Following the thread: Searching for turtle nests


Nest – Ben Stegenga
A gravid female Spotted Turtle.

This spring we were posed with a challenging question.  How do you go about finding Spotted Turtle (Clemmys guttata) nests when there is very little information available about where or when they are going to lay eggs?  Our first step towards answering this question was to capture female turtles and attach radio transmitters to their shells.  This made locating each turtle fairly straightforward after the initial capture.  By mid-April, we had attached transmitters to 17 female turtles at two sites in southern Georgia.  We located each turtle every few days, weighing and palpating them on each occasion to look for signs that turtles were gravid.  Palpating (feeling for eggs) female Spotted Turtles can be tricky because of their small size and tendency to not stick out their back legs.  With enough patience, it is usually possible to get both pinkies into the gaps between the bridge of the shell and the back legs.  Eggs feel like hard lumps in the body cavity, although it took some trial and error to figure out exactly what a gravid Spotted Turtle felt like.

Spotted Turtle with radio transmitter and threadspool attached.- Houston

By the second week of May, a majority of the turtles had been steadily gaining weight for a couple weeks, and we started to feel what felt like eggs in some turtles.  However, just feeling eggs does not present a good estimate of clutch size, which is a key metric when studying reproduction.  The best method to get a count of the number of eggs is to x-ray turtles after the eggs are shelled inside the female.  Thus, we gathered up all of the female turtles and transported them to our friends at the Georgia Sea Turtle Center for x-rays.  It turns out that our suspicions had been correct and 12 of the 17 female turtles were gravid, with clutch sizes ranging from 1–3 and an average of 2.3 eggs per gravid female.

Threadspool leading to a Spotted Turtle nest.- Ben Stegenga

With knowledge that a majority of the turtles in the study were gravid, it was time to come up with a plan for identifying nesting locations.  Other researchers have had success tracking turtles at night and catching them in the act, but the distribution of turtles and layout of our study sites made this option impractical.  The other primary method used to track animal movements around an environment for a short period of time is via threadspooling.  Cocoon thread bobbins can be attached to the back of a turtle’s shell and the end of the thread tied to a small tree or branch.  As the turtle moves through the environment, the thread will unwind and get stuck on various objects, marking the path that the turtle has traveled since being released.  Ideally, the thread would get buried during a nesting event making it easy to identify nest sites.

After experimenting on a couple turtles, we were pleased with the early results of the technique, and it seemed like we would be able to follow the threads to nesting locations.  We settled into a routine of daily checks on each threadspooled turtle, weighing turtles to document any decreases in weight from one day to the next (Spotted Turtles lose approximately 12% of their body mass during nesting).  After the first week, all we had to show for our efforts was one nest that we could not locate despite the female being threadspooled and a bunch of still gravid turtles that seemed to have little on their mind other than foraging (and constantly running out their threadspools).

As it turns out, it took almost two weeks for all 12 gravid turtles to nest.  We successfully located seven of the nests — the first to be scientifically documented in Georgia!  Uncovering the first nest was one of the most exciting field experiences I have ever had.  A majority of these nests were well-hidden and difficult to find even with the turtles attached to threadspools.  Eggs were buried in dirt and leaf litter on raised areas or edge habitat on the periphery of the wetland.  Many females moved in excess of 200m to nesting habitat and then immediately returned to their normal home ranges after nesting.  Further evidence that these small turtles have incredible spatial awareness.

Over the next two months, we will monitor all seven nests to estimate the number of eggs that survive through hatching and identify how many nests are raided by predators.  During the time that we spent searching for this initial group of nests, 3 of the 5 female turtles that were initially not gravid have developed eggs, increasing the total percentage of monitored females reproducing to almost 90%.  Later this month we expect many of the same turtles to produce a second clutch of eggs, effectively doubling their potential reproductive output this year.  A second clutch will mean more threadspooling and the opportunity to locate additional nests.  By the end of the summer, we will have dramatically increased our knowledge of Spotted Turtles at the southern end of their range.