Spotted Chorus Frog vs. Squirrel Treefrog
Pseudacris clarkii vs. Hyla squirella

If I had to choose the species whose calls give me the most trouble in Texas, I'm embarrassed to say it is these two species - the Spotted Chorus Frog and the Squirrel Treefrog.  The strange thing they don't look anything like each other, they don't overlap much in range and most of the time they are easy to tell apart by call.  But occasionally, I hear one of these species calling by itself and it stops me and leaves me confused.

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  My goal is to be able to distinguish them immediately 100% of the time.  I'm not quite there yet.


© Chris Harrison 2016

Mexican Spadefoot
Spea multiplicata
Another One off the List!

Mexican Spadefoot (Spea multiplicata)
Schleicher County, Texas
When I started recording anurans four or five years ago, I didn't really have a plan.  I just liked recording and documenting frogs and toads.   After a year or two, as I amassed more and more recordings it suddenly seemed worthwhile to try and get every species in my home state of Texas.  I did pretty well for a few years, but adding these last few Texas species to my "recording" lifelist has been harder than I thought.  Some of the species I need have restricted ranges or are rare in the state so their absence on the list is not unexpected.  But the Mexican Spadefoot doesn't fit in either category, I have seen lots of them, I have just whiffed on getting a recording of it calling multiple times.

Although it ranges widely across the desert Southwest of the US and down into the Central Plateau of Mexico, the Mexican Spadefoot (Spea multiplicata) is not a real "desert" dweller per se.   It is a grassland species that is found in the dry grassy desert grasslands.

A few days ago, I saw that it had rained 2 inches in the grasslands of far western Texas (Hudspeth County) and I decided that it was time to make another quick trip to west Texas to get this species.  While two inches of rain isn't enough to get frogs breeding in many areas, in the heart of the Chihuahuan desert where they might only get 5 inches of rain in a year, a two inch rainfall is a deluge.   Desert amphibians have to take advantage of that or not breed at all.  So I piled my gear into the car and headed out in to the high grassy desert of northern Hudspeth County.   

When I arrived I drove up to the area that had received the rain and spotted a  few anurans on the road.  That is always a good sign for me to stop and listen at that spot.   When I got out of the car all I could hear was the wind blowing across the grassland at 20mph from the east.....but then I heard it....I weak drumming sound from across the grasslands - Mexican Spadefoot calls!

I pointed my shotgun mic out into the darkness of the desert and managed to get a few weak recordings.  Here's what I heard in the distance -

The call of the Mexican Spadefoot is often described as a drumming or a dry trill sound.  It is like a short drum roll on a toy drum lasting about 3/4 of a second.  It reminds me of the sound made by a ball bearing dropped onto a hard surface from a few millimeters.   The ball bearing bounces with each bounce making a sound.  The bouches get a bit faster each time as the ball bearing bounces a little lower with each bounce and the pitch of the sounds goes up a bit toward the end. (Maybe that doesn't work for you, but that's the image I get?)

After I got back to my hotel that night, I was trying to decide where my next hunting ground would be.  Yes, the species was on "the list" but I wasn't very satisfied because it wasn't a great recording, I didn't have a recording of a single individual and I didn't have a photo of one calling.  I looked online and saw that central Schleicher County (central TX) had received heavy rainfall that night and I decided my next night would be there.  After all, I already had my spadefoot and it wasn't supposed to rain where I was so I might as well move on to wetter pastures.
When I got to Schleicher County, I was delighted to find the flooded fields held huge choruses of Mexican Spadefeet (Spadefoots?) that were easily approached, recorded and photographed.   Here's a single individual from Schleicher County -

Here's the sound of one of the Schleicher County choruses.  The nasal groans are Couch's Spadefoot and the higher pitched "crick, crick, crick" are Spotted Chorus Frogs. -

Here's a spectrogram of their call.  You can see how the later drum beats increase in pitch slightly -

And here is a short sequence of one of the little spadefeet in action.  As he called, the motion of expanding and collapsing his vocal sac seemed to propel him forward so he was hard to follow with the tripod-mounted camera!

So that's another species off the list, and more importantly my last panhandle/west Texas species.  This means I can focus my searches on the areas with missing species and not have to make the 6-10 hour drive up to those areas again!

© Chris Harrison 2016

Choosing A Recorder for Frog Calls

First off, let me start with the caveat that I am not a sound engineer or audio junkie. I am a herper who has taught himself a little bit about recording amphibian calls and wanted to share some of what I have learned through trial....and lots of error.

There is lots of great information about recording wildlife online, including advice on which recorders are useful for these tasks.  Wildlife recorders face some challenges that studio recordings and concert recordings including very quiet environments with quiet subjects.  This isn't the typical problem that people encounter recording a rock concert, for example.   Because of this, the sensitivity required by wildlife recording brings out the flaws in some otherwise useful recorders.

Recording calling frogs and toads adds another level of challenge that normal wildlife recorders don’t face. It is usually done in wet areas, sometimes hip-deep in water, usually in the dark, and often in the rain. You might be miles from your home or vehicle and have to carry everything with you.  And then you might have to stand/crouch for 10-15 minutes in the dark waiting for the darn frog to start calling again!

So how do you choose a recorder for this task?

Of course, if budget is no concern and sound quality is the foremost consideration, you will probably end up with a high end field recorder such as the those made by Sound Devices  and expensive microphones with the appropriate wind and weather protection . But for most herpers with just a casual interest in documenting frog calls, that is probably overkill and it won’t fit in your pocket.

So how do you choose?

Here are some variables to consider:


Frequency Range

First, some technical mumbo jumbo.  Hertz is the scale used to measure the frequency or pitch of a sound.  High pitched sounds are higher in frequency and have higher Hertz values.  For some scale, the lowest note on a piano is 27.5 Hz and the highest note is 4186 Hz.  The low C (C2) note sung bass singers is around 65Hz while the high C sung by a good operatic soprano is around 1046Hz.

These values are sometimes expressed in Kilohertz (KHz).  1KHz = 1000Hz.  

The generally quoted boundaries for human hearing are ~20Hz at the low end ~20,000Hz (20Khz) at the high end.

Sample Rate

Hertz can be used to express the frequency of the sound being generated, but confusingly, it is also used to measure the sample rate (or recording rate) of a recording.  These are not the same. 

The sample rate is a measure of how often the recorder samples the sound per second.  You can think of it as analogous to how a video captures motion.  A video doesn't actually capture motion, it captures a whole series of still shots (frames) of the action and by playing them back at a particular speed they appear to be continuous motion.  

The recording sample rate is similar.  The recorder is not measuring the sound continually, but is measuring the sound over and over again and these individual captured sounds are then played one after the other to give the whole continuous sound.  Sample rates are very high with most modern recorders sampling the sound at rates of 48Khz (or 48,000 samples per second).  Some more specialized recorders support sample rates of 96Khz, 192Khz and even 384Khz.  These recorders captures sounds well above the level of human hearing but can be useful in documenting some insect or bat calls.

Why do I need to know this stuff?   Because there is a theoretical limit we need to worry about.  A recording can only capture sounds up to a frequency that is approximately 1/2 of the sample rate.

So using a sample rate of 8Khz will only capture sounds up to a frequency of ~4Khz.  If you use a higher sample rate (say 48Khz), you will be capturing sounds up to ~24Khz frequency.

CD Quality recording is 44.1Khz which means CD quality recordings can capture sounds up to 22,050 Hz (22Khz).  Most humans have a hearing range that tapers off near 20Khz at least when they are young.  Older people have a hearing range that tapers off at 12Khz or even lower.  (My hearing is failing, so I don't hear much above 11.5Khz).  But because "CD quality sound" includes is sampled at 44.1Khz, it can include all the ranges capable of being heard by humans.

Lastly, larger sample rates means larger file sizes.  

OK, enough background already!......

So the obvious first thought might be to turn to a Voice Recorder or use an app on your cell phone or tablet since they are inexpensive and readily available. This can work in some circumstances but voice recorders or the voice recorder apps in many phones are optimized for capturing the human voice.  As we just saw, the typical frequency range of the human voice is usually between 80 and 2000 Hz..  Therefore a recording rate of 4KHz is usually enough for voice and and 8KHz sample rate will capture almost any sounds a human can make.

The frequency cutoff also depends on the particular mode you use in your recorder. For example, one common entry level voice recorder made by Olympus only records frequencies up to 3 kHz in certain space saving modes.   Why limit the frequency range in voice recorders?  Simple.  Higher recording rates = larger file sizes.  Large file sizes means you can't fit as many hours of recording on the recording medium and if you read the ads for voice recorders their main selling point is usually how many hours they can store.

So voice recorders are great for recording human voices, but once we step up record natural sounds many animals call outside of this range. Several species of frogs in the genus Eleutherodactylus, for example, have very high pitched calls that can be in excess of 8 kHz. The North American Little Grass Frog (Pseudacris ocularis) calls are in the 7.5 kHz range.  To capture those, you would need a recorder that records at at least a 16KHz recording rate.

To hear the difference, here's a recording of a chorus of frogs from Puerto Rico including the lower pitched calls of the Common Coqui (Eleutherodactylus coqui) and the Red-eyed Coqui (E. antillensis).  Over the top of this lower pitched background, you can hear the high-pitched whistle of the Whistling Coqui (E. cochranae) as it calls three times. The Whistling Coqui call has a peak frequency of 4100Hz.  (This recording is at 16KHz so it includes everything below 8Khz)

Now here is the same recording saved as an 8KHz recording as it might be picked up by an  inexpensive voice recorder.

See, no more Whistling Coqui!  It's call is just above 4Khz so it was not picked up in the voice recorder (emulated) recording.

Here are those two recordings in a row with the 16KHz recording followed by the 8KHz recording and the spectrogram for those recordings.  You can see (highlighted) the high pitched call of the Whistling Coqui and the fact that it is "missing" from the second 8KHz recording.

Even frogs whose carrier frequency (main note frequency) is below the 8 kHz threshold, the actual “sound” of their call is dependent on higher sideband or harmonic frequencies which can be well above the carrier frequency. 

A good example of a North American anuran call in which the sidebands/harmonics influence the overall sound of the call is the Ornate Chorus Frog (Pseudaris ornata). The following is a recording of a Pseudacris ornata taken from somewhere out on the world wide web (sorry, I don’t remember where!). I took a short section of the recording and copied it. The first time it plays, it is playing at “CD quality” sample rate of 44.1 kHz (with a maximum frequency of 22 kHz). It then repeats having the sample rate reduced to 8 kHz (max frequency of 4 hKhz). Here is a sonogram of what is in this recording. The highlighted calls in the first recording are all that is left in the second. All the higher parts of the call are lost.

You can hear the difference the loss of these over tones makes:

When you listen to the recording, the second time through, it sounds distinctly different. This is due to the loss of the harmonics/sidebands above 4 kHz. So if this frog had been recorded with a low end voice recorder at a high compression setting, the call would sound like the second part of the recording. While that is certainly enough to identify the species in question, it clearly loses some of the texture or tone of this particular species’ call.

So a dependable frog recorder needs to be able to capture the range of frequencies used by most anurans so your recordings be representative of what the frog sounded like in the field. I would generally want a recorder capable of 48Khz recording rate which would record sounds up to 24Khz.  There are anurans known to call above these frequencies, but those ultrasonic frequencies are above the range of human hearing anyway so probably aren’t of interest to most casual anuran recordists. If you were interested in ultrasonic calls, there are recorders that go well beyond this. Some recorders can capture ultrasound with sampling rates of 96kHz, 192kHz or even 384kHz but they generally require specialized ultrasonic microphones to do so. While these recorded “sounds” can’t be heard by the human ear, they can have their frequencies brought down into human hearing range after recording. This is how bat biologists record and analyze bat calls. 


Signal to Noise Ratios

A microphone works by picking up the vibrations in the air generated by the sound wave and generates a weak electrical current corresponding to that sound wave. However, that current is so weak that in order for the signal to produce an audible recording, it has to be amplified before it is recorded. This is called “preamplification” and most recorders have built in “preamps” for this purpose.

This introduces a new problem. The microphone converts the sound to a weak electrical signal and the preamp the increases the strength of this electrical signal.  But during the process of amplifying this signal, it is possible to add extraneous electrical "noise" to the amplified signal.   This electrical noise shows up as hiss or other distracting background noise in the final recording.  

Recordists describe the quality of a good microphone and preamp by their signal to noise ratio.  In other words, how much of the desired subject (signal) is present in the final recording when compared to the extraneous noise added by the amplification process.

Well designed preamps will minimize this noise and amplify mostly the "signal" that we are interested in but poorly designed preamps can add excess noise.  Less expensive recorders often have lower signal to noise ratios than more expensive recorders, but there are good entry level recorders with good S/N ratios.

When dealing with loud sounds like guitar music, drums, people talking close to the microphone, etc., it isn’t an issue as the signal is so loud that the additional preamp noise is inconsequential. But when the quieter sounds of the natural world even a small amount of added noise can really interfere in the ability to discriminate quieter frog calls.   So the wildlife sound recordist wants a microphone and recorder combination that has a high signal to noise ratio.

Here is a comparison of the two species of anurans (Hyla chrysoscelis and Incilius nebulifer) recorded simultaneously from the same spot with two different recorders. One is recorded with an older model Motorola Android cell phone and the other with an Olympus LS-11 Digital PCM recorder. The Olympus recorder has good preamps and produces less noise than the Android phone. You can actually see this in the sonogram for this recording.

On the top recording (the phone) the background is much darker. This dark background represents noise in the recording. In the bottom sonogram, the background is much ligher while the frog calls are still as dark. Therefore you can see there is more signal (the darkness of the frog calls) compared to the noise (darkness of the background).

Here is a shorter section of these recordings played one after the other. In the first part, you hear the phone recording followed by a second of silence then the recording made by the Olympus LS-11. Listen to the background hiss in both recordings and compare how well the calls stand out. It is easiest to hear in the short buzzy trills of the Cope’s Gray Treefrogs (Hyla chrysoscelis). The longer trill is a Gulf Coast Toad (Incilius nebulifer).

It is worth pointing out here that there has been a significant improvement in the quality of recording that you can achieve with cell phones as I outlined in a more recent recording here - 


Recording Format

Another consideration is the format in which the recording will be captured. In voice recorders, a selling point is often the maximum number of hours of recording which can be stored in the recorder. In order to maximize that number the recordings may be compressed into lossy formats and the frequency compressed into the range expected for the human voice. This means higher frequency calls may be lost or significantly degraded in the final recording. When choosing a recorder, it is preferable to have a recorder which will save the file in an uncompressed format (aiff, wav) rather than a compressed format (mp3).

There is a nice comparison showing the limitations of voice recorders here -


Other Practicalities

Beyond the technical specifications we need a recorder that is small, field hardy, and somewhat weather-resistant. Furthermore, it is preferable to have a recorder that is easy to adjust and monitor in the field. Some recorders require you to go down through menus to make simple changes like the input level (gain) of the microphone. That can hard to do when you are out in the field, knee deep in water in the dark. Also, since you might be holding a microphone in the other hand, it can be helpful to be able to make changes with one hand.

Storage media and connectivity

How the recorder stores its recordings is another important consideration. While older handheld recorders relied on cassetes or microcassetes, the noise generated by those recorders made them obsolete with the advent of digital storage. Some recorders store their recordings on internal flash memory while others rely on removeable media such as SD cards. Some older models use compact flash cards and others rely on internal hard drives. These methods are OK, but not as field hardy as more modern methods.

Getting the recordings off the recorder is easy with most modern recorders. They either have removeable cards which can be put into a computer and/or they have mini USB ports plugs on the side to allow direct connection. Either way, getting recordings into your computer is a breeze.


Another variable to consider when purchasing a recorder is what types of external microphones it will accept. There are two primary microphone plug types used in recorders. Inexpensive microphones and recorders use a 3.5mm TRS plug similar to a headphone plug on an MP3 player. These plugs as small and easy to use. There are inexpensive adapters and extension cords available for these sized cords at almost any electronics stores and even many Wal-mart type stores. Microphones with these types of plugs either require a battery in the microphone for power or rely on the recorder to provide a low voltage plug-in power than can power small microphones directly.

Professional microphones generally have a 3 pin XLR type plug instead. These plugs are larger and most XLR plug microphones depend on power to be supplied by the recorder itself. This recorder-based power supply is often called phantom power. Some recorders are capable of providing phantom power and some aren’t. Phantom power voltages vary from microphone to microphone but most recorders that supply phantom power can supply it at various voltages.

So why would you go the trouble of using an phantom powered XLR type microphone when a 3.5mm microphone would be easier? 

One difference is the sturdiness of the connection. TRS pins can become unplugged easily if pulled. I have on more than one occasion been recording with my 3.5mm plug microphone only to find out it wasn’t plugged in to the recorder and what I was actually recording with was the internal microphones of the recorder. TRS 3.5mm pins are also fairly thin and I have bent a couple when bumping (or dropping :-() my recorder when out in the field. XLR type connections are sturdier and often lock into the socket. In order to be pulled out, you have to depress a pin as you pull so accidentally pulling one out is more difficult.

The other difference is the quality of the sound. When a microphone has its own internal power supply and unbalanced inputs like many TRS 3.5 mm cables, that increases the chance that electrical interference will be produced in the line and show up on the recording as noise. With good microphones and cables this can be reduced, but it is never as quiet as a balanced phantom-powered, XLR type connection. The problem with noise in a recording is that you don’t notice it until you hear a recording that has less.

Part of the learning process is learning to hear the difference between a good (quiet) recording of an amphibian and a bad (noisy) one. Try going to online resources like's audio observations and listening to some of the frog and other recordings on there. You will hear a profound difference in the quality based on the different recorders, microphones and techniques used. 

You can also hear this difference by listening to the differences in the recordings used as vouchers in online databases like HERP,, and (for birds).

But most importantly, get outside, record some amphibians and have fun! 

And don't forget to upload your mp3 vouchers into a citizen science database somewhere like those mentioned in the last paragraph.  That way your recordings can live forever!

© Chris Harrison

Green Treefrog
Hyla cinerea

The Green Treefrog is a widespread, conspicuous frog over much of the southeastern US.  It seems that anywhere you can find permanent water with some vegetation in or around it, you will find Green Treefrogs.  
Their preferred habitat seems to be rushes, cattails and other emergent plants at the waters edge, but they aren't restricted to such areas.  These Green Treefrogs from near College Station, Texas were calling from a metal sign overhanging a flooded pond.

Their vocal sac is on their throat as is quite large.

The call of the Green Treefrog is a loud nasal "queehnk-queehnk-queehnk".  It is sometimes described as "bell-like".  The speed at which it is repeated depends on the temperature and number of frogs calling.

Here is an individual calling from a roadside pond in Fayette County, Texas.  You can hear other Green Treefrogs as well as the clicks of Blanchard's Cricket Frogs (Acris blanchardi) in the background.

Here is a large group of Green Treefrogs calling from Hog Lake at Aransas National Wildlife Refuge.  When there are this many calling, it makes a cacophonous noise.


Here's a Green Treefrog calling from a tree trunk in Gonzales County, Texas.  Notice how much air he breathes in an out with each call.  No wonder they are so loud!


© Chris Harrison 2012 (updated 2016)

Woodhouse's Toad
Anaxyrus woodhousii

Northeast of Fredericksburg, Texas is a narrow little road that loops through some pretty rocky canyon areas.  It is all private land, but it is a pretty drive and very popular during wildflower season.   My interest in the area, of course, is the potential for seeing reptiles and amphibians that are hard to find other areas in the hill country of Texas.

They received 2-3 inches of rain earlier this week so I thought I would make a drive up there to see if I could find some hill country frogs like Strecker's Chorus Frogs or possibly Barking Frogs calling.   I was surprised to find a much more widespread species that I had never seen in the area before - Woodhouse's Toad (Anaxyrus woodhousii)!

Woodhouse's Toad (Anaxyrus woodhousii)
Burleson County, Texas
Woodhouse's Toad is a toad of the Great Plains and desert Southwest.  However it does occur into east-central Texas and down into the southern tip of the state.  It's distribution in the eastern 1/3 of the state is very patchy and hard to predict.  I had spent considerable time in this particular area of Central Texas and never seen a Woodhouse's Toad.   On this night I saw 3 and heard one distant calling individual.

The recording isn't very good since it was a distance away and I had to filter out several other species (Incilius nebulifer, Lithobates berlandieri, Acris blanchardi) to make the Woodhouse's audible, but it is at least my first Woodhouse's Toad recording and another species off my Texas Recording Lifelist.

The call of the Woodhouse's Toad is a nasal trill.  You can hear it here above the background of the other species.

The call of this species is very similar to the call of its sister species, the Fowler's Toad (Anaxyrus fowleri).  Fowler's Toad and Woodhouse's Toad used to be considered the same species.

Hopefully I will be able to get a better recording for this species this summer, but for now its off my list of Texas misses at least.

© Chris Harrison 2016

Gray Treefrogs
Hyla versicolor vs. Hyla chrysoscelis

There is a problem with Gray Treefrogs...there are actually two species disguised as one.  The two species are physically identical from in external appearance but have different numbers of chromosomes and have different calls.
This has created a historical problem in defining their ranges.  Typically, the ranges of an animal are determined by looking at where specimens have been found.  Unfortunately, Gray Treefrogs in museum jars don't call so most older collections have large numbers of unidentifiable Gray Treefrogs.

They are both grayish frogs with a mottled "tree bark" pattern and they both vary.

Here is the Eastern Gray Treefrog (Hyla versicolor) from Grimes County, Texas -

and another from Houston County, Texas -

and here is the identical Cope's Gray Treefrog (Hyla chrysoscelis) from central Arkansas -

and another Cope's Treefrog from central Texas -

Both species can show varying amounts of green coloration, like this Hyla chrysoscelis from south-central Texas.

Some of these frogs have very little visible mottling on their backs and can appear almost solid green backed or solid silvery white.  
They can be told from other solid green frogs by the yellow/orange mottling on their hind legs.  Here is a pretty green-backed Hyla chrysoscelis from Karnes County, Texas.  You can see the yellowish-orange color on the hind legs  -

So if they look identical, how does one tell them apart?

Easy....sort of....

Hyla versicolor has twice as many chromosomes as Hyla chrysoscelis and so if you karyotype one you will have your identification.  But karyotyping isn't a very useful character for identifying them in the field.  

Because they have more chromosomes, H. versicolor have slighty larger cells as well.  Supposedly this can be seen to the "trained eye" with magnifying glass by looking at the cells on the toe pads.

The easier way to tell them apart is their calls.  Hyla versicolor's call is a pretty, bird-like whistled trill.  Here's one from Lavaca County, Texas. 

The whistled trill of Hyla chrysoscelis is similar, but it is drier and less bird-like.  This is due to the fact that the trill is actually faster.

The trill rate differences not only make the two species sound different, you can actually count the rate of trills per second to help distinguish between the two.  Of course, with trill rates of 25-65 pulses per second, you can't count them as you hear them, but you can see the difference in a spectrogram.

These two sections of spectrogram represent 0.25 seconds of each species call.  Hyla versicolor is on the left and Hyla chrysoscelis is on the right.  

Each of these vertical blue lines represents on pulse of the trill.You can see that the H. versicolor made 7 pulses in this time (28 pulses per second) while the H. chrysoscelis made 11 pulses (44 pulses per second).  The threshhold for distinguishing the two species is often quoted as greater or less than 30 pulses per second.

When you do hear them together in areas where there ranges overlap, you can hear the difference pretty clearly.  Here is a spectrograph of a recording I made in Davy Crockett National Forest in East Texas.  What you see is a Hyla versicolor being answered by a Hyla chrysoscelis (maybe 2?).  I have labeled each species calls (v for versicolor, c for chrysoscelis).  

So listening to the recording, you will hear (in order):
- Hyla versicolor
- Hyla chrysoscelis
- Hyla versicolor
- Hyla versicolor
- Hyla chrysoscelis
- Hyla versicolor
- Hyla chrysoscelis (different individual)
- Hyla versicolor

And here is the recording.  See if you can "follow along" the spectrograph and hear the difference in their calls.  (The higher pitched upward trilling call in the background is the Cajun Chorus Frog - Pseudacris fouquettei).

Of course, the pulse rate of frog calls varies with body temperature (warm frogs call faster).  So how could you tell the slower pulse rate of a cold H. chrysoscelis from the faster pulse rate of a warm H. versicolor?  Fortunately, there isn't much overlap.

Here's a potentially confusing call from Kendall County, Texas.  This is Hyla chrysoscelis, but its pulse rate is around 36 pulses per second, giving it an almost Hyla versicolor sounding call.  However the versicolor-like nature of this call is due to the relatively low temperatures while this frog was calling (58°F).

This graph shows the pulse frequency for a number of H. chrysoscelis and H. versicolor calls at different temperatures that I have recorded them (or in a few cases, where I have received recordings with temperatures from others).  
The blue dots represent Hyla chrysoscelis and the red dots represent Hyla versicolor.

Notice that the trend lines for each data set are roughly parallel (I'm not sure how MS Excel calculates the "linear trend line" mathematically).  This does help reinforce the idea that the increase in call rate versus temperature is roughly equivalent in each species.

From these data, you can see that Hyla versicolor and Hyla chrysoscelis do overlap in pulse rate, but only at the opposite ends of their temperature ranges. 
Just by chance, the lowest pulse rate I have been able to document for Hyla chrysoscelis is 34 pulses per second for a recording made in Cavalier County, North Dakota at 60°F.   The highest pulse rate I have ever documented for Hyla versicolor is also 34 pulses per second for a frog calling in Colorado County, Texas at 81°F.   So even though I have documentation of both species producing a pulse rate of 34 pulses per second, the Hyla chrysoscelis produced that pulse rate at an ambient temperature over 20°F colder.

Therefore we can expect that there would never be a situation where the two species would be exhibiting the same call rate in the same area at the same time.   The pulse rate of Hyla chrysoscelis is roughly twice that of Hyla versicolor at all temperatures.

(There is a published formula that has been used to standardize the call rates of the two species for temperature.  I will dig that out of my readings and post and cite it here soon.)

To help distinguish them even further, there is research to suggest that in areas where their ranges overlap (in Texas at least) the pulse rate of the Hyla chrysoscelis calls is even higher, helping to further distinguish their calls.

So since museum specimens don't call, the ranges of these two species are still being worked out.  In many states that used to assume they had both species, it has turned out that only one species occurs.  But there are areas where there is some overlap and some uncertainty still.  Texas is one place with some question marks since both species do occur here.

You can get a rough idea of their ranges by looking at the excellent National Amphibian Atlas from the Patuxent Wildlife Research Center.  You can search for any species range by choosing "View a Species Map" on the left menu and choosing the species you are interested in.  They show a county by county range map for the species although they acknowledge there is some uncertainty in some of the records for which species of Gray Treefrog it is.

But if you look at their maps, this is the range map for Hyla chrysoscelis -

and here is their map for Hyla versicolor -

What you see on these maps is that Hyla versicolor doesn't appear in the Southeastern US.  In the Northeast, it is the only species present.  

But Texas has an interesting distribution map for these species.  Here is their map for Hyla chrysoscelis in Texas -

You see the big "hole" in SE Texas around the Houston area where the only gray treefrogs are Hyla versicolor.  This is approximately what I have heard, although I think the "reappearance" Hyla chrysoscelis doesn't take place as far east or south (i.e. I have sampled several of the counties that this map implies have chrysoscelis but I have only been able to document Hyla versicolor).  There are also areas of overlap east of Houston not shown on this map.

Here are records by county for the last several years of my recording these species as well as a few records from the Inaturalist Herps of Texas project.

Obviously, I have a lot of counties to fill in on the map, but the data follow the general pattern shown in the Patuxent maps.  Hyla versicolor are generally found in the SE part of the state and west and north of there they appear to be replaced by Hyla chrysoscelis.  I have found that the boundary/area of overlap is a bit further west than the Patuxent map implies and it is quite narrow.  When you find one species calling, you can drive 20 miles east/west and you find only the other species.

Many field guides imply that their ranges overlap extensively in Texas.  This is due to their reliance on older maps that didn't distinguish the two species very well.  My experience is that the two species don't overlap in range very much (at least in the areas between San Antonio and Houston that I have sampled well).  I have only heard the two species together on a few places in the yellow shaded counties.  They don't overlap throughout those counties, just in a narrow band within those counties. I don't have enough data yet for the northern boundary of their range (it's a long drive from my home in San Antonio!).

I will acknowledge here that the absence of data is not data, but my conclusions in these areas of overlap area based on numerous samples at various times, not just single points.  Here's a map of my recording locations going east of San Antonio 150 miles or so.  Hyla chrysoscelis (green pins) is the only species heard until you get about 75 miles east of San Antonio then suddenly you hear nothing but Hyla versicolor (pink pins).  The few places I have heard the two species calling together are marked with yellow pins.

The broad red line is just there to help show my interpretation of the contact zone between the species.  Yes, the two species do occur together, but the zone of overlap is fairly abrupt in this part of Texas in my experience.  That line represents a contact zone that is less than 15 miles wide.  This isn't intended to be authoritative, but these records are all backed up by recordings available to anyone wanting to check them at and

As you move into far east Texas, Hyla chrysoscelis does occur in the counties east of Houston as well.  In Louisiana, Hyla chrysoscelis seems to take over again.  So there appears to be a little "pocket" of versicolor in Texas. My general feeling is that if you are within 70 miles of Sealy, Texas you are going to find Hyla versicolor.  If you go east of west of that, you are likely to find Hyla chrysoscelis.  The range of versicolor appears to extend further up into the forests of east-central Texas though.  I look forward to the opportunity to fill in some more gaps on this map.

So even more recent range maps appear to be less than accurate.  Part of the problem seems to be the way the data are reported and collected.  I have seen well-meaning people report species in citizen science databases (like Herpmapper, NAHerp, or Inaturalist) where they made their identification based on:
1. "my field guide shows both species present in my area, and we saw hundreds of them so I am assuming we saw both species" 
- The problem is that the field guides are based on historical records that weren't correctly identified to species.

2.  "we heard mostly species X, but there was one that sounded a bit different" 
- The problem with this argument is that the calls of each species vary between individuals and with temperature.  And sometimes a frog will make a random weird call!  You have to listen to a series of calls to be sure what you are hearing.

3. "I've heard lots of species X.  These were slower/faster" 
- Many people don't know about the difference in their calls based on temperature and how a cold Hyla chrysoscelis can sound like a Hyla versicolor.

These species can be identified by call, but it takes a little patience.  Hopefully with time we will gather a more precise view of their distributions in the Lone Star State.

© Chris Harrison 2013 (updated 2016)