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Causes and Rates of Mortality of Swift Foxes in Western Kansas

Methods


We used live traps (81 25 30 cm) to capture adult swift foxes beginning in March 1996. Traps were modified to provide a smaller (2.5 2.5 cm) mesh, thus reducing chance of injury to captured swift foxes. Traps first were placed within 3 randomly selected 10-km2 blocks in each study area; blocks were separated from each other and study area boundaries by at least 1.6 km. Trapping effort expanded outward from the 10-km2 blocks as capture of unmarked swift foxes from within the block subsided. Our goal was to radiocollar 25 adult swift foxes in each study area. We began trapping juvenile foxes in August 1996 near dens of radiocollared adults. Numbered eartags and a radiocollar (approx 39 g; Advanced Telemetry Systems, Isanti, Minnesota, USA) containing a mortality sensor detectable up to 3.2 km were attached to each swift fox. Captured swift foxes were sexed and incisor wear of adult foxes was appraised to index age groups (Rongstad et al. 1989. Ecology of swift fox on the Pinon Canyon Maneuver Site, Colorado. Unpublished final report to the U.S. Army, Fort Carson, Colorado, USA).

We used vehicle-mounted tracking equipment to monitor radiocollared swift foxes from March 1996 through January 1997. We monitored foxes for a minimum of 4 nights every 2 weeks on each study area. Monitoring never occurred on consecutive nights on a study area. We monitored foxes in 2 6-hr periods, approximately 1900-0100 and 0100-0700, with some variation depending on day length and weather events. We sampled the 2 6-hr periods an equal number of times in each 2-week period. We also attempted to obtain daytime locations approximately every 3 days, not coinciding with nighttime monitoring.

We determined cause of mortality for each radiocollared swift fox, using physical evidence at the site, necropsy results, and when appropriate, results from a veterinarian pathologist's examination. Physical evidence included presence of tracks of other species and location of carcass (e.g., cached). We used diagnostic puncture wounds and condition of the carcass as described by Disney and Spiegel (1992) to indicate possible mortality factors. If cause of death could not be determined at the site, carcasses were frozen and examined later.

We identified potential mammalian predators of swift fox present on the 2 study areas from predator surveys (scent-station surveys, track surveys, spotlighting surveys) conducted for a concurrent study (M. A. Sovada and C. C. Roy, unpublished data). Surveys were conducted 5 times each at regular intervals from March through early October. We conducted scent-post surveys (Linhart and Knowlton 1975, Roughton and Sweeny 1982) and spotlighting surveys (Frederickson 1979, O'Farrell 1987), using standard methods along 5 8-km sections of roads located throughout each study area. Track surveys were conducted as described by Sargeant et al. (1993) on 20 randomly selected quarter sections in each study area. Additionally, we kept daily records of any mammalian or avian predators seen in the study areas.

We estimated mortality rates for radiocollared swift foxes via the Kaplan-Meier estimation technique in a staggered entry design (Pollock et al. 1989). We report adult mortality rate for an 11-month period (Mar 1996 -- Jan 1997), and juvenile mortality rate for a 6-month period (Aug 1996 -- Jan 1997). For comparison to other studies, we calculated an annual mortality rate by subtracting from 1 the product of the 11-month survival rate and the average of winter monthly survival rates. The average of winter monthly survival rates was used as an estimate of the February survival rate. We computed mortality rates due to 3 specific causes: predation, vehicle collision, and other, which included poisoning, farming activities, and unknown causes. Observations of surviving foxes were censored (Lee 1992) on the date their collars were removed at the completion of the study. We assumed the status (dead or alive) of each individual was known for each day, which was generally true because we were monitoring animals systematically. The day of capture was considered the first day an animal was at risk. Exposure-day was defined as each day an individual swift fox was monitored and exposed to risk of mortality. We assumed a radiocollared swift fox was equally likely to be found whether it was dead or alive.

We compared swift fox mortality rates between sexes, study areas, and seasons (spring = Mar-May; summer = Jun-Aug; fall = Sep-Nov; winter = Dec-Jan), using CATMOD (SAS Institute 1988) as described by White and Garrott (1990:250). We examined the sites of mortality relative to the locations within the home range of the fox (minimum convex polygon method [Mohr 1947]; M. A. Sovada and C. C. Roy, unpublished data) and the distance away from dens.


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