How Grandad Taught Me Radio Tracking

Grandad O'Donnell giving me an early lesson in wildlife and fisheries management.

When I was young and we went to my grandparents' house for Thanksgiving or Christmas, my Grandad would play a game with us called "Hot or Cold."  He'd hide a few coins around the living room while we waited upstairs, then call us down.  As we slowly worked our way around the room he'd give us the play-by-play, to indicate how close we were to the coins and whether we were going in the right direction.  "You're very cold," "You're getting warmer," and when we almost found the coins, "You're burning up; you're almost on fire!"  Radio tracking an animal works exactly the same way.

This weekend, I got to help Tiffany Sprague, an Arizona State University graduate student, radio track Mexican Garter Snakes.  The Mexican Garter Snake is a federally protected species, listed as Threatened under the Endangered Species Act.  It once occurred along many still and slow-moving water bodies in central and southern Arizona and New Mexico, but it has declined dramatically because of competition and predation by introduced species and the development and draining of wetlands.  Tiffany is trying to help this species by studying its fine-scale habitat preferences.  Knowing exactly what kind of habitat is best for this species will help land managers bring it back to the areas where it has been lost, once the non-native predators and competitors like sport fish, crayfish, and bullfrogs are under control.

This fish hatchery specializes on raising native non-game fish and on making habitat for the endangered Mexican Garter Snake.

A selection of various radio transmitters that are used on the snakes, some for larger snakes (red), and some for smaller snakes to be implanted (silver antennae) or attached externally (black antennae).

Since May, Tiffany, Kellie, and a crew of volunteers have been catching Mexican Garter Snakes and having radio transmitters surgically implanted in them.  They then released the snakes where they found them, and have been returning each weekend to monitor what the snakes are doing and especially which habitats they are using.  They use a handheld antenna to listen for pings from each snake's radio, with each snake on a different frequency.  The antennae are directional, so by rotating them back and forth, they can find which direction is "hot" or "cold:" which direction they need to go to find their target snake.

Tyler and Kellie radio tracking a snake near the hatchery.  "You're getting warmer. . . ."

Scott and Tiffany try to pinpoint the exact underground location of a brumating Mexican Garter Snake.  "You're almost on fire!"

Once we tracked down a snake, whether it was underground or not, we measured a bunch of habitat variables in the area of the snake, including slope, aspect, ground cover, temperature, wind speed, and others.  Then we randomly selected a nearby point so that Tiffany could compare the habitats the snakes chose to the ones they didn't.

Tiffany starts measuring the habitat where a snake was seen earlier in the day (pink flag), while Scott tries to determine where it has moved to.

Kellie used this one meter diameter section of garden hose to delineate the area around a brumating snake where she would measure the microhabitat.

By about 4:00, few snakes had been seen on the surface and those that were on the surface had quickly escaped into nearby holes or blackberry brambles.  I hadn't seen a snake yet, so we went back to measure the habitat at a spot where Scott thought the snake might still be visible.  This was a unique microhabitat, where the snake was in a small undercut bank at the edge of a dry pond.  Although it was similar to a burrow, it didn't go deep enough that the snake could actually escape view.  It was here that I saw my first Mexican Garter Snake, or at least saw its side.  Because they are an endangered species and because Tiffany was studying their behavior, we wanted to be certain not to disturb the snake too much or change its activity.  Scott gently peeled the grass back just enough that I could see the snake in its hole, and then placed it back, leaving the snake just as we had found it.

Scott pulls back the grass to reveal my lifer Mexican Garter Snake under the cut edge of the bank in a dried pond.

A slightly closer look at the endangered Mexican Garter Snake.

Scott and Tiffany measure the habitat in a 5-meter diameter circle around the snake using these PVC poles to mark distances from the snake at half-meter intervals.

This south-facing rocky slope was a particularly popular place for the snakes to spend the winter.  Each pink flag (at least 10 visible here) marks the location a snake was detected underground.

As the sun set and the sky got dark, most of our tracking was done for the day.  However, Tiffany is only tracking the snakes once a week, and wanted to do a bit more intensive tracking of a few select snakes to be sure here weekly data were representative of shorter time scales as well.  So each weekend, two snakes get monitored every 3 hours, 24 hours a day.  It was nearly dark when Kellie and Scott set out to track the two target snakes, and they'd be waking up every three hours through the night to check on them.  I, on the other hand, shared a great dinner with the crew and then got to head home to my own bed!

As the sky turned from blue to black, Kellie and Scott headed out to check on the movements of the two Mexican Garter Snakes that were the subject of fine-scale monitoring this weekend.