Vegetative Characteristics of

Swift Fox Denning and

Foraging Sites

in Southwestern South Dakota

Daniel W. Uresk

Kieth E. Severson

Jody Javersak

United States

Department

of Agriculture

Forest Service

Rocky Mountain

Research Station

Research Paper

RMRS-RP-38

March 2003
Uresk, Daniel W.; Severson, Kieth E.; Javersak, Jody. 2003. Vegetative characteristics of swift

fox denning and foraging sites in southwestern South Dakota. Research Paper RMRS-RP-38.

Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station.

4 p.

Abstract—Vegetative characteristics of swift fox (Vulpes velox ) denning and foraging habitats

were studied in southwestern South Dakota. We followed 14 radio-collared foxes over a two-year

period and identified 17 den sites and 82 foraging sites. Height-density of vegetation (visual obstruction reading, VOR) was determined on each den and foraging site and on 81 randomly selected sites. Total vegetation VOR was higher (p=0.08) at den sites than on randomly selected

sites (11.7±1.4 and 9.5±0.6cm (SE), respectively). Swift foxes used foraging areas with vegetation

greater VOR (p=0.01) than that found on randomly selected sites (11.9±0.7 and 9.5±0.6 cm, respectively). Canopy cover for seven major plant species on foraging sites was different than on den

sites (p=0.055) but random sites were not different from either foraging or den sites. While previous

studies have described swift fox macrohabitats with little vegetative cover (e.g., plowed fields or

heavily grazed areas), our study showed that height-density of vegetation is important to these

foxes.

Keywords: Swift fox, Vulpes velox, foraging, dens, vegetation, habitat, South Dakota

The Authors

Daniel W. Uresk is a Research Biologist and Project Leader at the Center for Great Plains Ecosystem Research in Rapid City, SD. Kieth E. Severson is a Research Wildlife Biologist and Jody

Javersak is a Biological Science Technician. Uresk and Javersak work for the Rocky Mountain

Research Station.

Acknowledgments

This study was done in cooperation with South Dakota State University, Department of Wildlife and

Fisheries (28-C1-586). Partial funding was provided by the Nebraska National Forest and the Badlands Natural History Association, Interior, SD. Thanks are extended to Carmen Blumberg, Ryan

Tompkins, Steve Denison, and Daryl Mergen for data collection. Rudy King, Station Biometrician,

and Lakhdar Benkobi provided help with data analyses.

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USDA For. Serv. Res. Pap. RMRS-RP-38. 2003 1

Introduction

The swift fox (Vulpes velox) is currently considered

endangered in the states of Nebraska and Kansas and

is listed as a threatened species in South Dakota. This

fox, once abundant, was distributed from southeastern

Alberta southward into the Texas Panhandle and from

the Rocky Mountains eastward to the western edge of

the tallgrass prairie (Scott-Brown et al. 1987). Population declines began with the settlement of the Great

Plains and by 1900 the species was rare. There were

no reported sightings of swift fox from Nebraska between 1901 and 1953, from North Dakota between

1915 and 1970, or from South Dakota between 1914

and 1966. An increase in the number of sightings from

South Dakota since 1966 suggested that a population

was becoming reestablished (Hillman and Sharps

1978). However, more recent surveys show that swift

fox populations are decreasing.

Great Plains habitats of the swift fox have been generally described as gently rolling prairies dominated

by grasses, particularly wheatgrasses (Agropyron and

Pascopyrum spp.), blue grama (Bouteloua gracilis),

and buffalograss (Buchloe dactyloides) (Kilgore 1969,

Boggis 1977, Hillman and Sharps 1978, Hines and Case

1991). Hines and Case (1991) have been the only authors to report home range selection. Investigation of

swift fox use of the various habitat components within

home range, except for den site selection, has been limited. Vegetation and physical characteristics of den sites

have been described by Cutter (1958), Hines and Case

(1991), and Uresk and Sharps (1986).

The objectives of this study were to examine selection of den and foraging sites based on vegetation characteristics including cover and vegetation heightdensity (visual obstruction reading, VOR). The null hypothesis tested was that there were no differences

between available sites (randomly selected) and those

used by swift fox for denning and foraging.

Study Area

The study area was on lands administered by the

USDA Forest Service’s Buffalo Gap National Grassland in southwestern South Dakota. It included 4,144

ha of a mixed-grass prairie habitat dominated by western wheatgrass (Pascopyrum smithii) and blue grama

and encompassed several cattle allotments grazed in

either season-long or rotation grazing systems. Topography was gently rolling upland straddling the divide

between Mule and Horsehead Creeks. Small, first

order drainages flowed northwest toward the former

or southeast to the latter.

October 1 to September 30 precipitation was 50.5

and 41.2 cm during the two-year study. The 83-year

average for annual precipitation is 45.0 cm. Mean

monthly temperatures for both years averaged between

17o

 and 22o C during June-September.

Methods

Capturing and Monitoring Foxes

Swift foxes were trapped during June and July during 1991 and 1992. Tomahawk traps were set and baited

with canned mackerel near known den sites and on 10

selected 1.6 km transects spaced 1.6–3.2 km apart along

secondary roads used for management of the grasslands. Seven traps were placed surrounding each den

site approximately 10 m from the den. Ten evenly

spaced traps were placed on transects along the two

track roads. Trapped foxes were fitted with a radio with

an external antenna attached to a 2.5 cm wide collar.

Radiocollared foxes were released at the capture site

and were relocated at night with handheld antennas.

We used vehicle-mounted or handheld spotlights to note

behaviors (foraging, running, etc.) and marked locations on USGS 1:25,000 contour maps and on the

ground with steel posts. Only den sites and locations

where foraging behavior was noted were used in these

analyses. Since swift foxes are primarily nocturnal,

observations were conducted after sunset. Foxes were

tracked a minimum of twice per week during the summer months (June-September). One observation on a

given night per animal was used for a location but not

all animals were located each night.

Measuring Vegetation

Vegetation measurements were taken at den sites,

foraging sites, and on randomly selected sites. Random sites were selected by computer-generated random numbers (Universal Transverse Mercator coordinates). Canopy cover of major plants by individual

species and by total grasses, forbs, shrubs, and total

two-dimensional cover was estimated on 30, 0.1 m2

plots placed at 1-m intervals along a 30-m transect

(Daubenmire 1959). Two 30-m transects were sampled

at each den, foraging, and random site location. Those

at den sites were perpendicular, crossing at the center

of the main den entrance, while those at animal foraging and random sites were parallel, 30-m apart. Data
2 USDA For. Serv. Res. Pap. RMRS-RP-38. 2003

were collected on foraging and random sites within

one week of locating. On each site, a modified Robel

pole (Robel et al. 1970, Benkobi et al. 2000) with

2-cm alternating orange and white bands was placed

at 5-m intervals for a total of 13 stations along each of

two 60 meter transects. Four readings at each station

were taken at 90o points around the pole. Visual obstruction readings were taken and recorded at the first

ring above visual obstruction of the vegetation (heightdensity of vegetation).

Visual obstruction data and canopy cover data were

reduced to one mean per site for den, foraging, and

randomly selected sites. Mean visual obstruction of

random sites was compared with den and foraging sites

using the Mann-Whitney U test as an alternative to a

t-test in this situation to address non-normally distributed data. Canopy cover for seven variables (western

wheatgrass, blue grama, buffalograss, green

needlegrass [Nassella viridula], sideoats grama

[Bouteloua curtipendula], prairie Junegrass [Koelaria

macrantha], and scarlet globemallow [Sphaeralcea

coccinea]) for random, den, and foraging sites were

compared with Multi-Response Permutation Procedures (Mielke, et al. 1976, Mielke et al. 1982,

Zimmerman et al. 1985). A Bonferroni correction was

applied to the probabilities. Significance levels were

estimated using exact methods to address the small

number of dens (SPSS 1998). Biological differences

were considered at a =0.10.

Results and Discussion

A total of 14 swift foxes was captured: 6 in 1991

and an additional 8 in 1992. Twice-weekly tracking

and surveying of the foxes resulted in the discovery of

3 dens in 1991 and 14 dens in 1992. Each den was

used by an individual family unit. In 1991, 20 foraging areas were identified; an additional 62 foraging sites

were located in 1992; and 81 random sites were

sampled over the 2-year period. Years were examined

separately between and among den, foraging, and random sites but data were too limited in 1991 to reflect

differences. When years were combined, the results

were consistent with 1992 data. Canopy cover data

from 1991 and 1992 were combined for an overall

analysis and differences were detected between foraging and den sites (p=0.055). No differences were found

in comparisons of canopy cover between den and random sites or between foraging and random sites. The

null hypothesis was accepted for plant canopy cover

but was rejected for visual obstruction.

Vegetation

Percent canopy cover for seven major plant species

was evaluated to determine differences among denning,

foraging, and random sites for the swift fox (table 1).

Major species in order of decreasing cover were: western wheatgrass, blue grama, buffalograss, green

needlegrass, sideoats grama, junegrass, and scarlet

globemallow. The three treatments differed overall

(p=0.007), showing selectivity by the swift fox with

respect to availability and differences between den and

foraging sites. Characteristics of den sites were different from those of foraging sites (p=0.055). These sites

were primarily lower in western wheatgrass canopy

cover.

Den Sites

Previous papers describing swift fox habitats have

documented a variety of conditions where dens have

been found. These have included plowed fields, heavily

grazed areas, fence rows, on or near hilltops, and short

and mixed grass prairies (Cutter 1958, Kilgore 1969,

and Uresk and Sharps 1986). Some of these

macrohabitats may appear to lack cover (e.g., plowed

fields or heavily grazed areas). Our study, however,

focused on microhabitats (or at least a higher order of

habitat definition [see Johnson 1980]) and showed that

some vegetative attributes are important to these foxes.

Overall, vegetation around den sites differed (p=0.055)

from foraging sites (table 1). Den sites had less western wheatgrass canopy cover but were similar in cover

for the six other plant species and for total cover, total

forbs, and grass. However, there were no differences

in plants overall between den and random sites

(p=0.105). Visual obstruction of vegetation was greater

(p=0.08) at den sites than on randomly selected sites

(11.7+1.4 and 9.5+0.6 cm, respectively). Height-density of total vegetation, measured in terms of visual

obstruction, does influence use of sites for both denning and foraging (figure 1). Our results, that vegetation surrounding the den is more dense, may relate to

use of screening cover for burrow entrances or places

where foxes could loiter outside the den with greater

security. The magnitude of the differences (2.2 cm) in

height-density is important and represents the difference that completely obstructs view; maximum vegetation height would be greater. Hiding cover would

be significantly augmented by total vegetation height.

The presence of screening cover around den sites

has not been discussed in previous studies but may have

been present since even heavily grazed pastures may
USDA For. Serv. Res. Pap. RMRS-RP-38. 2003 3

when covered by plowing or discing. He concluded

that these dens were temporary.

Foraging Sites

Multiresponse permutation procedures detected no

differences in canopy cover of the seven plant species

between foraging and random sites (p=0.114, table 1).

Generally, canopy cover of the individual plant species and for the major categories was also similar between foraging and random sites. Swift fox did use

foraging areas with more dense vegetation than was

present on random sites; VOR=11.9+0.7 cm and

9.5+0.6 cm, respectively (p=0.01).

Uresk and Sharps (1986) studied swift fox food habits in other areas of western South Dakota and found

that black-tailed prairie dogs (Cynomys ludovicianus),

which are characteristic of early seral stages, and insects, primarily grasshoppers and beetles, were the most

common food items eaten. Prairie dogs did not occur

on our study area and voles (Microtus spp.) and northern pocket gophers (Thomomys) are found in areas of

more dense vegetation in the mixed-grass prairie. Insects were abundant in the diets of the swift fox in our

study (unpublished data). Cottontail (Sylvilagus spp.),

pocket mice (Perognathus spp.), and thirteen-lined

ground squirrel (Spermophilus tridecemlineatus) were

also common in the swift fox diets. These species are

generally found in less dense vegetation.

Table 1. Means and standard errors of percent canopy cover (Daubenmire 1959) for

key plant species on swift fox study area sites.

Denninga Foragingb Randomc

Western wheatgrass 27.0±2.7 38.8±0.7 35.8±1.8

Sideoats grama 3.4±1.6 2.6±0.7 5.1±1.2

Blue grama 19.0±3.1 15.2±1.3 14.9±1.5

Buffalograss 16.4±3.5 17.0±1.6 11.2±1.4

Junegrass 2.9±0.9 3.5±0.6 2.5±0.4

Scarlet globemallow 0.7±0.2 1.5±0.1 0.9±0.1

Green needlegrass 3.4±1.0 5.3+0.7 4.2±0.5

Total coverd 79.0±2.6 80.5±1.3 77.5±1.6

Total forbsd 4.6±0.6 5.6±0.4 5.7±0.7

Total grassd 73.7±3.9 77.0±1.4 73.8±1.6

a n=17

b n=82

c n=81

d Two-dimensional percent cover

Figure 1. Mean visual obstruction readings of denning,

foraging, and random sites. Different letters represent

significant differences between means (p=0.10).

contain patches of ungrazed or lightly grazed, less palatable species. The exception would be dens found in

plowed fields (Cutter 1958, Kilgore 1969). Kilgore

considered dens in plowed fields a function of the percentage of land under cultivation in his study area.

He noted that the dens were more shallow and less

extensive than other dens and only rarely reopened
4 USDA For. Serv. Res. Pap. RMRS-RP-38. 2003

Swift fox are adapted to a wide range of habitat conditions. Feeding habits are cosmopolitan and focus on

those animal and plant species that are available

(Kilgore 1969, Boggis 1977, Hines and Case 1991,

Uresk and Sharps 1986). Foraging relationships and

site characteristics such as height-density of vegetation (VOR) and plant species canopy cover may reflect local availability of potential prey items during a

particular season. Longer-term studies throughout the

range of the swift fox are needed to determine the effects of variation in yearly and seasonal precipitation

and temperature and prey base on microhabitat selection or use. Information on the intensity and duration

of livestock grazing as related to vegetation structure

and plant composition, and impacts to selectivity of

den and foraging sites of swift fox, are also needed.

Management Implications

Management for swift fox in this area will require

vegetation with greater visual obstruction than that

which occurs randomly throughout the summer

months. We recommend visual obstruction readings of

11–12 cm for den sites and foraging sites. Maintaining

visual obstruction readings of 11–12 cm may require

lighter grazing use by livestock.

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Protocol for monitoring standing crop of extensive grasslands

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Cutter, W. L. 1958. Denning of the swift fox in northern Texas.

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Daubenmire, R. 1959. A canopy-coverage method of vegetation

analysis. Northwest Science. 33: 43–64.

Hillman, C. N.; Sharps, J. C. 1978. Return of swift fox to northern

Great Plains. Proceedings South Dakota Academy of Sciences

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Hines, T. D.; Case, R. M. 1991. Diet, home range, movements,

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