When the time of year and conditions are right, the sound is nearly impossible to miss.
Biologists describe the melodic call of boreal chorus frogs as something like running a fingernail along a fine toothed-comb.
On spring days near shallow Northwoods wetlands, their chorus drowns out most other sounds.
“Chorus frogs are some of the first ones that we hear in the spring,” said Rhinelander-based DNR District Ecologist Carly Lapin, “as soon as it starts getting warm and sunny and the ponds start melting.”
Ephemeral ponds form during spring snowmelt and rains, providing a perfect habitat for chorus frogs. A frog-seeker might hear hundreds of them at a time without ever seeing one.
“They’re found in grassy wetlands and typically will call from clumps of grass on the edge of the wetlands,” Lapin said. “They’re really difficult to find. You hear them, typically, without seeing them.”
The turning of spring triggers chorus frogs to begin calling to attract a mate.
The time of year, however, is but one factor contributing to calling and intensity, according to Emma Brinley Buckley.
Brinley Buckley is a researcher currently employed by the National Park Service. In 2020, while working at the University of Nebraska-Kearney, she published a study highlighting the other conditions necessary for peak chorus frog calling.
“We found that temperature was an important factor,” Brinley Buckley said. “Boreal chorus frogs increased their calling drastically when temperatures were above about five degrees Celsius, or 41 degrees Fahrenheit.”
Using audio monitors at wetland sites in Nebraska, Brinley Buckley’s team also found the amount and timing of water inundation in the wetlands correlated to calling activity.
“If a wetland is too temporary, it risks drying before the larvae can metamorphose,” she said. “On the other hand, more permanent waters will increase predation from fish or bullfrogs.”
In other words, the loudest calling is a result of nature hitting a sweet spot – the right time, the right temperature, and the right amount of water.
In the Wisconsin spring, the calls chorus frogs share the spotlight with another common and rhythmic frog song.
Spring peepers are found “anywhere where there’s forest and wetland edges,” said DNR herpetologist Andrew Badje.
Badje calls Wisconsin frogs like spring peepers and chorus frogs “indicator species.”
“They’re kind of like a canary in a coal mine. They’re a good gauge of what’s actually happening in these wetland and upland systems,” Badje said.
Frogs are particularly sensitive to changes in things like water quality, habitat, and climate. Indeed, a warming climate is likely having an impact on when listeners hear their first frogs in the spring.
“Spring arrival is getting more erratic. Things are definitely getting warmer. I think, probably, in the long haul, frogs, on average, are starting to call earlier,” Badje said.
Among Wisconsin frogs, spring peepers and chorus frogs are likely to be pretty resilient to continued climate change.
But mink frogs, which can be heard in northern Wisconsin, are already near the southern extent of their range.
“What we think is going to happen is that range is slowly going to shift northward,” Badje said. “That’s really the species that we’re most concerned about losing.”
Using the small sample of a tennis-court-sized pond near Rhinelander’s airport, frog diversity in the Northwoods remains impressive.
After catching two spring peepers on a sunny May afternoon, my two-year-old son Rhys and I later returned to catch a tiny wood frog, a brightly spotted northern leopard frog, and a softball-sized American bullfrog.
But chorus frogs?
We still only heard them, their calls a reminder of how time, temperature, and water levels were aligned just right to produce their natural symphony.
“Yes, there’s a phenological timing to things, and we can expect it in the spring,” Brinley Buckley said. “But is that changing? Is that habitat, that local habitat changing? All of those factors coming together have to hit that sweet spot for this to occur where you’re hearing it.”
Carly Lapin is a WXPR Board Member. The Board of Directors is responsible for management and operation of White Pine Community Broadcasting, Inc. and has no influence over programming.
