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Gray tree frogs prefer to call under cover of darkness on warm nights. Early in spring they will call during the daytime, too, when temperatures rise above 60 degrees. Have you heard their vibrant trill? Photo by Larry Stone.

Brilliant white trilliums nodded in the breeze and black flies chased me through the woods. My knobby tires crunched over sand and rocks on the Hatchery Creek mountain bike trail near Hayward. From the dense, green tree canopy overhead, dozens of warblers, vireos, flycatchers, and scarlet tanagers belted out their beautiful (though aggressive) songs.
Curving along a contoured trail cut precariously into the hillside, I caught a glimpse of sparkling water out of the corner of my eye. A small wetland thrummed with life at the toe of the slope. Through the cacophony of birds, a new sound filtered into my consciousness. Somewhat bird-like, these short trills seemed to add to the heat and humidity of the morning. The first of the gray tree frogs had made their entrance in the symphony of spring!
Gray tree frogs freeze solid over the winter, just like wood frogs and spring peepers. They spend the winter as “frogsicles,” with over 80% of their body frozen, and breathing and heart beat suspended. Although they thaw out early in spring—at the same time as peepers and wood frogs—gray tree frogs need to build up their energy reserves before they can start the strenuous business of attracting females. In contrast, the other two frogs take only a few days to recover from hibernation and start singing.
As with most frogs and toads, females choose a mate based on the length and strength of the male’s call, as well as the quality of his territory. Therefore, it is worthwhile to a male frog to put a lot of energy into his in calling. Gray tree frogs spend most of the night shouting aerobically at about 60% of their maximum output. But when a female is near, they bump it up to near 100% for a short time.
In order to accomplish these athletic feats of song, male frogs and toads have highly developed body-trunk muscles. Packed with mitochondria, the powerhouses of the cells, the singing muscles have the capacity for high aerobic metabolism. Frogs and toads call for such a long time that their muscles must switch from burning carbs to burning fats, just like human endurance athletes. (I can only hope that my mountain bike ride is long enough to start burning some fat!)
Those muscles are used to drive air over the vocal chords, producing the surprisingly loud calls. Some frogs and toads can be as loud as a lawn mower. Luckily, they have an internal pressure system that keeps their own ear drums from vibrating excessively and therefore prevents hearing loss in the shouter himself. In contrast, the silent female frogs and toads have much less body muscle. Their specialization, after all, is quietly laying eggs.
The most intense frog choruses occur on warm, cloudy nights, from dusk to midnight. But the air temperature needs to be at least 60 degrees F for a tree frog to call, so the early singers often call during the warmer daylight hours instead.
As you might imagine, loudly calling tree frogs can be vulnerable to predators. That’s one reason they prefer to sing at night, under cover of darkness. Males stay hidden in thick plants next to the shallow ponds where they prefer to breed. Gray-green, bark-like patterns on their skin make excellent camouflage.  A tree branch overhanging the water is the perfect stage.
Large, moist toe pads covered in mucous glands create enough surface tension to support the frog’s body mass as it climbs a tree—or your window. Climbing not only allows them to access safe singing stages and avoid predators all summer, it is also their main mode of hunting. Gray tree frogs search for tasty insects, larvae, mites, spiders, and snails in the understory of wooded areas. They need a lot of food to fuel their spring chorus, and will even cannibalize a smaller tree frog if it fits in their mouth!
If a predator—like any number of birds, snakes, other frogs, and small mammals—goes after a tree frog, it will leap away, revealing bright yellow-orange skin on its inner thighs. The flash of aposematic coloration may startle the predator and allow the frog to escape.
Pedaling off down the trail in my own flash of color (neon yellow!), I didn’t stop to search for one of the well-camouflaged singers. The bugs would have eaten me alive. Away from the wetland, bird songs dominated the airwaves again. But the frogs were not forgotten.
Later that evening, after washing mud spatters off my calves and fixing a flat tire, I sat down with my phenology journal to record the day’s new seasonal sightings. Tree frogs topped the list, in company with least flycatchers, scarlet tanagers, and American toads. Closing the book and turning out the lamp, I let the noises of the house fade away. Then—through the open window—streamed the bird-like trill of a tree frog, singing near my neighborhood pond. The symphony of spring continues.

For over 45 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new phenology exhibit: “Nature’s Calendar: Signs of the Seasons” is now open.

Find us on the web at www.cablemuseum.org to learn more about our exhibits and programs. Discover us on Facebook, or at our blogspot, http://cablemuseumnaturalconnections.blogspot.com.