Most folks just hear the croaking of frogs. But for a group of keen-eared Maplewood volunteers, it’s the soundtrack of summer.

The citizen scientists hear as many as nine different varieties of frogs and toads with their own distinctive songs, ranging from a chorus of sleigh bells to the quacking of ducks and the pluckings of a banjo. They venture into Maplewood’s wetlands, listen and then report back their findings to the city’s Frog Monitoring Program.

This year, 13 volunteers listened for and documented frog calls at nearly 40 locations in the city. Frog-calling season starts in April with the high-pitched call of the spring peeper, and extends through July with the long trill of the American toad. The calls are intended to attract a mate.

The count, once part of a now-discontinued larger effort led by the Minnesota Department of Natural Resources, is one way to gauge the health of the city’s wetlands. When a wetland become polluted or destroyed, the amphibian population plunges, Maplewood Naturalist Carole Gernes said. In some parts of the country, affected frogs and toads have exhibited deformities such as extra limbs and missing eyes.

“They are the canaries in the coal mines. When you see these things, you know something is wrong” with the wetland, Gernes said.

The frog count — along with bluebird monitoring, open space and pollinator programs run through the Maplewood Nature Center — helps people connect and care for nature in small, everyday ways. You don’t have to portage a canoe in the Boundary Waters to experience Minnesota’s flora and fauna.

Before heading out in the field, the volunteers have done their homework memorizing all 14 distinctive frog calls for species found here in Minnesota, nine of which live in the Twin Cities. They even take an online quiz.

After donning reflective vests, volunteers head out 30 minutes after sunset, on clear nights with little to no wind. Once they arrive at their destination, Gernes said, “They just stand still and listen.”

For five whole minutes.

That’s the part that volunteer Daniel Sadoff enjoys the most.

“It’s an opportunity to be quiet and listen. It’s solitary,” said Sadoff, a veterinarian. “You go out at night and you feel like you are out in the middle of nowhere. Some of the places, you can see tens of thousands of fireflies. You hear owls calling. You might hear a coyote.”

Sadoff said he often hears several species of frogs calling at once, though he rarely spots them.

Gernes said they have tracked measurable changes in wetlands where the city has improved conditions and water quality.

“In Maplewood, we’ve done a lot to support and improve water quality,” she said.

That includes buffering wetlands and ponds from salt, sediment, grass clippings and pesticides that hurt amphibians and their tadpoles.

Gernes pointed to a small wetland near John Glenn Middle School, at County Road B and Hazelwood Street. Groundskeepers had been mowing all the way down to the water line. Grass clippings and other pollutants promoted algae growth, turning the wetland into a green, lifeless bowl.

Then crews stopped mowing to the water line and allowed prairie grasses and wildflowers to grow instead, acting as a natural buffer.

“Now this area is full of frogs,” Gernes said. “It’s a nice little spot to listen.”