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Chorus frogs are singing. Even after cold nights that leave a skin of ice on the breeding ponds you can hear this lovely sound of spring as the sun begins to do its warming work. The songs of chorus frogs--to be precise, our local animals are western chorus frogs--are like the rasping noise made by somebody dragging a thumbnail across the teeth of a comb. This is a generalized description. According to Conant and Collins's A Field Guide to Reptiles and Amphibians, the "sound may be roughly imitated by running a finger over approximately the last 20 of the small teeth of a good-quality pocket comb, rubbing the shortest teeth last." Your cheap pocket combs are not going to fool anybody.

Chorus frogs could not produce the sound in that fashion because they have neither nails nor claws. The absence of claws or nails is one of the defining characteristics of amphibians in general. Chorus frogs belong to the tree-frog family, and like other members of that group, their toes end in adhesive pads, which are useful for hanging on to the trunks and branches of trees and shrubs. These pads are not very well developed in chorus frogs, which fits their habits well. They do climb some, but they generally stay on or very near the ground.

Their principal habitat in ages past was undoubtedly the prairie, a place that offers few opportunities for climbing, but they have been able to move into farm fields and even suburban neighborhoods where pesticides are not used too heavily. You can hear chorus frogs in the city at North Park Village and in various parks and cemeteries. They're the most likely frogs to hang on in an urban environment, though how potential mates can hear the songs of the males over all the traffic noise is beyond me.

The classic image of the singing frog has it sitting on a lily pad, and indeed chorus frogs will do just that. They also sit on other kinds of floating vegetation or stay hidden in clumps of grass or sedge in very shallow water. I have spent a lot of time standing absolutely still listening to chorus frogs singing just a few feet away. But I have almost never seen one, unless I flushed it and caused it to hop into deeper water. They are cryptically colored--dull grays and browns variously striped and spotted with darker shades--and a big one is only about 1.5 inches long, so they are easy to miss. They usually respond to disturbances such as the approach of heavily booted humans by shutting up. A whole pond full of them can fall silent in an instant. It is as if their chorus had a conductor giving them signals.

Male frogs sing to attract females, and females seem to respond strongly to large choruses. A single chorus frog in a tiny pond may get no response at all. A massed chorus of a large number of males apparently signals listening females that the pond is a really great place to have a family. The massed chorus also makes it impossible to tell where one frog's song leaves off and another's begins. Imagine 50 people with good-quality pocket combs all simultaneously scraping their thumbnails along the last 20 teeth.

The image of a frog on a lily pad leads many people to believe that frogs live in and around water all the time. But only some do that. Bullfrogs, for example, live in permanent bodies of water. Many frogs--and all toads--come to the water only to breed. They may favor moist habitats at other times, but they don't require standing water. Some frogs have adapted to desert conditions. In the southwest, desert frogs breed during the brief period of summer rain. When things get too dry they burrow into the ground and estivate. Estivation is like hibernation, but the animals do it in the summer instead of the winter.

Vernal ponds are vital to the success of chorus frogs. Low spots that fill with water in late fall and early winter and stay wet until midsummer give amphibians enough time to mate and lay eggs and give the eggs enough time to hatch and the tadpoles enough time to grow legs, lose their tails, and move out into the larger world. The frogs could use permanent bodies of water as breeding sites, but permanent bodies of water tend to have fish in them, and fish are very fond of frog eggs and tadpoles.

Last Sunday I was hearing chorus frogs in three vernal ponds at Somme Woods in Northbrook. All of these ponds are likely to be dry by the Fourth of July. I also saw a couple of great blue herons passing over. Their arrival from the south is nicely timed to coincide with the emergence of frogs--and they hunt quite happily in vernal ponds.

Primeval Chicago must have been a paradise for frogs. It was practically nothing but vernal ponds. People probably couldn't sleep for all the frog noise. Of course as settlers began to pour in, pond after pond was drained. With the development of field tile in the late 19th century, every landowner could become his own hydraulic engineer, converting wet prairies, sedge meadows, and pothole marshes into cornfields--and wiping out population after population of frogs and toads.

You often hear complaints these days about how the government is forcing landowners to preserve tiny wetlands, the implication being that vernal ponds that cover only a couple of acres can't be of much use. But the frogs I heard last Sunday were singing in ponds of less than an acre. One was only a few thousand square feet. The value of these tiny wetlands is considerable, and it increases if the small wetlands are part of a complex of scattered wetlands. Frogs are mobile enough to get from one of these wetlands to another, and so are turtles. A couple of years ago a female Blanding's turtle with a small radio transmitter attached to her carapace was tracked wandering north from Bluff Spring Preserve, a Lake County forest preserve north of Zion, into the Chiwaukee Prairie in Wisconsin. The animal covered about half a mile from one marshy pond to another, and in the process became a threatened species. Blanding's turtle is considered "threatened" in Wisconsin but not in Illinois.

I was most pleased to hear chorus frogs singing in Oak Pond at Somme Woods. The first time I visited Somme, nearly 20 years ago, trees were growing right out of the middle of the pond. Their shade was so dense that nothing grew under them. The pond was a mud hole practically devoid of life. There were no frogs in those days.

The restoration work that has been going on at Somme since that first visit has made a dramatic difference. The trees were girdled. That killed them, but it let sunshine reach the water. Now the pond is covered with greenery throughout the summer. Smartweeds and pickerelweeds, bulrushes and wild irises, and many other wetland plants live in the shallow water. Dragonflies hunt for insects. Mallards and wood ducks rest on the waters in spring, and on some of my visits I have scared up a green-backed heron, which was probably hunting for chorus frogs. Oak Pond is nowhere near large enough to support a green-backed heron, but as part of a cluster of small wetlands it can make its contribution.

Those ducks may be responsible for bringing in some of the wetland vegetation. Seeds can cling to mud on a duck's foot and hitch a ride from one pond to another. Plants and animals adapted to life in scattered habitats often develop methods of getting around. At a time when most stories about amphibians tell of declines and disappearances, one can take some comfort in the arrival of frogs at a pond where they could not previously live.

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