A Tiny Frog with a Bird-like Voice

The unique nature of the bald cypress and water tupelo swamps that are the hallmark of the Coastal Plain Natural Division also provide this area of extreme southern Illinois the distinction of providing habitat for a small but loud species of frog. In these swamps one encounters the northernmost part of the range for a frog that sounds more like a bird than a frog.

Hearing the distinct musical, bird-like sound of the call, many people ask: “What bird is making such a racket all night?” It is not a bird at all, but rather the bird-voiced tree frog. It is primarily found only in closed-canopy cypress and tupelo swamps and wet bottomland forests.

When the great swamps of the Coastal Plain Natural Division were altered, and drained, the resultant habitat loss included habitat for the bird-voiced tree frog. The loss of habitat and coinciding drop in population numbers have resulted in the bird-voiced tree frog being listed as an Illinois threatened species.

The bird-voiced tree frog is a small gray-brown or greenish tree frog averaging about 1 to 1.75 inches in length. It has a light spot under each eye. The inner concealed portions of its hind legs, or what we humans might consider “the groin” area, have a pale green to white coloring. The body color is gray, brown or green. The tiny little frogs have adhesive pads present on the tip of each toe. The tadpoles of the bird-voiced tree frog can be quite striking and differentiated from others due to their dark brown coloring decorated with red “saddles” and bronze stripes.

This frog is easily and often mistaken for the larger gray treefrog, which is similar in coloration, including a light square below the eye. The bird-voiced treefrog is most easily distinguished from its look-alike by the greenish-white flash of colors on the concealed portion of its hind leg. On the bird-voiced tree frog the inner hind leg colors are usually less obvious than on the gray treefrog. While most often observed as a mottled gray, like many treefrogs, it is capable of changing its coloration to brownish or green depending on temperature and activity.

The unique feature of this tiny frog is certainly its call. While often challenging to spot in the cypress and tupelo trees and swamps that make up the majority of the Coastal Plain Natural Division, its call is unmistakable.

A recent paper authored by southern Illinois resident John Palis theorizes that the populations may be starting to recolonize previously extirpated sites within the very narrow confines of their northernmost range. Palis wrote, “Since European settlement, southern Illinois has lost an estimated 95.9 percent of its bald cypress and mixed bald cypress-hardwood swamps. Bird-voiced tree frogs (Hyla avivoca)—thought to be swamp obligates—were detected at only 24 southern Illinois remnant swamps in the 1990s and were subsequently listed as state-threatened. I conducted systematic acoustic surveys for vocalizing male bird-voiced tree frogs during May and June of 2019 and 2020 and accrued supplementary observations from 2008 to 2017. My efforts yielded 32 new detections of bird-voiced tree frogs in three southern Illinois watersheds, including 14 swamp remnants, a stream channel and 17 human-made water bodies. As a result of habitat restoration, bird-voiced tree frogs appear to be recolonizing portions of their former Illinois range from which they were previously extirpated by forest clearing and wetland drainage.”

Additional research is currently under way through an ongoing State Wildlife Grant Project (T-129-R-1) directed at this species. Dr. John Crawford, Terrestrial Wildlife Ecologist with the National Great Rivers Research and Education Center, is one of the principal investigators. OutdoorIllinois Journal readers were previously treated to a look inside some of the research associated with that project in the November 2021 article, “Research Interns Hear the Call of the Swamps.”

One interesting aspect of Crawford’s research is exploring if the bird-voiced tree frog can be utilized as an indicator species to help determine the success of restoration efforts in the cypress and tupelo swamps and the overall success of restoration efforts under way in the Coastal Plain Natural Division.

For nature enthusiasts who would like an opportunity to hear or see this fascinating little amphibian, Scott Ballard, Illinois Department of Natural Resources (IDNR) Southern Region Endangered and Threatened Recovery Species Specialist/Herpetologist, suggests a visit to Heron Pond from mid-May through June.

“Usually, the bird-voiced tree frogs are most active during mid to late May,” Ballard noted. “Heron Pond is an excellent location where visitors may hear their distinct calls. While the tree frogs are stirring around, they can be seen on the cypress knees and tree trunks next to the boardwalk and sometimes on the boardwalk or boardwalk railings. Visitors should plan their visits as a night hike and listen closely for that unique call. Arriving a bit before dusk enables visitors to have a wonderful look at an outstanding example of the Coastal Plain Natural Division and its habitats.”

Ballard cautions visitors NOT to handle, touch or try to pick up the tiny frogs.

“These frogs are very susceptible to anything that might be on our skin,” explained Ballard. “When we touch tree frogs they can absorb chemicals from lotions, soaps or insect repellents on our skin. This can be very detrimental to the frogs.”

While the bird-voiced tree frog may be a tiny inhabitant of the Illinois Coastal Plain Natural Division, it has a huge role in helping us to understand the health of the ecosystem. We are lucky to have this primarily southern species here in the Prairie State.

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Gretchen Steele hails from Coulterville, Illinois. Steele is a freelance outdoor communicator. Her award-winning work appears as a regular columnist and contributing feature writer for Heartland Outdoors, Illinois Outdoor News and several Illinois newspapers. She enjoys spending her time afield as a volunteer for the Illinois Department of Natural Resources, Delta Waterfowl Foundation, Retrievers Unlimited and the Illinois Federation of Outdoor Resources. She is currently Vice President of Missouri Outdoor Communicators and a former board member of Association of Great Lakes Outdoor Writers.