November 1, 2021
Bird-voiced treefrogs have the unique capability of metachrosis (the ability to change color), therefore the best way to identify them is by their unique inner thigh flash of green coloring. Photo by Catherine Nguyen.

NGRREC Research Interns Hear the Call of the Swamps

By Jessica Mohlman, Brooke Prater, Catherine Nguyen, Dr. John Crawford

The swamps of southern Illinois are enchanting, often making visitors feel as if they have been transported to the most remote areas of the southeast. Within these beautiful remnant bald cypress (Taxodium distichum) and water tupelo tree (Nyssa aquatica) swamps lies a unique inhabitant, the bird-voiced treefrog (Hyla avivoca). The calls of the bird-voiced treefrog may perk up any birder’s ears, but those familiar with the species know that it is not a feathered friend making those rapid bird-like whistles. Rather it is a small, understudied state-threatened amphibian. 

Two researchers sit on a rocky platform for sign for the Shawnee National Forest. In the background is a forest to the left and a road to the right.
NGRREC interns Brooke Prater (left) and Catherine Nguyen (right) at the Shawnee National Forest. Photo by Jessica Mohlman.

Each summer the National Great Rivers Research and Education Center (NGRREC), located in East Alton, offers upper-level undergraduate students from across the nation the opportunity for hands-on experience in the world of ecological research. Over the summer of 2021, Brooke Prater and Catherine Nguyen had the opportunity to learn more about the state-threatened bird-voiced treefrog and its habitat. Their research will help researchers and wildlife managers better understand the small treefrog species with the bird-like whistle. 

Brooke Prater, Project Title: Tree Preferences of Treefrogs

My NGRREC research project focused on the tree species preferences of the state threatened bird-voiced treefrog in southern Illinois to determine if these preferences affected population persistence. My research was supported by my mentors Jessica Mohlman, Dr. John Crawford, Dr. Lyle Guyon, and NGRREC intern Catherine Nguyen.

A plastic PVC pipe trap for tree frogs is attached to a tree trunk. A small rectangular sensor is to the right of the pipe on the tree trunk. In the background is a clearing amongst the trees.
PVC-pipe traps used to capture treefrogs. The passive design of the traps allows the frogs to freely enter and leave the traps. The white box on the tree allows researchers to record the temperature, relative humidity and dew point of the swamp. Photo by Jessica Mohlman.

Bird-voiced treefrogs are habitat specialists, meaning they are only found in one specific habitat type—swamps. Other treefrog species are also found in these habitats, such as green treefrogs (Hyla cinerea) and gray treefrogs (Hyla versicolor and Hyla chrysoscelis). However, green treefrogs are limited to the southern quarter of the state and and gray treefrogs are more statewide in distribution. 

The goal of this study was to understand the habitat preferences of bird-voiced treefrogs in hopes of learning how to improve current conservation practices to benefit their persistence and success. I hypothesized that more bird-voiced treefrogs would be caught in higher quality sites and on water tupelo and bald cypress trees, which are tree species typically associated with higher quality swamps. I focused on tree species and size selection at 10 pre-selected sites in and around the Shawnee National Forest. The selected swamp sites varied in quality, ranging from fully degraded to partially degraded to pristine. Site quality was determined by flooding frequency, tree size and health, and tree species diversity. Each site I visited had a total of 50 passive PVC-pipe traps attached to trees for a total of 500 traps across the study. Treefrogs were attracted to these traps because they provide a moist and dark environment, and treefrogs are able to enter and leave the traps freely. At each trap, I measured the tree’s diameter at breast height (DBH) using a DBH tape. This measurement is useful in determining individual tree quality and age. Then, I measured tree height using an electric hypsometer, which uses a laser to calculate how tall the tree is. Finally, I identified the tree species using its leaves and bark. For every frog captured, the species and size of the tree was recorded. 

A researcher looks through a sensor as she stands wearing waders in a swamp.
Brook Prater measuring tree height using an electric hypsometer. Photo by Jessica Mohlman.

The fieldwork with this internship was physically challenging, and was conducted in all weather conditions, meaning there were some days I was catching frogs in heavy rains. Not to mention, the temperatures were always high and the insects relentless. However, the hard work was worth it as these findings will be useful to the future of bird-voiced treefrogs as the research continues over the next few years. Because of this research, I am more confident than ever that wildlife biology is the career I want to pursue. Every day during this internship was a challenge, which I found to be gratifying because I learned to push the limits of both my mind and my body. I would like to thank my team at NGRREC and all others I met along the way for this amazing opportunity. I have found my calling, and I am so excited to see what the future holds. 

View Brooke Prater’s symposium presentation at

Catherine Nguyen, Project Title: Population Estimates of Bird-Voiced Treefrogs

I had the great pleasure of working with my NGRREC mentors Jessica Mohlman and Dr. John Crawford, and intern Brooke Prater, in assisting with their ongoing research on the state threatened bird-voiced treefrog in southern Illinois. Bird-voiced treefrogs are primarily found within the remaining bottomland forest swamps in southern Illinois that are dominated by bald cypress and water tupelo trees. In Illinois, wetland habitat loss due to water impoundment and wetland drainage has resulted in the decline of bird-voiced treefrog populations. 

A close-up of a researcher examining a tree frog by gently stretching out its right leg.
Outside of their unique call, bird-voiced treefrogs can only be identified by a light green flash pattern found in their inner thighs. Photo by Jessica Mohlman.

To assess population sizes in the state, a capture-mark-recapture study is being conducted across 10 swamp sites from spring to fall for three years (2021–2023). Each site has 50 passive PVC-pipe traps attached to trees, meaning that the treefrogs can freely enter and leave the traps. During my time assisting with this project, I checked more than 500 traps for treefrogs each week for four weeks. Once we found a treefrog, I was able to observe and learn what information and measurements were gathered on each specimen. To identify any recaptures of treefrogs, each frog was given a uniquely numbered Passive Integrated Transponder (PIT) tag and then released back into the swamp. Their unique tag allows researchers to know if a frog is recaptured, which can provide information on growth and survival rates for this species. I have never worked so closely with treefrogs before, and I now know how to identify and determine the sex of three different species.

A researcher examines a white plastic PVC pipe trap for treefrogs attached to the trunk of a tree. In the background is a clearing in the trees.
Catherine Nguyen checking a PVC-pipe trap for treefrogs. Photo by Jessica Mohlman.

After collecting all our field data, I analyzed and graphed my results using software programs that are frequently used in the field of ecological research. I was familiar with using these software programs beforehand, but now I feel more confident in these skill sets and my ability to use them prior to graduate school. Going into this internship, I was most excited about the fieldwork, and it did not disappoint. The best part about this research was exploring the beautiful, protected swamps in Illinois that very few people get to see. Before this internship I didn’t know Illinois had swamps and I was enthralled by every gorgeous insect, salamander and cottonmouth snake (Agkistrodon piscivorus) we came across. During this internship I met a lot of wonderful like-minded people who are also passionate about the environment, and I received significant preparation for my future education and career path in wildlife ecology. Before this, I was firmly set on a career in marine biology, but this research opportunity has inspired me to rethink my career goals and I am now considering pursuing a career in herpetology instead. I am extremely grateful that my mentors and NGRREC provided me with this fantastic opportunity, for the people I met, and the new experiences I made along the way.

View Catherine Nguyen’s symposium presentation at

Conservation and Management Implications

A graphic including a photo of a tree frog sitting on a researchers hand. Text on the graphic includes, "For more information on the summer undergraduate internship opportunities at NGRREC, please visit"
Bird-voiced treefrog. Photo by Jessica Mohlman.

The overarching goal of the work Brooke Prater and Catherine Nguyen contributed to is to determine bird-voiced treefrog population sizes and demographics in the state of Illinois. The data collected from their work will help to inform both ongoing and future research and management actions regarding bird-voiced treefrog conservation in the state. This research is critical as bird-voiced treefrogs are understudied throughout their range, which adds to the difficulty of creating management and conservation action plans for this species. Moreover, the beautiful remnant bald cypress and tupelo tree swamps that bird-voiced treefrogs call home are disappearing due to habitat degradation and alteration by humans. We hope this study will provide a better understanding of bird-voiced treefrog populations and habitat use, and that we may also be able to help conserve and restore the rare and beautiful habitats they call home. 

Jessica Mohlman is an Assistant Scientist and Research Coordinator with NGRREC. Her research interests lie in creating innovative interdisciplinary ways to address wildlife management and conservation-related concerns. She grew up in northern Illinois and earned her B.S. at Northland College in Ashland, WI and M.S. at the University of Georgia in Athens, GA. 

Brooke Prater is a senior at Southern Illinois University Edwardsville (SIUE) pursing a Bachelor of Science in Ecology, Conservation, and Evolution. In the summer of 2021 she did research with NGRREC where she studied microhabitat selection of bird-voiced treefrogs in southern Illinois. After she graduates in May 2022, she plans to work towards a Master’s degree in wildlife biology.

Catherine Nguyen is a senior at the University of Wisconsin-Madison in Madison, WI pursuing a Bachelor of Science in Zoology with a certificate in Environmental Studies. This summer she was a research intern through NGRREC where she examined population estimates of bird-voiced treefrogs in southern Illinois. Moving forward from this research internship, she aims to obtain a Master’s degree and pursue careers in wildlife conservation research, particularly focused in ichthyology and herpetology.

Dr. John Crawford is a Terrestrial Wildlife Ecologist with NGRREC. A primary goal of his research is to conduct projects that answer important contemporary questions related to the conservation and management of lower vertebrates. He prefers to conduct research that bridges disciplines and combines current thinking and needs in multiple areas to approach and answer these questions. His primary research interests have been and continue to be on the effects of habitat alteration, habitat degradation, and global climate change on amphibian and reptile populations. He earned his B.S. from the University of Illinois (Urbana-Champaign), his M.S. from Illinois State University, and his Ph.D. from the University of Missouri (Columbia).


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