Spring 1999

[TEXT ARCHIVE WEB-PUBLISHED MARCH 2002.
ORIGINAL PRINT PUBLICATION DATE: SPRING 1999.]

Western Chorus Frog: Audible but Seldom Seen

By Bill Glass

It's sprrreeeng! Let the chorus begin. The first sound most people will hear in the spring is a "prrreep" sound, much like someone running a fingernail over the small teeth of a pocket comb. This is the call of the western chorus frog (Pseudacris triseriata). Each "prrreep" call lasts one to two seconds. At first, only a few hardy individuals call, but as the days get warmer, large choruses can be heard from little ponds both day and night.

Western chorus frogs are often the first frogs to become active in the spring. Even when the nights are still quite cold with temperatures near freezing, these small hardy frogs can be active. In Chicago Wilderness, western chorus frogs are usually heard in early March with increasing numbers until they peak in April then diminish in May. Cold weather delays the calling and early warm weather causes them to call early in the season. These small creatures are probably among the most frequently heard frogs in the Chicago Wilderness region, but they're elusive and seldom seen.

Western chorus frogs are tiny, usually around an inch long (from their snout or nose to their vent or rear). They're gray or tan in color with dark stripes down the back and along the sides. They usually have a dark triangular area on the top of their heads right between the eyes. One consistent distinguishing characteristic is a light line along the upper lips. The females tend to be larger than the males, while the males tend to have dark throats during breeding season. These frogs are members of the tree frog family, which is characterized by adhesive toe pads, allowing them to climb. But this species has small, poorly developed pads, so they aren't as agile at climbing as many tree frogs, although they can and do climb.

Western chorus frogs are true prairie frogs. Originally, they were found in the many temporary prairie wetlands of the Chicago Wilderness area. Although their numbers have probably decreased due to diminishing habitat, they've hung on by being adaptable. Today they're found not only in prairie remnants and marshes, but also temporary wetlands in agricultural areas, in drainage ditches, and in shallow ponds.

The loud "choruses" are in and around temporary water sources (ephemeral ponds). In order to attract females, the males sing day and night. The males often call from under vegetation, like a clump of grass, although sometimes they'll also call from out in the open. The males take in air, inflating their lungs and throat pouch. Then they close their nostrils and force air back and forth between the lungs and throat pouch passing the air over their vocal cords. The throat pouch can expand to the point where it looks like it will burst. When filled with air, the vocal sac serves as a resonator that will increase the volume of the call. The throat pouch remains inflated during the call. Each frog and toad species has a different call and can be identified easily by that particular call. It's amazing how loud some frog species can be.

The choruses attract receptive females as well as other males. Some research has shown that the larger the chorus (the more male frogs calling), the greater success there will be in attracting females. The calls may also serve as a territorial song warning off other males from the immediate area. In other words, the males are proclaiming that they are western chorus frogs, they're interested in mating, and that the small area from which they are calling is their territory.

To hear chorus frogs in the Chicago Wilderness area, visit just about any wet prairie, sedge meadow, or shallow marsh area in March and April. Because they're probably the most common frog calling at this time of the year, it'll be difficult to miss them. Listen for the "prrreep" sound. To see these special creatures, you'll probably need to get out into the wetland so rubber boots are a necessity. Try to disturb the water as little as possible as you walk through the wetland. The vibrations in the water from your movements may disturb the frogs and they may stop calling. If individuals or the entire chorus stops calling, stand still for a little while and they'll usually start back up.

Listening and viewing frogs and toads can be fun and challenging. The limited number of frog species makes learning their calls fairly easy. With a little practice, you'll be able to identify all the frogs in the Chicago Wilderness area. So get out there this spring. Look, listen, learn, and enjoy your neighbors.