Use of Radio Tracking to Improve the Estimation by Track Counts of the Relative Abundance of Red Fox

1. What they did

Counting the trails that animals leave where they cross a transect is a common way to estimate the relative abundance of carnivores. These counts usually assume that each animal crosses the transect at a steady daily rate. The authors set out to measure how often red foxes actually cross a transect, and how the way foxes move and where the transect lies affect the count.

• Radio tracking. In the Doñana Biological Reserve, southwestern Spain, they caught four adult male foxes with neck snares and padded steel leg-hold traps, fitted them with radio transmitters and released them where they were caught. They then followed them for seven continuous 24-hour periods (four for one fox, one each for the other three), taking one bearing an hour, and mapped each day’s movements.
• Counting crossings. From the maps they counted how many times a day each fox crossed a straight 3.3 km fence that ran through all four home ranges.
• Track counts. Once a month, from December 1984 to February 1985, they counted fox trails crossing a 1.3 km stretch of the same fence. The day before each count, they cleared the route with a sweep mounted on rollers, or they waited three days after the last rain.
• Testing transect position. They averaged the hourly radio locations into a “typical” daily path of a fox, drew eight evenly spaced imaginary transects across it, and counted how often the path crossed each one.

2. What they found

• Each fox crossed the fence between zero and six times a day, on average 2.86 times (standard error 0.7). The foxes crossed about equally often in each direction, suggesting they were resident, and all crossings were at night.
• The track counts found 1 trail in December, 5 in January and 8 in February: 0.77, 3.85 and 6.15 trails per km.
• Dividing by the average daily crossing rate turned these into 0.27, 1.35 and 2.15 foxes per km.
• The number of crossings changed a lot depending on where the transect was placed across the typical daily path (Figure 1 shows between about 2 and 6 crossings on the imaginary transects, against 3 on the fence).

3. What they concluded

• How foxes move during the day and where the transect lies both strongly affect the results of track counts. This bias matters most when transects are not permanent or are imaginary lines, such as ski paths.
• Used as they usually are, track counts give only a rough measure of population trends. To do better, the method needs a large sample, knowledge of how often each animal crosses the transect, a cleared route to avoid counting old tracks again, and an assumption that animals are neither drawn to nor put off by the transect (for example a road or fence). This study focused on the crossing rate.
• Radio tracking can provide correction factors for each species, habitat and season, making track counts more reliable and helping to design better surveys.
• The authors acknowledge two weaknesses: only males were radio tracked (although males and females had about the same home range sizes and daily movements in the area, from unpublished data) and there were only a few track counts, used as an example.

4. How tracking was or was not used

The researchers counted fox trails crossing a fence transect on sand, after sweeping the route or waiting three days after rain so that only fresh trails were counted. These counts were then corrected with the daily crossing rates measured by radio tracking. The paper does not describe how fox trails were identified or told apart from those of other animals. No trailing was reported.

5. Tracker or observer certification

Not reported. The paper does not say who counted the trails or how experienced they were.

6. Other technologies, skills or innovations

• Radio transmitters working in the 151 megahertz (MHz) range (Biotrack, England), with one bearing an hour over continuous 24-hour periods.
• A sweep mounted on rollers to clear the census route before each count.
• Combining radio tracking with track counts to turn a count of trails into an estimate of foxes per km.

7. Implications for conservation (from the paper)

• Correction factors from radio tracking could improve track counts and survey design. This is especially relevant for rare or endangered species, where even small changes in numbers may need particular attention.