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Frogs Are Courting & Vocalizing in May

Apr 29, 2024 ● By Sheryl DeVore
Bullfrog on water lily leaf.

Bullfrog on water lily leaf. Photo by Steven D. Bailey.

On a typical May day near wetlands, and especially at night, we may hear the constant whir of American toads, the rattling comb sound of chorus frogs or even the low “varoom” of a bullfrog, all vying for a mate. It’s a key time for volunteers to foray into the night to document the presence of some of the 13 species of frogs, ranging from an inch to 8 inches long, that live in northeastern Illinois. This research is helping scientists understand what frogs need to survive and their role in our local wetland ecosystems.

Spring peepers call mainly at night and sound like tiny sleigh bells. “When there is a full chorus, their decimal level can hurt your ears,” says Susan Lawrence, co-steward of reptile and amphibian monitors for the Morton Arboretum. It’s amazing, she says, because “the sound is coming from animals about the size of the tip of your thumb.”

Spring peeper with throat sac inflated. Photo by Reni Akande.

Frogs have vocal sacs that expand and deflate like a balloon to create their unique calls, made only by males.

As with other amphibians, frogs spend part of their lives on land and the other in water, making them particularly important to ecosystems. “They help transfer energy and nutrients back and forth from breeding ponds to uplands,” explains Allison Sacerdote-Velat, curator of biology and herpetology and vice president of conservation research at the Chicago Academy of Sciences/Peggy Notebaert Nature Museum.

“Frogs have permeable skin which can dry out in the heat of the sun. That’s why they restrict their activities to night or times of low light,” continues Sacerdote-Velat, who runs the Calling Frog Survey in the Chicago region. The permeable skin also makes frogs, as well as other amphibians, susceptible to changes in water and land regimes as well as climate. For example, Sacerdote-Velat says, “Frogs breathe and drink through their skins, absorbing not only water and air, but also salt spread in winter that ends up in their wetland habitats.” 

More than 150 frog species have gone extinct in the past 25 years, she says, explaining that “as wetlands are destroyed and habitat becomes more fragmented, that puts the frogs closer together, and in turn, there’s more completion, predation and disease.”

Illinois has lost 85 percent of its historic wetlands. Increased droughts and flooding also contribute to the loss of habitat for frogs.

Some of the region’s smallest frogs, measuring up to about 1.5 inches long at the most, become active in early spring as vernal pools are formed when snow melts. These are fishless wetlands where the frogs have a better chance of successfully reproducing, Sacerdote-Velat says. These include the rare wood frog, which has already mated and stopped singing by May, as well as the chorus frog and spring peepers, which start singing in March or April and can often be heard in May. Another small frog, the gray treefrog, starts singing in May into June. These frog species mate in ponds, lay eggs and then retreat to dry land to hunt for insects and other delicacies during summer. 

Two young northern green frogs. Photo by Sheryl DeVore.

Three larger frog species—including the northern green frog, about 2 to 4 inches long, and the bullfrog, which can grow to roughly 8 inches in length—use permanent ponds and are more active later in the season, but they also can be heard singing in May.

Recently, biologists have noticed a decline in the number of cricket frog populations in the Chicago region. In the 1960s, it was the most common amphibian in the state, but today it has nearly disappeared from the northern third of Illinois. The decline could be due to the buildup of pesticides and heavy metals in the region, according to the Illinois Department of Natural Resources. A survey of museum specimens collected in Illinois over the past 100 years showed that more frogs from the Chicago region displayed signs of hormonal imbalances compared with the rest of the state.

The most commonly heard frog in spring is the chorus frog, whose calling sounds as if someone were running a finger over a comb. It’s one of few small frog species in the region that can be heard during the day even when it’s sunny.

“Last year, there was a drought early in spring and it seemed there weren’t very many chorus frogs singing,” Sacerdote-Velat says. “Then we had rain in July and the chorus frogs actually started singing again. It’s interesting to wonder if they are adapting their breeding cycles to more frequent droughts.” 

Less common and usually only heard at night in appropriate habitat are the spring peepers. They have adhesive toe pads allowing them to climb up trees, she says. Individually, a spring peeper sounds like a high-pitched single “peep” on two different pitches. When many sing together, they sound like sleigh bells.

Northern leopard frog. Photo by U.S. Fish and Wildlife Service.

Northern leopard frogs are much less vocal, singing typically in April through early May. Careful observers may be lucky enough to actually see one during the day. This frog species is about 3 to 4 inches long and has large, dark ovals all over its greenish back.

A common amphibian song around small bodies of water in May and even in some urban areas is the whirring of American toads. “Adult toads spend a lot of time hopping around in the woods, and you can find them away from water, especially as the season heads into summer,” Sacerdote-Velat says. The American toad has one of the longest and most melodic calls of frogs in the region. It’s a fast, mysterious-sounding high-pitched trill, according to Sacerdote-Velat.

Two species of gray treefrogs live in the region, which look identical but are genetically distinct.

Gray treefrogs may hang around in suburban yards where there is appropriate habitat. “If you leave a garage light on, a gray treefrog might climb to the light to feed on moths and insects attracted to the light,” Sacerdote-Velat says. “They have a slow musical trill, which gets faster as it warms up.”

Bullfrog in a pond. Photo by Steven D. Bailey.

Two species of frogs that live in permanent ponds call in May, June and into summer. The green frog sounds like a banjo being plucked and the bullfrog sings “varoom, varoom, varoom.”

If one sees these frog species poking their heads out of the water, they’ll look very similar. The bullfrog has a ridge around its eardrum, while the green frog has a ridge that extends all the way down its back.

Volunteer frog monitors survey a particular route at least three different periods from March through June, so they can record species that sing at different times. In the past few years, more than 100 volunteers have observed and contributed their data. “This year, we have more than 200 monitors,” Sacerdote-Velat notes.

While earning a master’s degree at Northern Illinois University, Lawrence, who leads frog monitoring at the Morton Arboretum, used Calling Frog Survey data to identify habitat characteristics that promote spring peeper females to lay their eggs. In 2018, Lawrence, now an adjunct professor of biology at Aurora State University, analyzed 19 sites in Cook, DuPage, Kane, Lake and Will counties.

She surveyed sites where spring peepers had been documented as well as places where they were not. “We found that spring peepers persisted in ponds with a more open canopy,” Lawrence relays. “Spring peepers lay eggs singly instead of in masses like other frog species; therefore, they need plenty of submergent vegetation for egg laying.” Like other frog species, spring peepers can lay up to 900 or more eggs per season.

“Land-management decisions are taking amphibians into consideration,” Lawrence says. Biologists are girdling some trees around ponds so that the trees won’t soak up all the water the frogs need. In addition, land managers are removing invasive species in wetlands and uplands and doing prescribed fires in ways that don’t harm the amphibians.

Frogs have been on Earth for at least 200 million years, living alongside dinosaurs. Frog experts in the Chicago area hope their research and that of volunteers can keep these important species thriving.

Sheryl DeVore has written six books on science, health and nature, as well as nature, health and environment stories for national and regional publications. Read more at