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Frogs Are Courting

Mar 31, 2020 ● By Sheryl DeVore

Chorus frog Photo courtesy of Chicago Academy of Sciences.

Tiny Amphibians Sing to Woo Mates and Reveal Ecosystem Health

by Sheryl DeVore

Even when it’s only 45 degrees near the smallest body of water with a bit of ice still on top, the chorus frogs are singing. The song sounds like someone running a finger across a comb—and when they are in full force, typically in early April, they can be heard at least a half-mile away from their temporary wetland homes. The chorus frog is among 13 species ranging in size from one to eight inches long that live in the Chicago area and are being monitored by biologists and community scientists.

Frogs serve as indicator species, according to Allison Sacerdote-Velat, curator of herpetology at Chicago Academy of Sciences/Peggy Notebaert Nature Museum. “Because of their permeable skin, just like other amphibians, including salamanders, they’re very sensitive to water contamination,” she notes. “Since frogs are vocal at certain times of the year, it’s easy to monitor them.” Documenting their presence is one way for biologists to determine if habitat is improving or becoming degraded.

Chorus frogs, spring peepers and northern leopard frogs are typically heard between late February and the end of April. American toad, Fowler’s toad (rare) and gray tree frog are typically heard singing in May. Green frogs, bullfrogs and cricket frogs (very rare) sing from about late June to mid-July, although many overlap and some linger farther into summer. “The disappearance of ubiquitous cricket frogs from most areas in this region really was part of why we started documenting frogs,” Sacerdote-Velat says.

Over nearly two decades, they’ve documented the decline and return of spring peepers to certain areas. The calling frog survey began in 2000 and has been managed by the Chicago Academy of Sciences since 2014. Today, an average of about 120 community scientists monitor frogs at 100 sites in 10 counties in the Chicago region, including Lake, Cook, Will, DuPage, McHenry, Kane, DeKalb and Winnebago counties.

Frogs are a link in the middle of the food web. “The larval forms (tadpoles) are eating a lot of plant material and fungus,” Sacerdote-Velat says.  Juvenile and adult frogs eat invertebrates such as flies and soil arthropods. Adult frogs of the larger species such as the bullfrog eat small vertebrates, including other frogs.

Green frog in duckweed Photo courtesy of Chicago Academy of Sciences.

Frogs get eaten by larger vertebrates like wading birds and carnivores. “When you cut out the middle of the food web, it can create problems for ecosystems,” Sacerdote-Velat observes. Frogs have been on Earth for at least 200 million years and shared space with the dinosaurs, according to the American Museum of Natural History.

Frogs typically have excellent night vision. Their bulging eyes help them see in the front, at the sides and even a bit behind them. They were among the first land animals with vocal cords. Male frogs have vocal sacs—pouches of skin that fill with air. The noises attract females to mate with them and lay eggs in temporary, semi-permanent or permanent bodies of water, depending on the species.

American toad Photo courtesy of Chicago Academy of Sciences.

The first and most common frog to start singing in spring, sometimes even in late February, is the chorus frog, about 1.5 inches long and difficult to see, although easy to hear. The chorus frog, which is in the tree frog family, is about the size of a thumb, with an air sac that blows up to nearly half its size underneath the throat when singing, often at night or on cloudy days.

Mary Busch, who began monitoring frogs four years ago, says calling frogs means spring is coming. “At the end of winter, when there’s still snow on ground, we’re anticipating the first spring peepers and the chorus frogs,” says Busch, who works as volunteer program specialist for the Forest Preserve District of Cook County, and lives in Evanston. They monitor an hour after dusk when the frogs are most vocal. The temperature has to be above 45 degrees at night, and the wind needs to be less than 11 mph.

Frog monitors are required to have a permit and alert conservation officers to know when they will be monitoring. “When you’re out in the preserve after dark, it’s very exciting for people who like natural areas and are a little adventurous,” Busch says.

Bullfrog Photo courtesy of Chicago Academy of Sciences.

“We can hear the choruses as we enter the woods even before we get to our listening points.” As they get closer, the frogs go silent. “It’s exciting to stand still and listen until one frog starts it off again and the chorus grows and grows.”

Busch monitors at Somme Woods, in Northbrook, where she’s heard chorus frogs, spring peepers and northern leopard frogs. Spring peepers individually sing a high-pitched, “Peep. Peep.”  When they’re in full chorus, they’ve been said to sound like sleigh bells jingling. Spring peeper numbers, about the same size as a chorus frog, but with an X on their backs, have declined at some sites. The decline may be linked to forest canopy closure above their breeding ponds, reducing the amount of emergent vegetation in the pond.

Recent research by Susan Lawrence, of Northern Illinois University/Aurora University, used calling frog survey data to identify sites with and without spring peeper breeding activity across multiple counties. “Her research identified canopy openness and emergent vegetation as important variables associated with spring peeper breeding sites,” Sacerdote-Velat says.

Gray tree frog Photo by Steven D. Bailey

Busch and Lalainya Goldsberry, who works at the Chicago Academy of Sciences, have documented three of the 13 Chicago region species. She doesn’t expect to hear green frogs and bullfrogs because they require permanent bodies of water, which her site does not have. “The green frogs sound like they are plucking a banjo string. I love that. I was at Cranberry Slough, in the Palos Region on an ecology walk—oh my gosh, when you hear that banjo thing, it’s just so exciting. I love the green frog. That’s got to be my favorite,” says Busch.

Data shows that land management practices can positively affect the diversity of frog species. Sacerdote-Velat has been working with the Lake County Forest Preserve District to introduce the rare wood frog into areas where scientists are restoring oak woodlands. “In 2018, we documented 375 breeding adult wood frogs where there once were hardly any before the restoration,” she says.

Sheryl DeVore has written six books on science, health and nature. She also writes nature, health and environment stories for national and regional publications.


Learn About Frogs, Monitoring and Creating Ponds

Before becoming a frog monitor, volunteers undergo training in early winter. Workshops are held throughout the Chicago region so they can learn the frog calls, as well as how to set up a route, gather data and enter it into a database. They also need permits to enter forest preserves and other natural areas at night. For more information and to hear the calls of 13 frogs vocalize in the Chicago region, visit

For information on building a frog-friendly pond in the back yard, visit