disking to 2 to 4 inches is recommended no more than every 3 to 5 years with no more than 33% disked annually to maintain an early successional state or form a fire break [63].
A review recommends that activity should be excluded within 660 feet (200 m) of occupied swift fox dens in the kit rearing period of 15 February to 31 July, and industrial and resource development activities should not occur within 1,640 feet (500 m) of natal dens [10].
Threats: Swift foxes face many threats including trapping, poisoning, vehicle collisions, red fox range expansion, coyotes, and habitat loss and degradation.
Although trapping and poisoning are not as common now as they were from the late 1800s to the mid-1900s [22], swift foxes are vulnerable to unintentional trapping and poisoning, including predator, rodent, and rabies control efforts [10,93,100,106]. Recommendations to minimize these impacts include monitoring, coordination with agencies responsible for trapping and baiting [10], and carefully targeting control programs ([1], see Predator control). Intentional trapping in areas where swift foxes are abundant can apparently occur at light intensities without negative impacts [10].
In some areas the losses due to vehicle collisions can be substantial [38,100] (see Survivorship). See Roads for more information on the potential positive and negative impacts of roads on swift foxes.
There is limited evidence of exclusion of swift foxes by red foxes [34], which suggests that the range expansion of red fox could pose a threat to swift foxes [1,10,106,122]. See Predators for information on the impacts of coyotes and Predator control for potential benefits and consequences of controlling predators.
The loss and degradation of shortgrass prairie habitat by grazing, urbanization, or conversion to agriculture are also major threats to swift foxes [10,93,106,122]. Conversion of habitat to irrigated agricultural land was probably the major cause of the extirpation of a swift fox population near Denver, Colorado [59]. Grazing can have both positive and negative impacts on swift foxes (see Habitat management, below). However, reviews note that overgrazing degrades habitat and results in increased shrub cover [34,122]. In addition to habitat-related impacts, agriculture [100] and grazing [11] can result in accidental swift fox deaths.
Coyote predation: Coyote predation is usually the predominant source of swift fox mortality. Swift foxes in several studies died of unknown causes [39,49,83]. If any of those deaths were due to coyotes, the values reported for these studies in the table below would be underestimates. Dietary overlap (see Food Habits) and the irregularity of coyote consumption of swift fox carcasses suggest that competition, rather than food requirements, may explain coyote predation on swift foxes [43].
Percent of mortality likely caused by coyotes in study areas throughout the swift fox's range Location Percent mortality due to coyote predation Montana 46% [3] Wyoming 46% [83] Colorado 48% [49] 82% [95] 63% [2] Colorado, New Mexico, Texas 69% [39] Kansas 62% [100] Texas 89% [41] southern Alberta and Saskatchewan, and northern Montana 56%, 44%, 17% in each of 3 years [76]Coyotes can impact swift fox survival, recruitment, and density. Swift fox density was negatively associated with coyote abundance in northwestern Texas [40] and in 2 temporally distinct studies in a southeastern Colorado study area [95,112]. Survival rate and density of swift foxes were higher in a population with significantly (P<0.01) less coyote-related mortality than a nearby population [42]. Removal of 227 coyotes from the site with high coyote-related swift fox mortality resulted in swift fox survival rate increasing from 0.47 to 0.63, swift fox density increasing from 0.24 to 0.31 fox/km² to 0.68 fox/km², and recruitment increasing from 0.25 young/adult to 1.2 young/adult [40]. A study investigating the impacts of coyote predation on kit and swift foxes concluded that in combination with prey abundance and territorial behavior, coyotes can lower the maximum density obtained [118]. Coyotes can also influence home range, den, and habitat selection; and, due to effects on swift fox density, mating systems.
Factors influencing predation rates: Swift foxes are apparently at higher risk of predation in unfamiliar areas. In Colorado 100% [49] and in Wyoming 89% [83] of swift fox mortalities due to predation occurred on the periphery or outside of the swift fox's home range. In southern Alberta and Saskatchewan, and northern Montana half as many predation-caused deaths occurred in the core of a swift fox's home range (50% fixed kernal) as in less used portions of the home range [76]. In western Kansas half of predation-caused mortalities occurred outside the swift fox's home range [100].
Vegetation structure [81,112] (see Preferred Habitat) and den [2] and food availability may also influence exposure of swift foxes to predation.
Landscape characteristics: Topography of swift fox locations is generally flat. In big sagebrush-shortgrass prairie transition habitat in Wyoming, swift foxes selected (P<0.001) areas with slopes 3% or less and avoided areas with slopes of 3% to 9%; this included the steep topography and thick vegetation of creek drainages [81]. Rugged, steep terrain is generally avoided [10,90], although swift foxes have been observed in "badland-like" regions of Wyoming [10].
Swift foxes may select habitat at a large scale. The best supported model of habitat selection by reintroduced swift foxes in and around Badlands National Park in South Dakota incorporated habitat variables such as distance to roads and prairie dog towns and topography at a 4,000-foot (1,200 m) scale [90].
Vegetation structure/cover type: Swift foxes typically use habitats comprised primarily of short (<12 inches, 30 cm) grasses and in some cases a few short shrubs. According to a review, swift fox densities are greatest where shrubs are >50 feet (15 m) apart, decline where shrubs are 16 to 50 feet (5-15 m) apart, and do not occur where shrubs are <16 feet (5 m) apart [122]. During the winter and spring, swift foxes in New Mexico selected areas of grama (Bouteloua spp.) rangeland (P=0.005) with low shrub density (P=0.004) [33]. On a site in southeastern Colorado with shortgrass prairie and Colorado pinyon-oneseed juniper woodland, swift fox home ranges occurred only in open prairie habitat [49]. In eastern Colorado, swift fox occupancy was associated with amount of shortgrass prairie [23,64]. Big sagebrush in suitable swift fox habitat at the transition between shortgrass prairie and big sagebrush cover types in Wyoming occurred at less than 16% cover and was less than 12 inches (30 cm) tall [81]. In the Badlands National Park area of South Dakota, swift fox occupancy in areas with visibility to 490 feet (150 m) was more than 7 times the occupancy of areas with visibility to only 160 feet (50 m). Within home ranges, the best supported habitat selection model for this area was based on a negative association with vegetation height [90]. Swift fox densities in southeastern Colorado were negatively associated with basal percent cover and shrub density, and significantly (P=0.02) negatively associated with grass height [112].
The swift fox's association with open, short cover is likely due to greater visibility in these habitats reducing predation on swift fox. In South Dakota in fall of 2005, coyote predation on reintroduced swift foxes in areas with low visibility (160 feet, 50 m) was 5.3 times that in areas with high visibility (490 feet, 150 m) [90]. In the transition between shortgrass prairie and big sagebrush steppe of Wyoming, swift foxes that had been killed by predators were located in big sagebrush habitat significantly (P=0.01) more and in sagebrush-grassland significantly (P=0.03) less than would be expected at random [81].
Swift foxes generally avoid crops and planted tall grasslands. In northwestern Texas, swift foxes used dry land agricultural fields of grain sorghum or wheat less than expected based on availability, and completely avoided irrigated fields of corn (Zea spp.) and winter wheat (Triticum spp.). Only 1 juvenile was observed in grasslands planted with tall grasses such as bluestems (Andropogon spp.) and sideoats grama (Bouteloua curtipendula) [38]. Similar tall grass communities in western Kansas were rarely used by swift fox. Croplands used by swift foxes in western Kansas were generally fallow or small grain fields [102]. In northwestern Texas, average dispersal distance from private ranches was significantly (P=0.04) further than from national grasslands, and swift foxes from private ranches tended (P≤0.001) to move away from cropland and residences [79]. Swift foxes in rangeland of western Kansas weighed significantly (P=0.01) more and were in better condition than those in cropland [65].
Reproduction: The swift fox's breeding system varies, with monogamy being most common. The spatial arrangement of swift foxes in southeastern Wyoming implied a monogamous mating system [84], and a low-density population in northwestern Texas exhibited monogamous behavior [42]. In addition, several reviews note the predominance of monogamy in swift fox populations [1,10,21]. However, the occurrence of trios including 2 males or 2 females has been noted [10,21], and social units with nonbreeding adults have been observed in Colorado, New Mexico, and northwestern Texas [39,95]. In shortgrass prairie habitat in Colorado, trios were observed as well as extrapair mating and mate switching [51,95]. In northwestern Texas, a dense swift fox population that included nonbreeding females exhibited polygyny and communal denning. It is speculated that differences in mating system between this site and an adjacent, low-density swift fox population that exhibited monogamy were due to the densities of swift foxes, which were likely influenced by different levels of coyote (Canis latrans) predation [42].
Following the winter breeding period and a 49 to 55 day pregnancy, litters of up to 8 pups are born in spring. Breeding occurs from late December to March, with populations in northern regions breeding later [1,21,63,106]. Gestation periods of 49 to 55 days have been reported for kit and swift foxes (Thacker and Flinders 1999, cited in [122]). Pregnancies of 51 days [63,106] or 52 to 53 days [21] may be most common for swift fox. Litter sizes are often from 3 to 6 pups ([3,83,102], reviews by [1,21,106,122]). However, average litter size upon emergence from dens in southeastern Colorado was 2.4 pups [95]. A review notes litter sizes as small as 1 pup and as large as 8 [106] pups. Swift fox pups in the southern portion of their range are typically born in March or early April [21,122], while in northern regions pups are born in April and early May [21].
Data from southeastern Colorado suggest that swift foxes do not breed every year. At the Pinyon Canyon Manuever Site, not all females reproduced during an 18-month study in 1986 and 1987 [2], and 28% of social units on the site did not produce pups during a late 1990s study [95].
Sex ratios of swift fox populations in Colorado [95] and New Mexico [31] were similar to 1:1. A review also notes that sex ratio is typically 1:1 [21].
Survivorship: Annual swift fox survival rates generally vary from 40% to 75% for adults and substantially less, 5% to 33%, for juveniles [106]. The following table illustrates the spatial and temporal variability in swift fox survival rates.
Swift fox adult and juvenile annual (unless otherwise noted) survival throughout their range. Blank cells indicate that data was not available. Location Adult survival Juvenile survival Texas 44% [42]Swift fox populations are generally dominated by young individuals, but individuals over 3 years old can comprise more than a quarter of a population. Juveniles were the most abundant age class in swift fox populations in shortgrass prairie of northeastern New Mexico [31]. The average age of 8 live-trapped swift foxes in Nebraska prairie dominated by sedges, needle-and-thread, and blue grama was 1.25 years [36]. Of 22 swift fox carcasses from western Kansas that were aged, 54.5% were 1 year old or less [65]. A review also notes the high proportion of young individuals in swift fox populations [21]. However, swift foxes from 3 to 7 years of age can comprise a substantial portion of populations. Of 22 swift fox carcasses from western Kansas that were aged, 6 (27.3%) were from 3 to 7 years old [65]. Of 30 swift foxes aged after death in southeastern Colorado, 26.6% were from 5 to 7 years old [95].
Causes of mortality include predation, vehicle collisions, disease, and incidental trapping and poisoning. Predation is the most common source of mortality (see Predators). Vehicles were responsible for 42% of mortality in northwest Texas [38] and 29% of juvenile mortality in western Kansas [100]. Diseases, such as canine distemper, occasionally result in swift fox deaths [39,83]. Incidental trapping and poisoning cause mortalities [31,39,100], although these were more common in the late 1800s and early 1900s [22]. For more information on factors that may negatively impact swift fox populations, see threats.
Vulpes velox is the scientific name of the swift fox, a member of the Canidae
family [5,119].
There is disagreement regarding the taxonomic status of the swift fox.
As of 2008, most literature considers the swift fox and kit fox (Vulpes macrotis)
separate species. Research based on mitochondrial DNA led to the conclusion
that species differentiation of swift and kit fox is appropriate [72]. Samuel
and Nelson [93] cite Snow (1973) for specific status of swift and kit fox being
justified. However, some evidence suggests that swift fox and kit fox should
be considered subspecies, Vulpes velox velox and Vulpes velox macrotis,
respectively. The evidence includes interbreeding in a portion of the area of western
Texas and eastern New Mexico where kit and swift fox ranges overlap [19] and morphometric
and protein electrophoresis data [18]. Halls' Mammals of North America [29] also considers
kit and swift foxes the same species but lists 10 subspecies. According to a FaunaWest
review [21], swift fox was split into 2 subspecies, Vulpes velox velox and
Vulpes velox hebes, by Merriam (1902). However, this distinction is not generally
recognized and occurs rarely in the literature. Reviews of these issues are provided by
Allardyce and Sovada [1] and Dragoo and others [19].
The swift fox (Vulpes velox) is a small light orange-tan fox around the size of a domestic cat found in the western grasslands of North America, such as Montana, Colorado, New Mexico, Kansas, Oklahoma[3] and Texas.[1] It also lives in southern Manitoba, Saskatchewan and Alberta in Canada, where it was previously extirpated.[2] It is closely related to the kit fox (V. macrotis) and some mammalogists classify them as conspecific.[4] However, molecular systematics imply that the two species are distinct.[5] Interbreeding between the two species does occur where their ranges overlap (eastern New Mexico and western Texas), but this hybridization is quite restricted in scope.[6]
The swift fox lives primarily in short-grass prairies and deserts. It became nearly extinct in the 1930s as a result of predator control programs, but was successfully reintroduced later. Currently, the conservation status of the species is considered by the IUCN as Least Concern owing to stable populations elsewhere.[2]
Like most canids, the swift fox is an omnivore, and its diet includes grasses and fruits as well as small mammals, carrion, and insects. In the wild, its lifespan is 3 to 6 years, and it breeds once annually, from late December to March, depending on the geographic region. Pups are born anywhere from March to mid-May, and are weaned at six to seven weeks old.
The swift fox has a dark, grayish, tan coloration that extends to a yellowish tan color across its sides and legs. The throat, chest, and belly range from pale yellow to white in color. Its tail is black-tipped, and it has black patches on its muzzle. Its ears are noticeably large. It is about 12 inches (30 cm) in height, and 31 inches (79 cm) long, measuring from the head to the tip of the tail, or about the size of a domestic cat. Its weight ranges from around five to seven pounds.[7] Males and females are similar in appearance, although males are slightly larger.[8]
The swift fox lives in short-grass prairies and western grassland. They form their dens in sandy soil on open prairies, in plowed fields, or along fences.[8] It is native to the Great Plains region of North America, and its range extends north to the southern Alberta and Saskatchewan, Canada, and south to Texas. It reaches from western Iowa to Colorado, Kansas, Wyoming, Nebraska, and Montana.[7]
The swift fox was once a severely endangered species, due to predator control programs in the 1930s that were aimed mostly at the gray wolf and the coyote.[8] The species was extirpated from Canada by 1938,[2] but a reintroduction program started in 1983[9] has been successful in establishing small populations in southeast Alberta and southwest Saskatchewan, despite the fact that many reintroduced individuals do not survive their first year.[8] Nonetheless, by 1996, 540 foxes had been released around the Alberta-Saskatchewan border and Milk River Ridge areas, parts of the species' original native range.[10] Four years later, those introduced foxes had tripled in number, making the program one of the most successful endangered species reintroduction programs in the world.[11] In May 1999, the Species at Risk Act listed the swift fox as an endangered species in Canada, giving the species further protection for growth.[12] A small, but stable and growing population continues to live freely in the southeastern regions of Alberta, and southwestern regions of Saskatchewan. Canada's national recovery strategy plan was revised in 2008 by the National Swift Fox Recovery Team, with projections of a long-term goal by 2026, "to restore a self-sustaining swift fox population of 1,000 or more mature, reproducing foxes that does not experience greater than a 30% population reduction in any 10-year period".[13]
Exact population numbers of the swift fox are unknown, but it is known that they currently inhabit only 40% of their historic range.[7] In addition to its populations in Canada, there are also swift fox populations in the United States, ranging from South Dakota to Texas. In 1995, the U.S. Fish and Wildlife Service determined that the fox warranted an endangered listing, but other higher priority species precluded its listing.[14] This prompted state wildlife agencies within the fox's range to create the Swift Fox Conservation Team, which worked to implement better swift fox management and monitoring programs.[14] Populations in the United States are stable in the central part of its range,[8] and it is not considered endangered in the United States. The IUCN Red List characterizes it as of Least Concern.[2]
In the wild, the swift fox usually lives 3–6 years, but may live up to 14 years in captivity.[10] It is primarily nocturnal, spending only evenings and nighttime above ground in the summer. Daytime activities are usually confined to the den, but it has been known to spend the warm midday period above ground during the winter.[8] Due to the harsh winter conditions where the swift fox can be found, there is an increase in mortality rates.[15] The swift fox is more heavily dependent on its den than most North American canids, using them as shelter from predators. These dens are usually burrows that are two to four meters in length.[8] It has been known to run very fast, at speeds of over 50 km/h (30 mph).[8] or up to 60 km/h (40 mph) [16] The coyote is the swift fox's main predator, but often chooses not to consume the swift fox.[17] Other predators include the badger, golden eagle, and bobcat.[17] It is also vulnerable to trapping and poisoning, as well as death on highways.[18]
Swift foxes are a socially monogamous species, although multiple breeding strategies have been observed.[19] The adult swift fox's breeding season varies with region. In the southern United States, it mates between December and February with pups born in March and early April, while in Canada, the breeding season begins in March, and pups are born in mid-May. The male swift fox matures and may mate at one year, while the female usually waits until her second year before breeding. Adults live in pairs, and although some individuals mate for life, others choose different partners each year. Gestation takes around 51 days, and four to five kits are born.[7][8]
The swift fox only has one litter annually, but may occupy up to thirteen dens in one year, moving because prey is scarce or because skin parasites build up inside the den. Sometimes it makes other burrows from other bigger animals, even though it is completely capable of digging one on its own. Pups are born in the den and typically remain there for approximately one month. A newborn pup's eyes and ears remain closed for ten to fifteen days, leaving it dependent on the mother for food and protection during this time. It is usually weaned around six or seven weeks old and remains with its parents until fall.[8] Recent research has shown that social organization in the swift fox is unusual among canids, since it is based on the females.[20] Females maintain territories at all times, but males emigrate if the resident female is killed or removed.[20]
Like most foxes, the swift fox is an omnivore. Rabbits, mice, ground squirrels, birds, insects, other arthropods, lizards, amphibians, fish and eggs are staples.[7][8] Grasses and fruits round out its diet. However, like any efficient forager, the swift fox takes advantage of seasonal foods.[8] During the summer, adults eat large amounts of insects, including beetles and grasshoppers, and feed their young with larger prey items. Deer and other carrion killed by other animals may also be important food sources.
The swift fox (Vulpes velox) is a small light orange-tan fox around the size of a domestic cat found in the western grasslands of North America, such as Montana, Colorado, New Mexico, Kansas, Oklahoma and Texas. It also lives in southern Manitoba, Saskatchewan and Alberta in Canada, where it was previously extirpated. It is closely related to the kit fox (V. macrotis) and some mammalogists classify them as conspecific. However, molecular systematics imply that the two species are distinct. Interbreeding between the two species does occur where their ranges overlap (eastern New Mexico and western Texas), but this hybridization is quite restricted in scope.
The swift fox lives primarily in short-grass prairies and deserts. It became nearly extinct in the 1930s as a result of predator control programs, but was successfully reintroduced later. Currently, the conservation status of the species is considered by the IUCN as Least Concern owing to stable populations elsewhere.
Like most canids, the swift fox is an omnivore, and its diet includes grasses and fruits as well as small mammals, carrion, and insects. In the wild, its lifespan is 3 to 6 years, and it breeds once annually, from late December to March, depending on the geographic region. Pups are born anywhere from March to mid-May, and are weaned at six to seven weeks old.
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