American Toad vs. Fowler's Toad
Anaxyrus americanus vs. A. fowleri

The two US toad species that are probably confused more than any others would have to be the American and Fowler's Toads (Anaxyrus americanus vs. Anaxyrus fowleri).

The problem is that they overlap in range across much of the eastern/northeastern half of the US and that they occupy much of the same habitats.   They are both highly variable in color pattern and there is significant overlap in their characteristics.  These two toads are confusing and difficult to tell apart even for experienced herpetologists (people who study reptiles and amphibians).

Fowler's Toad
Cherokee County, Texas
American Toad
Scott County, Arkansas
One thing that can be helpful occasionally for distinguishing the two species is that American Toads can sometimes have a reddish ground color.  Fowler's Toad is usually gray/brown/black without the reddish tint.  However, many American Toads do not have this reddish coloration and some Fowler's do.  So if you find one of these two species and it is very reddish, it is probably an American Toad,...maybe.  (It should be noted that other toads can be reddish, so not every reddish toad is an American Toad, it is just less likely to be a Fowler's Toad.)

American Toad
Ouachita County, Arkansas

Fowler's (East Texas) Toad
Tyler County, Texas
Making this matter even more confusing is the fact that the Fowler's Toads of southeast Texas are sometimes regarded as a different species (the East Texas Toad).  Furthermore, Fowler's Toad has previously been treated as the eastern "morph" of the widespread Woodhouse's Toad (Anaxyrus woodhousii).

So...how do you tell them apart?

One character that is often quoted in comparisons is the number of tubercles ("warts") in each of the dark spots on the back.  If it has only a single (or two) warts in each dark spot, it is probably an American Toad.   If it has three or more in some of the spots, it is probably Fowler's Toad.

Another characteristic used to "differentiate" them is the contact between their parotoid glands (big gland behind the eye on most toads) and the bony cranial ridges behind the eye.  In American Toad there is a small spur on the cranial crest behind the eye that points posteriorly and touches the front of the parotid gland.  You can see the small spur connecting the cranial crest to the parotoid gland in this American Toad from North Carolina.

American Toad
North Carolina
In Fowler's (and Woodhouse's) Toads, the spur is usually missing and the parotid gland bumps right up against the transverse part of the cranial ridge.  There is a nice diagram of this from the Frogs of Tennessee page here - 


Another character that is sometimes mentioned is the color of the underside of the toad.   American Toad is whitish underneath while Fowler's Toad can have some dark spotting particularly near the throat and front limbs.   

Yet another characteristic that is used to distinguish them is the size of the tubercles (warts) on the tibial part of the leg.  American Toad has prominent "warts" here while Fowler's are less noticeable and not larger than the rest of the leg tubercles.

So by looking at the tibial warts, cranial crests, parotid glands, belly coloration, and warts-per-spot on the back, you might be able to identify one of these toads to species.   But these variables are hard to see in some individuals and apparently vary between species as well!

Now, in Texas the American Toad has a very limited range so the area of possible overlap is fairly small.  These two species (and maybe Woodhouse's Toad) would only be expected to occur together in the counties of far NE Texas.  The area of overlap is not completely defined as the known range of the American Toad in Texas is rather spotty.


So the experts can sometimes have difficulty telling these species apart, so 
1.  how do we tell them apart?
and
2.  how do we know they are two species?

Well, the answer to both of those questions is their calls.  Frog calls serve as reproductive isolating mechanisms.  They generally prevent species from interbreeding.  A female American Toad is not likely to be attracted to or approach a male Fowler's Toad because their calls sound so different.
Fortunately for us, we can also use these calls differences to identify which species of toad we are dealing with.

Both species have typical toad-like trilling calls.  The difference is the length and tone of the calls.  Fowler's (and Woodhouse's) Toads have short, buzzy, nasal  trills that rarely last more than 3-4 seconds.  
American Toads have a similar trill that is "cleaner/prettier" and a little less nasal sounding and often last between 5 and 30 seconds or more.
Here's a spectrogram comparing the two call lengths (American Toad on top, Fowler's on the bottom).  You can see the American Toad trill continuing for almost 14 seconds while the Fowler's tapers off within 3 seconds.  You can also see that the Fowler's Toad call is "messier" on the Y axis (frequency axis).  This tells you the call will be buzzy rather than the clean almost single pitch call of the American Toad.


And here's what that difference sounds like.  These 15 second recordings include a single call by an American Toad while the same time period includes three calls of Fowler's Toads.  The difference is so obvious that in spite of their physical similarities, you aren't likely to confuse a calling American and Fowler's Toad.

First the long pretty, whistled trill of the American Toad:



then the short, buzzy, nasal trill of the Fowler's Toad.  :





Next year, I hope to be able to post an American Toad recording from Texas.

© Chris Harrison 2017

Dwarf American Toad
Anaxyrus americanus charlesmithi

Dwarf American Toad
Scott County, Arkansas
The American Toad (Anaxyrus americanus) is a widespread and well-known toad of the northeastern United States.  



Here in Texas however it has a much more restricted range.  It is only known from a few counties in north-eastern part of the state.  It's range in Texas isn't fully understood because it is very difficult to distinguish from the more common Fowler's Toad.  
I still haven't seen an American Toad in Texas :-( so these photos and this recording come from nearby Arkansas.


In many areas of the northeastern US, this is a common "yard toad".  They are variable in general color ranging from brownish-black to bright reddish.  They often have dark spots on their back.  There is usually a light mid-dorsal stripe visible and a lighter stripe running from the tympanic membrane (ear) down towards the hind leg.  These lighter stripes are sometimes obscured by the ground color.

American Toad
Fairfield County, Connecticut
American Toad
North Carolina
Our Texas and nearby populations of American Toad are smaller than the typical American Toads of the Northeastern US and have been identified as belonging to a separate subspecies, the Dwarf American Toad (A. americanus charlesmithi). 

Dwarf American Toad
Ouachita County, Arkansas
They breed in spring into early summer.  When they breed successfully, you can often find their distinctive helical strings of black eggs in nearby ponds and streams.

American Toad Eggs
Scott County, Arkansas

The call of the American Toad is a very long, high-pitched trill.  It lasts from 5 seconds to over 30 seconds.  The Dwarf American Toad subspecies has a slightly higher pitched call than the typical larger northeastern subspecies.
Here is an American Toad from the Ouachita National Forest in Arkansas calling from the middle of a forest road low water crossing.


Dwarf American Toad call - Arkansas


© Chris Harrison 2017


Rio Grande Leopard Frog
Lithobates berlandieri - revisited



Rio Grande Leopard Frogs (Lithobates berlandieri) will usually call while floating in shallow water.  Their vocal sacs protrude laterally from the sides of their heads.  This individual was calling from a flooded riparian area underneath a bridge in McMullen County, Texas.



Here is a recording of a Rio Grande Leopard Frog from Bexar County, Texas.  It's call could best be described as a series of "croaking" calls.  
The higher pitched clicking heard behind the Leopard Frogs are the calls of Blanchard's Cricket Frog Acris blanchardi




This spectrogram represents the first three "croaks" of the preceding call.



2016 Addendum -

Like most frog species the sound of the call of the Rio Grande Leopard Frog can change with ambient temperature.  At lower temperatures, the tempo of the snoring call slows down and sounds a bit different. You can hear that by comparing these calls from one of my favorite roadside ponds near Quihi, Texas.
One summer evening in August I stopped by there to record some Rio Grande Leopard Frogs along with other species.  Here's what they sounded like in this warm shallow pond at an air temperature of 75°F (and a water temperature probably a bit higher than that) -



I returned to this pond on a cool winter evening (for south TX that is) after it had rained heavily and was 56°F.   The only species calling that night was the Rio Grande Leopard Frog.
Notice how much less "snore-like" it sounds.




© Chris Harrison 2012 & 2017

Stereo Frog Recording

Most of my frog recordings are made in mono. There is only a single channel of the recorded call and it plays exactly the same over the left and right speakers. The reason for this is that I generally use a shotgun microphone to help isolate individual frog calls and most shotgun microphones do not capture stereo. Furthermore, when trying to focus on an individual frog you are recording sound coming from what is in all practicality, a point source. Because the sound is coming from a single point, there is no difference in what it sounds like coming from the left and the right. 

But we normally hear in stereo because they have two ears on the opposite sides of their head. Most sounds coming towards your head are picked up by both ears, but sounds coming from the left side reach the left ear slightly sooner than they do the right ear. Yes, the difference in arrival time is very small at the speed of sound, but there is a difference.

Furthermore, your head blocks some of the sound coming from the left side as it moves over to the right ear. So the right ear receives the sound slightly later and slightly less loud. Your brain is able to interpret these differences in arrival times and volume to give you a stereoscopic auditory image.

To some degree, the relative asymmetry of the pinnae and auditory canals also allows you to differentiate sounds coming from above and below as well. So by comparing the signals between your right and left ears, you can localize where a sound is coming from in space. (It is more complicated that this as studies have shown that it isn't simply the position of the ears that determines individual's ability to discriminate the source of a sound. - for example, Claes, et al., 2015).

So your perception of space in the auditory landscape is dependent on the position of the sound source and the difference in arrival time and intensity at each ear and the ability of your brain to assimilate and interpret that information correctly.


Localizing Space in Recorded Sounds


But now stop and think about listening to a recording.  For simplicity, let's imagine you are wearing headphones and listening to a stereo recording.  The relative position of the speakers does not change so to your ears, the sound source for each ear is the same.  Furthermore, your left ear and right ear are hearing different recordings (tracks).  Therefore, the only mechanism you have for localizing the source of a sound is to compare the sound arriving in each ear and extrapolate distance information from those differences.

Here's a simple demonstration of how this works.

Here is a recording of a Cajun Chorus Frog




It is playing exactly the same recording in both of your ears (both channels) therefore you can't localize where the sound is coming from or your brain will tell you it is coming from right in front of you.

Now listen to this version of exactly the same recording.  In the first call, the frog is clearly in the center.  In the second it sounds like it is coming from the right and in the third it is coming from the left.  




So why does it sound different? It isn't just a matter of me turning down the signal from the left then right speakers.  The sound is coming into both speakers.  You can prove this to yourself by removing the headphone from your left ear and just listening through the right ear and vice versa.

So why does exactly the same recording sound like the frog has moved from the center to the right and to the left?  Simple.  I took the two channels of the mono recording and added an extra 1/1000th of a second of silence in front of the left channel in the middle call and the right channel in the last call.  So at first the two channels are playing simultaneously, then during the second call the left channel is playing 1/1000th of a second later than the right, and in the third call the right channel is playing 1/1000th of a second later than the left.  So when the sound is getting to your right ear a fraction of a second later your brain tells you the sound is "coming from" your left.

Last little bit of brain trickery....now listen to this frog (again same original recording).

 



It starts off in front of you but gets further and further away to the left with each call. Of course the frog and your speakers aren't moving.  The first call is arriving at both ears simultaneously and at equal volumes.  The last three calls are arriving 1/1000th of a second later to the right ear (putting the frog on your left) and then each subsequent call is reduced in volume on the right channel 1db.  Your brain interprets that reduction in sound in the right ear relative to the left as a measure of distance.

So your brain is localizing the position of a sound source by the difference in arrival time and volume between your ears.  Of course, your ears (pinnae) point forward on your head, so you can actually differentiate whether sounds are coming from behind your or in front of you (sounds in front of you will sound louder).  Therefore sophisticated sound systems often rely on 5 or even 7 speakers to create that sense of sound localization called surround sound.


Recording in Stereo


Recording sounds in stereo is really a bit of trickery.  You have to record the sounds in such a way that the human ears will feel like they are there in the actual environment.  So you have to control the timing and volume of the sound coming into each microphone in such a way to approximate the experience the listener would have had in the field.

This is achieved by using multiple microphones and positioning the microphones so you can control the timing of the arrival of sound to each microphone.  Some of the different mechanisms have involved positioning two microphones close to each other but facing the opposite directions (at various angles), facing each other at various angles, spaced apart different distances, etc., etc., etc.  Then there are the various methods of baffling the sound between the two microphones in order to approximate the effect of the human head.  Other approaches have included things like making fake heads and putting mics in the ears.  There are lots of good discussions of the pros and cons of different methods online.

One popular approach among nature recordists is to place the microphones in an arrangement called a SASS (Stereo Ambient Sampling System) array.  This type of arrangement is known for producing a realistic stereo effect and also has the added advantage of amplifying the signal at the same time.  Vicky Powys has a great discussion of SASS and how it works for field recording on her wonderful blog, the Capertee Birder.


My Stereo Setup


My first forays into stereo recording were accomplished by unplugging my shotgun microphone from my Olympus LS-10 or LS-11 recorders and recording using the XY stereo microphones which came on the recorder itself. 
Here is a recording of a chorus of Hurter's Spadefeet calling from a flooded area in DeWitt County, Texas. 





This gave a satisfactory stereo image and you can clearly hear that the sounds are coming from two sides.  But the image is rather "narrow".  The frogs don't seem to occupy much space in the auditory landscape.  I found myself wanting to get a better sense of space and more amplification.  Inspired by examples built by Vicky Powys, I decided to make my own "field hardy" SASS unit.  Of course, I am not any kind of handyman and the idea of cutting wood at precise angles, etc., was very unappealing.  So I decided to try to make a "sort of" SASS unit out of a dense foam Yoga Block.  I downloaded the SASS dimensions from Vicky's blog and shrunk them down to fit on the largest yoga block I could find and I made my unit from there.  After I made it, I was happy to see that Curt Olson had already established that smaller units will provide good stereo images.  



My SASS array isn't pretty (I'm wondering is I shouldn't call it the HALF-sASS ;-)).  But it does give me a much better stereo image as well as a boost in gain (volume).  And I invested all of about 20 minutes making it and $4 for the Yoga Block.   Furthermore, it is one unbreakable piece and only weighs 5oz (142 grams) total so it is great for travel.
The green foam in the "nosepiece" is intended to allow a bit more sound to pass between the left and right sides than the dense foam of the yoga block.  The nosepiece is essentially hollow but blocked with that more open foam (I just hollowed out the center with a knife).  The pink things sticking out under the microphone are sections of rubber bands that hold the microphones flush in place at the edge of the yoga block (without these, the mics are a bit loose and too easily pulled out while positioning the unit).  The blue rubber band is just to hold the excess microphone cable out of the way while in use.  I thought about attaching a tripod screw or quick release plate, but I generally just put it on the ground, a stump or even hold it in my hands for shorter recordings.
  
For microphones I use a stereo pair of small EM-172 based microphones I bought from Micbooster.com.  These EM-172 mics are very popular among nature recordists for being very quiet.  You could buy the small mic capsules yourself and make your own microphones for less money but as I said, I am not handy and the Micbooster set comes pre-assembled - no soldering required!

The results from the SASS array are much more pleasing to my ear.  This is a chorus of several species of frog (Gray Treefrogs, Spring Peepers, Cajun Chorus Frogs and Southern Leopard Frogs) in Davy Crockett National Forest in East Texas in March,  2016 with version I of my foam SASS unit.  You can hear how much more "width" there is in the recording and I think an improved sense of space.  I am not sure there is enough center in the stereo image so I think some modifications might be in order once I do some research.



To try and give me a better "center" image to my stereo recording, I have also been experimenting with the the ORTF technique (named for the Office de Radiodiffusion-Télévision Française who developed it).  For this technique two cardioid microphones are place 17cm apart (approximating the distance between the human ears) and at 110° angles to each other.   To my ear, this produces a more natural stereo sound that better approximates the experience you hear in the field.  Here's an ORTF recording of a chorus from Guadalupe County, Texas in February 2017 (Cope's Gray Treefrogs, Blanchard's Cricket Frogs, and Southern Leopard Frogs) -




Frogs in Stereo - Is it worth it?


Of course, this begs the question "Is it useful/valuable to record frog choruses in stereo?".   

There are two issues with this:
1.  A frog chorus is generally fairly localized to a small area such as a small pond or drainage ditch.  If you are any distance from this source you really don't get much stereo image since all the calls are effectively coming from one spot.
2.  If you do get a large area or get close to the chorus, the sounds coming from each side/area are roughly the same.  So even though there is a difference in timing and volume in the stereo image you don't get as much sense of space due to the uniformity across the space.   

Here's an example of a situation where recording in stereo adds very little to the ambience of the recording.  This was a very loud chorus of Mexican Spadefeet, Texas Toads and Spotted Chorus Frogs in Schleicher County, Texas.

Here's the stereo version



And here's the mono version of exactly the same recording.



You can hear that there really isn't a huge difference in sound between the two.   So recording this in stereo probably wasn't necessary.

In contrast, when there are fewer frogs more widely spaced, such as these Canyon Treefrogs in the Davis Mountains of West Texas, a stereo recording does add more "information" to the recording.
Here is a stereo recording of these Canyon Treefrogs. You can tell where they were in space and which ones were closer to the recorder (Olympus LS11 by itself in this case). 




And here is the same recording recorded to mono. I think you can agree there is a loss of auditory information in this recording.




In the right situations, stereo recordings of frogs can add a lot to your sense of "being there" which is really the point of any amibence recording.  It is a bit more work to get a good stereo recording, but when it works it is worth the effort to capture the essence of the "frogscape" you experienced.


© Chris Harrison 2017

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 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


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!
video


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:


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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. 


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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 -
http://frogcalls.blogspot.com/2015/03/recording-with-phones-2015.html 

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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 - http://www.wildlife-sound.org/equipment/newcomersguide/usenospeechrec.html


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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.


Microphones


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 Inaturalist.org'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, Herpmapper.org, and Xeno-canto.org (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