Let's start with a little introduction to our players:
The Spotted Chorus Frog (Pseudacris clarkii) is a small hylid of the grasslands and prairies of the south-central US. The are found in prairies and marshes but also can be found in some agricultural fields and roadside ditches. In these areas, they are usually only seen after heavy rains flood their fields and they come out to breed. During dry periods they apparently burrow into the soil and are rarely seen. They are small light grayish frogs with green spots and can be quite attractive little critters.
Interestingly, Spotted Chorus Frogs change color slightly between the day and night. In the day the frogs are pale gray with brownish-green spots. At night, their background color lightens making them a whitish frog with brighter green spots. I don't have a great photo of this, unfortunately, because Spotted Chorus Frogs tend to call from deep within mats of flooded grass.
Squirrel Treefrogs (Hyla squirella) are quite a different looking little frog. Like other "treefrogs", they have enlarged toe pads and vary from brown to bright green. They also can change color depending on their activity states. When they are very active and calling they are often bright green with a white stripe on their upper lip. When cool or resting they tend to be tan or brown with or without brown markings.
So looking at these photos, you can see that these species don't really look that much alike. And over most of their range, they do not overlap. But in east-central Texas and down along the central Texas coast both species can be found together.
Where they do overlap in range, the problem is not their appearance but the sound of their calls. I had been recording calls for several years in east central Texas before it occurred to me that their calls were similar enough to be confusing.
One day in June 2015, I was at Aransas National Wildlife Refuge recording some frogs after a heavy rainfall and I made the following recording. The temperature was 72°F:
In my notes, I documented this as a chorus of Spotted Chorus Frogs since this is a species I frequently hear during rainy summer periods in this area. However, as I listened to the recording later that evening, I started to wonder if these could have been Squirrel Treefrogs which also occur in these coastal marshes. I began to doubt my ability to discriminate the calls and that made me go back and check all my recordings from their region of overlap. (It turns out I was correct at first, and these are Spotted Chorus Frogs).
The call of the Spotted Chorus Frog is a upward "fingernail over comb teeth" call. But the speed of the call is correlated to the ambient temperature, so when it is cool out, the call is slower. Here is a recording of some Spotted Chorus Frogs in San Antonio, Texas at 62° in March -
The call of the Squirrel Treefrog is an almost duck-like quacking sound and is typically faster than the trill of the Spotted Chorus Frog.
So even though the quacking sound is "different" than the trill of the Spotted Chorus Frog, as the temperature increases the trilling of the Spotted Chorus Frog becomes faster. As that happens, they two species sound more similar.
If you hear them together they are easier to tell apart.. Here is a Spotted Chorus Frog calling at the same time as a Squirrel Treefrog. There is also the slow upward "fingernail on comb" call of the Cajun Chorus Frog (Pseudacris fouquettei) in this recording.
Here is a spectrogram showing you what you are hearing in this recording.
Here's a recording of a pair of Squirrel Treefrogs followed by a recording of some warmer Spotted Chorus Frogs. You can see that although they are clearly different when heard with each other, they are similar enough that in the area of their overlap in range, you might have to listen carefully.
Fortunately, the area of overlap in range is pretty small. Here's a map showing which counties have records for Spotted Chorus Frogs (yellow), which counties have records of Squirrel Treefrogs (blue) and which counties have records of both species.
So I'm working on these two species to try and be better about telling them apart. I do that by getting out in the field when I can, but also by listening to the recordings posted on Inaturalist.org. My goal is to be able to distinguish them immediately 100% of the time. I'm not quite there yet.
© Chris Harrison 2016