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Causes and Rates of Mortality of Swift Foxes in Western Kansas

Results and Discussion

We radiocollared 41 adult and 24 juvenile swift foxes: 10 adult females and 9 adult males and 10 juvenile females and 8 juvenile males in the Cropland Area, and 9 adult females and 13 adult males and 2 juvenile females and 4 juvenile males in the Rangeland Area. We estimated from tooth wear that 2 females and 2 males in the Cropland Area and 2 females and 3 males in the Rangeland Area were 1 year old; other captured adults were ≥2 years old. One adult female captured in the Cropland Area was not included in analyses because of radio failure the day following initial capture. We also suspected radio failure when we lost contact with 2 adult male foxes in the Cropland Area after 214 days and 52 days of monitoring, and 1 juvenile male fox in the Rangeland Area after 123 days of monitoring. Our sample included 8,919 exposure days for all foxes.

Mortality rates were 0.55 (11-month period) for adult and 0.67 (6-month period) for juvenile swift foxes (Table 1). Our annual mortality rate estimate for adult swift foxes was 0.57. Highest rates of mortality overall were attributed to predation for both adults (0.44) and juveniles (0.42). There were no differences in mortality rates between study areas for adults (P = 0.52) or juveniles (P = 0.17). We did not observe differences in mortality rates between sexes for adults (P = 0.99) or juveniles (P = 0.11). We observed seasonal variation in adult mortality rates (P < 0.01; spring = 0.33 0.08 [Mean of X SE]; summer = 0.22 0.08; fall = 0.05 0.05; winter = 0.10 0.07).

Eighteen adult swift foxes died during the study: 13 by predation, 4 by poisoning, and 1 undetermined (Table 1). Fourteen juvenile swift foxes died during the study: 7 by predation, 4 by vehicle collisions, 1 by agricultural activities, and 2 undetermined. Additionally, we discovered 5 dead juveniles (not radiocollared) belonging to radiocollared adults: 3 deaths caused by vehicle collision and 2 deaths caused by agricultural activities (these juveniles were not included in mortality rate estimation).

Using evidence at sites of mortality and necropsy reports, we determined that coyotes were responsible for all 13 adult and 7 juvenile swift fox depredations, based on evidence at sites of mortality and necropsy reports. Determinations were further supported by results of predator surveys and daily observations: coyotes were the only medium-sized carnivore detected on each study area (M. A. Sovada and C. C. Roy, unpublished data). Coyotes and coyote tracks were observed throughout both study areas. We had no evidence of red foxes or bobcats in either study area, American badgers were uncommon, and dogs were only observed at occupied farmsteads. Avian predators such as great horned owls or golden eagles were not present or very rare. Only 1 depredated swift fox appeared to have been eaten, as part of its skull was missing. Four adult and 2 juvenile swift foxes that had been depredated were cached; the others were found aboveground. Most depredated foxes had puncture wounds in the skull, scapula, neck, and thoracic area where ribs usually were broken. Several had damage to skin and bones on rear legs, suggesting coyotes initially may have captured them by a hind leg.

Table 1. Estimated mortality rates a, total exposure days (Days), number of mortalities observed (m), and cause-specific mortality rates of swift foxes by age group and by Cropland and Rangeland study areasb in western Kansas from March 1996 through January 1997.
          Cause-specific mortality rates
Total observed Mortality rates Predation Vehicle collision Other
Days m Mean of X SE Mean of X SE Mean of X SE Mean of X SE

Adults 6,873 18 0.55 0.08c 0.44 0.09 0.0   0.18 0.06
   Cropland 3,865 7 0.47 0.12 0.47 0.12 0.0   0.0  
   Rangeland 3,008 11 0.65 0.10d 0.43 0.08 0.0   0.35 0.12
Juveniles 2,046 14 0.67 0.08 0.42 0.11 0.23 0.10 0.17 0.09
   Cropland 1,737 9 0.59 0.10 0.31 0.12 0.29 0.11 0.06 0.06
   Rangeland 309 5 0.87 0.09 0.64 0.16 0.0   0.36 0.17

aKaplan-Meier mortality estimates from staggered entry design (Pollock et al., 1989).
bCropland study area composed of approximately 76% cultivation; Rangeland study area composed of approximately 87% pasture.
cIf poisoned animals not included, mortality rate = 0.46 0.08.
dIf poisoned animals not included, mortality rate = 0.47 0.12.

We had a sufficient sample of radio locations for 9 of the 13 depredated adults to permit home range estimation (M. A. Sovada and C. C. Roy, unpublished data) and examination of depredation site with respect to home range and den locations. Eight of 9 depredation sites were located outside or bordered the 95% minimum convex polygon estimate of home range. Sites of depredation for adults killed by coyotes were always >1 km from dens the swift foxes had most recently occupied.

Four adult swift foxes died of organophosphate toxicosis. The pesticide Thimet, also known as Phorate, was detected in lethal concentrations (23.5 - 58.9 ppm; Smith 1987) in stomach contents of 4 Rangeland Area foxes . Thimet is an organophosphorus insecticide and acaricide, and is registered for use on several types of crops, but primarily corn. Foxes with organophosphate toxicosis were found dead within 4 km of each other. Three of the 4 were found aboveground, with no visible sign of injury or struggle. The fourth fox was alive when discovered but was unable to walk and was salivating excessively.

In the Rangeland Area, a den containing 4 juvenile foxes was partially collapsed on 13 September, and the soil was packed during construction of a field terrace in cropland. We opened the collapsed den and retrieved 1 radiocollared and 2 unmarked juveniles that had been crushed and killed, and 1 radiocollared juvenile that was not injured. The uninjured fox was monitored for an additional 10 weeks until its transmitter failed.

Vehicle collisions had a greater influence on mortality rates of juveniles (0.23) than adults (0.00). All 7 juveniles that died as a result of vehicle collisions were in the Cropland Area, and all vehicle casualties occurred between 29 August and 9 October.

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