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Dispersal Patterns of Red Foxes Relative to Population Density

Study Areas and Methods

Field work was conducted in Barnes, Kidder, and Stutsman counties (3-county area): about half of the 3-county area was in the Drift Plain and half in the Missouri Coteau physiographic regions of North Dakota (Sargeant et al. 1984). This part of North Dakota is intensively farmed for small grain and cattle production. The climate is characterized by hot summers and cold winters (Stewart and Kantrud 1972). The wild canid community during our study was almost exclusively red foxes; only 1 coyote (Canis latrans) and no coyote dens were observed. Six 93.2-km2 townships were selected (2 in each county) for intensive study (Sargeant et al. 1984). The 2 most eastern townships were in the Drift Plain physiographic region and the 4 most western townships were in the Missouri Coteau physiographic region. A 4-lane east-west interstate highway (I-94) bisects the 3-county area, and in each county, 1 study township was north and one was south of I-94. The 6 study townships were located nearly equal distances from I-94. The most distant study township boundary was 19.2 km from the highway. Additional description of the study area was provided by Sargeant et al. (1975).

We captured, tagged, and released red fox pups at rearing dens (nearly all during mid-May-Jun). Dens on the 6 study townships were located by systematic aerial searches (Sargeant et al. 1975) and by interviewing landowners. We located dens opportunistically off the study townships in the 3-county area during irregular aerial searches and by contacting landowners. We captured pups with nets after chasing them from dens with a wire ferret (Storm and Dauphin 1965) or by hand during den excavation. Dens were excavated in a manner that allowed at least partial restoration. One numbered tag similar to that described by Storm et al. (1976) was placed in each ear of every pup. Pups were handled so as to minimize stress and injury. Nearly all pups were released back into their rearing den. At landowner request, a few pups were transplanted to rearing dens occupied by other red foxes. We considered transplanted pups to have originated from the den into which they were released. Andrews et al. (1973) found that recovery rates for transplanted pups were not significantly different and recovery distances were not markedly different from those of pups released in their natal ranges.

We alerted fur harvesters and fur buyers throughout North Dakota to the presence of tagged foxes and asked them to notify the Game and Fish Department of recoveries. Because dispersal of young red foxes begins about 1 October annually (Storm et al. 1976, Pils and Martin 1978, Tullar and Berchielli 1980), we classified foxes recovered before 30 September of their first year of life as pups, and those recovered after that date as adults (Trewhella et al. 1988). Straight-line distances between release and recovery locations were determined for foxes whose recovery locations were known. Foxes recovered ≥8 km from their release site were considered to have dispersed following the criteria of Storm et al. (1976). Dispersal directions (direction of recovery from release site) were: north (316-45), east (46-135), south (136-225), and west (226-315). Numbers of fox families on each intensive study township were determined each spring by systematic aerial searches (Sargeant et al. 1975) and averaged annually to reflect overall populations in the 3-county area (Allen et al. 1974, Allen and Sargeant 1975). The average number of fox families/km2 for the 6 study townships was our measurement of fox density. We defined a study year as the period April through March (e.g., 1969 refers to 1 Apr 1969-31 Mar 1970).

We used a 2-way ANOVA to compare recovery distances among age-classes and between sexes. A natural log transformation, ln(y + 1), was used to normalize recovery distances. Orthogonal polynomials (Snedecor and Cochran 1980) were used to test recovery distances for linear and quadratic trends across age-classes. We compared the pairwise distribution of angular differences in dispersal directions of littermates recovered as adults with the pairwise distribution of angular differences in dispersal directions of nonlittermates recovered as adults. This analysis reduced effects of potential biases associated with uneven harvest and tag reporting rates and with the tendency for all foxes to disperse in the same general direction.

We used Chi-square analysis to examine differences in recovery directions of all foxes tagged and released as pups and recovered as adults, and to examine effects of I-94 on dispersal directions of pups tagged and released within 19.2 km of I-94 and recovered as adults. We restricted this analysis to foxes tagged within 19.2 km of I-94 because most foxes were tagged in the study townships. We tested for differences in dispersal directions associated with the 2 physiographic regions among foxes tagged north and south of I-94. We used Pearson correlation coefficients to assess the strength of the relationships between the annual fox population density and the annual percentage of foxes that dispersed and were recovered their first year.

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