If you are ever lucky enough to spot a swift fox, don’t blink, as they will quickly vanish into the vast expanse of the prairie.
Since January I have been conducting a survey on the Chico Basin Ranch looking for active swift fox dens. In particular, I was trying to locate possible natal dens where foxes would later give birth and raise their young.
Then in April, I found a rather interesting burrow about 300 yards from the main ranch road. Dirt had been newly excavated from the tunnel of what appeared to be a badger den, but the dirt was spread out in a 20-foot long tailing. That puzzled me at first, never having seen or heard of a tailing that long. I studied the trough and pattern of the way the dirt was spread out in scalloped paw marks.
Identifying the occupant of a den site (e.g., fox or badger) based on the size and shape of the burrow and the habitat is not conclusive. Other signs such as claw marks, tracks, scat, and debris are important clues. I did find some tracks, recent prairie dog and pocket gopher skulls as well as the upper and lower jaw of a swift fox in the tailings at this site.
This was the particular case that led me to realize I could often tell if the occupant was a badger, coyote, or a fox by closely examining the excavated tailings. Badgers and coyotes don’t worry about concealing their den tailings and will leave the tailings in a mound. And badgers don’t leave carrion by a burrow entrance; they consume their prey underground. On the other hand, swift foxes will typically spread out the tailings to at least some degree to avoid leaving a mound. They do this to better conceal the den from predators, coyotes in particular.
Even though it looked like an old badger den, I determined the newly dug tailing was done by a swift fox. To confirm this I quickly placed a game camera on the site. A few nights later I was watching a video of a female fox, a vixen. She was digging another den entrance with a shallow trench. She diligently, even obsessively, spread the dirt out flat in a long flaring arc. From what I could tell, most foxes don’t go to such a great length to spread out the dirt. Yet a number of fox dens have no tailing at all. Dirt spread out in this manner has likely been dispersed with the wind and the rain. This den now had three entrances, and as such was likely to be a natal den. I called this den site “Long Tailing.”
A vixen dispersing the soil at the site of the “Long Tailing” den.
After a week, I started watching the den before sunrise lying on the prairie next to a cholla cactus about 100 feet away. I saw a fox leave and then return to the den around 6:30 AM. I confirmed over a couple of weeks that there was a pair of foxes.
And then the foxes disappeared. A week went by, there was still no sign of them. They had just vanished, but why? Where did they go? They did not appear to be bothered by my presence nor by the game camera over the first two weeks. I hoped I had not disturbed them, but that is always a risk with field biology. As is often the case with red foxes, they probably had more than one den. If the vixen had kits at all, they were at a different site.
I have learned just how elusive swift foxes can be. They are almost exclusively nocturnal, live in underground dens year around, usually hunting after dusk and in the pre-dawn hours. They are incredibly fast, quiet, and stealthy, blending into their surroundings. Yet my biggest challenge in studying them was their tendency to change dens frequently. Swift foxes have several dens so that if pursued by a coyote they can escape. Swift foxes may move dens for several reasons — the family outgrows the den; the den is soiled; they are disturbed, perhaps by coyotes; they decide to move closer to new hunting grounds; or it is simply time to move. This behavior probably evolved in part to proactively elude predators.
Several weeks had passed. I was scouting about half a mile south of the Long Tailing site, when I found a den with a male fox sleeping outside the entrance. I watched him until he woke up and went hunting. He later returned and crawled into the den. After watching him for a couple of days, I saw a female fox near the den and followed her south as she hunted. I soon located two other dens, both with multiple entrances and long tailings and trenching similar in construction to Long Tailings. One was perhaps eighteen feet long. Based on the similar construction and proximity, I called it “Long Tailings II.” I figured these were at least related foxes, if not the same ones.
The next couple of mornings I managed to briefly observe a fox leaving to hunt. I was excited by the prospect of finding them again. Then one afternoon a few days later, an intense thunderstorm swept across the prairie, a wind squall hit followed by a driving rain that flooded the roads and landscape. When I returned in the days that followed, it seemed the foxes had vanished with the storm. I had lost them again.
Some time later, one day in August, I rechecked the Long Tailings II site, and I noticed tracks entering and exiting the den. The foxes had finally returned, if only for a while.