Findings from the North Lincoln Street Amphibian Crossing Study

June 15, 2014   |   Brett Amy Thelen
Intern Alex Kirk checks the North Lincoln Street pitfall array for amphibians. (photo © Brett Amy Thelen)

A Closer Look at an Amphibian Crossing in Keene

This past spring, we received funding from the Robert and Patricia Switzer Foundation for a new conservation research initiative focused on our Salamander Crossing Brigades. As part of this project and in partnership with a postdoctoral research ecologist from UMASS-Amherst, we installed and monitored a pilot pitfall array – comprised of silt fencing and aluminum can “traps” – at our North Lincoln Street amphibian crossing in Keene. The goal was to begin to figure out how many wood frogs and spotted salamanders actually cross the road at that site each spring, and what percentage of them are helped along by the Crossing Brigades. Read on for highlights from this pilot study.

What We Thought Would Happen, and What Actually Happened

A map showing the approximate location of the pitfall array.

Approximate location and extent of the pitfall array at North Lincoln Street, Keene, NH.

Here is how the pitfall array was supposed to work: any amphibians who successfully migrated across North Lincoln Street would encounter the silt fence, walk along it looking for an opening, and eventually fall into a trap, where they would stay until they could be recorded and released on the other side of the fence.

Here is how the pitfall array actually worked: Rocky roadside fill prevented us from digging the fence in along the entire length of the crossing, leaving a gap through which some amphibians inevitably crossed without ever encountering our traps. The persistence of frozen ground along the road edge long after adjacent forest soils had thawed – a consequence of snow compaction caused by wintertime plowing – prevented us from installing the array in time for the first amphibian migration of the season. In addition, we have good reason to believe that, later in the season, after warmer temperatures had energized our cold-blooded friends, adult wood frogs were sprightly enough to simply hop out of the traps.

One of the goals of pilot studies like this one is to evaluate research methods, and we’ve clearly learned a lot about installing pitfall arrays along New England roads during the spring thaw!

Proceed with Caution

Because of the limitations imposed by the survey methods (see above), we must exercise extreme caution when interpreting our data.

The Numbers

Amphibians wait to be recorded and released from a pitfall trap. (photo © Brett Amy Thelen)

Amphibians wait to be recorded and released from a pitfall trap. Holes were drilled in the bottom of each trap for drainage, and each trap contained a sponge for moisture and a wooden dowel to serve as an escape “ladder” for small mammals. (photo © Brett Amy Thelen)

With that said, we did record 1,963 amphibians crossing or attempting to cross North Lincoln Street in the spring of 2014. This figure is certainly an underestimate of the total amphibian population utilizing North Lincoln Street (again, see above). It includes 263 road-killed amphibians – also likely an underestimate, as our volunteers were largely focused on saving living animals and the dead are scavenged and/or pulverized beyond recognition by passing cars rather quickly.

Approximately 90% of the recorded amphibians who successfully crossed North Lincoln Street this past spring were moved across by our Salamander Brigade volunteers. Surely, many more amphibians crossed on their own and were not captured in the traps (in other words: the total number of recorded amphibians is likely an underestimate of the actual population using the road, and therefore the percentage crossed by Salamander Brigadiers is likely an overestimate), but it appears that the Crossing Brigades may indeed be making a difference at North Lincoln Street!

The Need for Long-Term Data

A Salamander Brigadier looks with wonder at a spotted salamander on a Big Night. (photo © Brett Amy Thelen)

Over 20 volunteers helped move amphibians out of harm’s way at North Lincoln Street on “Big Nights” in 2014.
(photo © Brett Amy Thelen)

Remember that weather plays a major role: amphibian migrations are highly weather-dependent events, so the timing of the migration can vary greatly from year to year. In the spring of 2014, most of the season’s warm rains occurred before midnight, when volunteers were awake and therefore more likely to assist with the crossings. During other years, the warm rains that spur amphibian migrations may occur in the middle of the night, when many of our volunteers are sleeping; at those times, most of the amphibians who successfully cross the road likely do so without the assistance of the Salamander Brigades.

Because of this variability, it would take many years of data to understand the long-term effectiveness of our Crossing Brigade efforts.

Spotting Spot Patterns

A spotted salamander captured both on its inbound migration on 4/11/14 (left) and its outbound migration on 4/27/14 (right). (photos © Brett Amy Thelen)

This spotted salamander was captured on its inbound migration on 4/11/14 (left) and its outbound migration on 4/27/14 (right). Note the distinctive spot pattern on the animal’s head and upper back, which is unique to this individual salamander. (photos © Brett Amy Thelen)

As a side project, we photographed the spot patterns of spotted salamanders encountered at the North Lincoln Street crossing – both in the pitfall array and by Salamander Crossing Brigade volunteers – and were able to identify five individual salamanders on both their inbound and outbound migrations by their unique spot patterns.

Documenting spotted salamander spot patterns won’t provide the same level of in-depth data that you’d get from a pitfall array, but it does have the potential for providing meaningful information on year-to-year survival in spotted salamander populations that must cross roads to reach their breeding pools. It’s also way cool. We hope to expand this initiative to more amphibian road crossing sites in the spring of 2015.


Jian Giglia, Alex Kirk, Nikki Leeman, Mary Kate Sheridan, and Brad Timm help to install the pitfall array. (photo © Russ Cobb)

BIG THANKS to Jian Giglia, Alex Kirk, Nikki Leeman, Mary Kate Sheridan, and Brad Timm for helping to install the pitfall array! (photo © Russ Cobb)

Finally, a note of gratitude: this project would not have been possible without the support of the Robert and Patricia Switzer Foundation, the City of Keene, Keene State College, and the Department of Environmental Conservation at the University of Massachusetts-Amherst, or the dedication of numerous volunteers, who helped us install the array, remove the array, and carry thousands of amphibians across the road. In our book, these volunteers – and the hundreds more like them who take to the roads on dark, rainy nights each spring to move frogs, toads, and salamanders out of harm’s way – are nothing short of heroes.

Contact Us

For more information on this study or the Salamander Crossing Brigades, contact Brett Amy Thelen at (603) 358-2065 or by email.