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The last light of the day was an orange blaze seen through the filigree of leafless branches of the forest. The air was still and cool without a bug to be slapped. Four fearless “froggers,” my group descended into the gloaming of the oak and maple woods at Lee and Rose Warner Nature Center in Marine on St. Croix to conduct a frog survey.

As a citizen-science project, we were there to collect data on what species of frogs are calling, in search of mates, at several spots along a trail called listening posts. We were also tasked with estimating the number of that species at each post — frogs being a bellwether species to watch in this time of climate change.

We found them to be amazing animals.

Frogs’ bodies and their strategies to adapt to dramatic temperature changes are fascinating. In winter, unlike turtles, aquatic frogs such as the green frog need to be near oxygen-rich water to keep their metabolism going, so they squiggle into mud at the bottoms of ponds or lakes. Terrestrial frogs, such as the wood frog, secure themselves in deep crevices of rocks or wood, or simply burrow under layers of leaves or snuggle into topsoil. Still, they freeze, but their metabolism has infused muscle, blood and joint with glucose, a sugary antifreeze, to cradle them in their sleep. And toads, aka diggers, are able to drill down below the frost line.

We froggers were here in early April after the thaw, when frogs and toads revive from their suspended animation, to listen to their songs that give life to future generations.

Structured approach

The survey has a simple protocol modeled after the system used by the state Department of Natural Resources’ Frog and Toad Calling Survey. It requires a shift from eyes to ears. As we followed a trail with several key posts, we listened, identifying frogs by their call and estimating their number by the collective intensity of their calls.

For one minute, we stood quietly. Apparently, the frogs can sense visitors and will stop calling until they are comfortable with their presence. Then, a timekeeper gave us a nod to begin active listening for five minutes. Our ears scanned for the calls of distinctive frogs. At time’s end, we shared what frog was heard, confirm the frog call, discuss the sound intensity, and settle on number designation: 1, for intermittent, singular frogs with no overlapping calls; 2, for singular but overlapping calls; and a 3, for continuous, overlapping calls.

Of course, nature is not going to yield its mysteries easily to a simple survey. To complicate things, at each post, several different frogs might present their calls at the same time. Intermingled among a torrent of sound from western chorus frogs, a person might hear only the banjo twang of one green frog. Frogs also are sensitive to weather. They don’t “present” when winds are more than 13 mph, or when it is raining or snowing and cold.

Unlike me, the other volunteers in my team were seasoned frog surveyors with ears keen to the voices found in the bogs, ponds, and lakes of the hilly and forested surrounds. They knew the frog species likely to be out: western chorus frog, spring peeper, wood frog, and, perhaps, an early northern leopard frog.

However much I wanted to carry my weight and not be too much of a student, I knew I was in for a learning experience that would take me beyond the hardness of fact and into a deep appreciation of how huge the natural world is when discerned by the sounds of the forest at night.

Listening posts

On this evening, we heard at our first listening post only the call of a few distinct spring peeper frogs, their voices intruded upon by a dog barking in the distance, an airplane overhead, the silence-ripping roar of a motorcycle, each defiant against the soft enclosing night. And then, as if to heal the wounded silence, our own voices, sibilant as if in a holy place, entered the data.

At the second listening post, we stood mute, ears tuned to the frogs typical for the time: western chorus frogs sounding like the thumbing of a comb; spring peepers, well, peeping; and wood frogs, sounding like rattled mallards. But, alas, at this stop, we noted only a few plaintive peepers.

At our next stop, a vernal pond, the sound of peepers was deafening. We all agreed on a number 3, and ambled on to the next post with the feel of harmonic sound still pressing in our ears.

We felt honored by discovering other beings are alive in the dark. A beaver tail smacked the water of the lake, painted turtles plopped into the water off limbs of fallen trees, coyotes howled and yapped. Someone appeared to hear a barely audible trill of an American toad on the far side of the lake. A flashlight beam briefly caught a small snake slithering through the leaves. A pair of sandhill cranes bugled during their descent into a meadow for the night. A surprised turkey exploded out of an oak treetop just above us.

Around 11 p.m., the teams met back at the center. We summarily listed the data and put the data charts in a box. The conversation became animated as the members unpacked their experiences.

After listening to the frogs over the years for four or five surveys (April through July), surveyors can name them by their call; and by their calls, their numbers; and by their numbers, how much we can hope to hear them next spring.

Over the summer, the calls will trail off as the frogs prepare their stores to see them through winter. Their body chemistry converts their protein diet of insects into their antifreeze, glucose, which will sustain them in their subnivean slumbers.

Through our winter wait, the thought of hearing all these voices again lightens our hearts with the expectation of them surviving a snowbound world to prevail in song during gentler days of spring and summer.

There, in Marine on St. Croix on an April night, to collect data from the natural world, we found that the world had gathered us in.

Then we went home and crawled into bed. Sleep came easily.

Don Wendel is a Minnesota master naturalist and a St. Croix master watershed steward. Currently, he is volunteering at Lee and Rose Warner Nature Center as a research assistant, studying the effect of water quality on the plant life of Bernie’s Bog.