Volunteers listen for chorus frogs, spring peepers, northern leopard frogs, American Toads and Eastern gray treefrogs. 

To identify them, listen for these sounds: The chorus frog sounds like running your finger over a fine-tooth comb; the spring peeper sounds like the "peep" of a baby bird; the northern leopard frog, like a rubbery, creaky door on a haunted house or a "long, rubbery snore with occasional chuckles"; the American toad, like a "long, musical trill," the gray treefrog a more "relaxed, laidback, musical, short trill." These frogs all call in the early spring, in April. 

Courtesy of the Iowa Department of Natural Resources.