Category Archives: bats

Listening to Bats, Part 1: The Joy of Heterodyne

Two weeks ago I wrote a guest blog for the ‘Dispatches from the Field’ blog about a typical night of listening to bats in the field. In that post I briefly mentioned the equipment I use to listen to bats, including my trusty heterodyne bat detector, but I thought it would be worth following up with some more detail about how we can listen to bats’ echolocation.

The Daubenton's bat has it's mouth open to emit echolocation calls before taking off. Normally, we cannot hear the high frequency calls, but bat detectors let us hear what it is saying!
The Daubenton’s bat has it’s mouth open to emit echolocation calls before taking off. Normally, we cannot hear the high frequency calls, but bat detectors let us hear what it is saying!

The vocalisations that bats produce for echolocation – to find their way around in the dark – can range from 20 kHz up to around 120 kHz. Sounds above 20 kHz are known as ultrasound. That just means they are above the range of human hearing, which ranges from about 20 Hz to 20 kHz, although our upper range drops off with age and loud music! There are a variety of technologies that can covert the bats’ ultrasound into our hearing range, or allow us to record it.

The most relevant options are heterodyne, which I will write about today, and full-frequency (also known as full-spectrum or just high frequency recording), which I will write about in another post. You may also hear of a few other options, namely frequency division, zero crossing and time expansion. These last three are still in use, but have been somewhat superseded by lower cost full-frequency devices, so are probably of less interest to someone learning about bats for the first time.

What is heterodyne?

Heterodyne bat detectors are among the cheapest bat detectors available, but perhaps conceptually the most difficult to explain. Heterodyning is a method of signal processing where two frequencies are combined to create new frequencies, which is exactly what a heterodyne bat detector does. It combines the frequency emitted by nearby bats, as detected by an ultrasound sensitive microphone, with an internally generated signal, which is usually adjustable for reasons that should be come clear.

The heterodyne circuit inside the detector multiplies the two signals together and, due to the physics of wave multiplication, this results in signal comprised of a high frequency wave equal to the sum of the two signals, superimposed on a low frequency wave equal to the difference between the two original signals. If you don’t follow this, don’t worry – you don’t need to know how the detector works in order to use it, I just wanted to give some background. If you’re interested to read more about this mechanism, you can read more here.

A rough simulation of a heterodyne bat detector I made in Microsoft Excel. The bat signal at 45 kHz (yellow) is multiplied with the detector's internal frequency at 50 kHz (white). The result is a wave of 95 kHz superimposed on a wave of 5 kHz.
A rough simulation of a heterodyne bat detector I made in Microsoft Excel. The bat signal at 45 kHz (yellow) is multiplied with the detector’s internal frequency at 50 kHz (white). The result is a wave of 95 kHz superimposed on a wave of 5 kHz.

Even if you don’t fully understand how the detector works, the important point to take away is that the frequency of sounds you hear is equal to the difference between the sound detected by the microphone, and the frequency to which you have tuned the detector. For example, if a bat flies past echolocating at 45 kHz, and your detector is tuned to 50 kHz, the sound you hear will be ~ 5 kHz. In theory, if the bat and the detector were tuned the same, the difference would be 0 Hz, but in reality that doesn’t come up when using a heterodyne circuit with bats.

How to use a heterodyne detector:

While the physics behind a heterodyne bat detector may seem complicated, using them is very simple. Most have only two controls to fiddle with. Firstly, a gain (sometimes volume) dial to adjust amplification, which often incorporates the on/off switch. Secondly, there is a frequency that can be adjusted through the range of frequencies the detector can cover (although microphone sensitivity often varies across this range).

To use the detector, one simply turns it on and adjusts the gain to a comfortable level. I usually adjust my gain so that the background hiss is audible, but not intrusive. If there are no obliging bats around, it’s easy to test the gain level by tuning the frequency to the 30 – 40 kHz range and lightly rubbing your fingers together in front of the microphone. You should hear the sound clearly through the detector, confirming that it is functioning and you have sufficient gain to hear bats.

Another bat echolocating in the hand - this bat is using echolocation to examine its surroundings before taking off.
Another bat echolocating in the hand – this bat is using echolocation to examine its surroundings before taking off.

Once the gain is set, you simply adjust the frequency dial to the frequency you wish to listen to. The detector only converts a narrow band of frequencies close to that selected, and so you should tune to the approximate frequency of the bat species you hope to hear. The best frequencies to try will vary depending on your local bat species, but in Europe I would suggest starting at around 45 kHz, but occasionally tuning down through the 30s and 20s as well. In North America, 25 – 30 kHz is a safe bet in many places.

With everything set, it is a case of finding some bats. While listening with the bat detector you will find that many things produce ultrasonic noise. Some ultrasonically ‘noisy’ things include: your clothes rustling, moving through long grass, camera flashes, computers, bicycles and jangling keys. However, once you hear a bat, the sound is quite unmistakable. The exact sound of a bat through a heterodyne detector varies with the detectors tuning, but you can listen to an example of bat echolocation here.

The final step in properly using a heterodyne detector is learning to tune once you hear a bat. When you hear a bat, you should gently adjust the frequency dial back and forth, looking for the deepest tone. Due to the heterodyning mechanism, the frequency with the deepest tone is approximately equal to the peak frequency of the bat’s echolocation. The peak frequency often varies between species and can be useful for identification. I find that the best way to approach this is to tune without looking at the detector, and then only looking at the frequency setting once I am confident that I have found the peak. Of course this step is much easier with a bat circling obligingly above your head, and more challenging with a bat that races past only audible for a few seconds!

Identifying Species:

Once you know the sound, it becomes very easy to identify the presence of a bat using a heterodyne detector. However, we often want to know more that this, in particular the species of bat that we’re listening to. This is where using a heterodyne detector becomes more difficult. With practice, it is often possible to identify bats with reasonable confidence. Unfortunately, gaining this practice can be difficult, as ideally you need to learn the different sounds with something to tell you what species they are! If you live in the UK, the best option is probably to join your local bat group, where you can likely find someone willing to teach you. If you live elsewhere, and do not have any local group to join, you make have to do your own research and figure things out for yourself. A good place to start is to look up the peak echolocation frequency of species known to be in your area.

Disadvantages:

While I like to preach the virtues of heterodyne detectors, it would be wrong to ignore their disadvantages. There are three major disadvantages to heterodyne:

Firstly, although a practiced ear can often get an idea of species from heterodyne, it is not always possible. Species with larger overlaps in their call repertoires are particularly problematic. It is also difficult to identify bats that pass by only briefly and aren’t audible for long enough to reach a solid conclusion, which is compounded because…

Secondly it isn’t possible to record from a heterodyne detector. Or at least, it is possible to record the output, but it’s mostly useless because the output depends on what the detector happened to be tuned to, information that is not included with the recording. Without the ability to record bats, brief passes cannot be recorded and reviewed later, and identifications cannot be verified later.

Finally, heterodyne detectors are disadvantaged by their narrow frequency range. Because the detector only converts sounds close to the frequency it is tuned to, it will miss bats echolocating at other frequencies. This is particularly problematic in an area where species using a wide range of frequencies are present.

Buying Options:

There are a range of companies manufacturing relatively affordable heterodyne bat detectors. I have not personally tested them all and so cannot give an objective view of the full range of options. However, I will mention two companies whose products I have extensive experience with, and wouldn’t hesitate to recommend.

My thirteen year old Magenta MKII
My thirteen year old Magenta MKII

Magenta Electronics supplied my first bat detector, the MKII (apparently still available as a kit), which is a good starter detector and is still going thirteen years later as a spare detector that I can lend to people at public bat walks. The MKII is reasonably robust, but probably not as much as other heterodynes. The most significant limitation is limited sensitivity at lower frequencies (~ 20 – 30 kHz) making it less suited for North American bats in particular. However, Magenta also produce two newer detectors with significantly improved robustness and quality. I don’t own one of the newer Magentas, but they are very popular in the UK, and I have recommended them for multiple public outreach projects in Canada where they have proved very effective (if you happen to live in Toronto you can even borrow one from High Park Nature Centre’s ‘Bat Detector Library’!).

My beloved Batbox Duet.
My beloved Batbox Duet.

I also own, and still use after more than 10 years, a Duet detector manufactured by Batbox. The Duet has travelled around the world with me for more than a decade, and has never once let me down, even after a few rainy nights. The Duet is so named because it has both heterodyne and frequency division capability. In my opinion the decreasing cost of full-spectrum detectors has made frequency division largely obsolete (though I’m sure someone out there disagrees). That being so, these days I would buy a Batbox III D, which looks identical to the Duet but without frequency division, and put the money saved toward a full-spectrum detector (which I will write about more next time).

Conclusion:

I strongly believe that watching bats fly while listening with a heterodyne detector is one of the best introductions to bats, as well as an indispensable tool for experienced bat surveyors. Heterodyne detectors are simple to use, and provide an instantaneous translation of the sounds the bat is making while leaving your eyes free too look up – as you should be doing. With the volume up loud, the bat can be heard by a group of people together, and the speaker causes the detector to shake in the listener’s hand. A heterodyne detector provides a novel and visceral interaction with some amazing wildlife.

Heterodyne detectors are also relatively affordable, so if you’ve never tried looking for bats before, or you’ve seen them in flying around your garden and want to know more, it’s well worth looking into getting one!

What did the bat say? A night of listening in the dark.

I was invited to write a post about my fieldwork experience with bat for the excellent ‘Dispatches from the Field‘ blog and am cross posting it here. Or you can read it on their site here!

Bats are rather mysterious creatures. Most people I talk to about bats think they are interesting, but don’t know much about them. It’s not all that surprising; unlike more obvious wildlife such as birds, people don’t encounter bats very often. After all, they are active at night, when people like to sleep, and even then it’s hard to spot small and flying creatures in the dark.

A Natterer’s bat taking from a tree roost in the UK.
A Natterer’s bat taking from a tree roost in the UK.

Happily, surveying bats is easier than you might think. Most bats (and all the ones found in Canada), find their way around in the dark using echolocation: emitting sounds and listening to the echoes to avoid obstacles and find prey. We don’t hear these sounds because they are at higher frequencies than we are able to hear, but a variety of devices known as ‘bat detectors’ can convert the sound into our hearing range, or record the bats (there are several more affordable bat detectors available if you want to learn more about your own bats!). With a bat detector in hand it is as if the bats are flying around shouting ‘I’m here! I’m here!’ What’s more, different species make different sounds, so it’s sometimes possible to identify the species of a bat based on its echolocation calls – though some species are more difficult than others, and the technologies, although improving, are far from perfect!

A big brown bat – in my experience the most commonly encountered bat in southern Ontario. If you have bats in your garden or home there’s a good chance it’s one of these! Please note that the bat’s mouth is open to facilitate echolocation more than aggression.
A big brown bat – in my experience the most commonly encountered bat in southern Ontario. If you have bats in your garden or home there’s a good chance it’s one of these! Please note that the bat’s mouth is open to facilitate echolocation more than aggression.

I’ve listened to bats for a variety of projects, along with my own enjoyment, including recording several million calls during my master’s research. Last year (2015) I was approached by Thames Talbot Land Trust to survey for bats at Hawk Cliff Woods, a site they are in the process of acquiring. Before heading out to survey, I used maps and satellite images in order to pick a transect route to walk. Hawk Cliff has a mix of forest, meadow and a field that is currently being farmed, and I wanted to cover as much as possible as different bat species have differing habitat preferences.

I arrived a little before sunset, giving myself some time to look around in daylight. I also took the opportunity to record a few waymarks that I thought would be difficult to locate in the dark with my GPS. Then it was time to get my kit together, and wait for the bats to arrive. I had a heterodyne ‘bat detector’, which allows me to listen live to bats’ echolocation and get a constant idea of activity – even when I can’t see anything. I also had an ultrasound sensitive microphone that records onto a tiny laptop in my backpack, I can review these recordings later to confirm species that I missed or couldn’t identify on heterodyne. I also had a smartphone with GPS and maps, to find my way and record it so I can geotag my recordings later. Lastly, I had a notebook and a couple of flashlights – nocturnal biologists love flashlights!

At Hawk Cliff setting off on my transect.
At Hawk Cliff setting off on my transect.

I try and time the beginning of my survey to be when bats start to become active, although this varies depending on how close you are to a roost and how dark it is, cloudy vs. brightly moonlit light for example. Activity at twilight is perhaps the most important. At this time, it’s unlikely that bats have travelled far from their roosts, so hearing them early suggests they are living close by. Also, for a brief time at twilight enough light remains that I can see the bats. This is something anyone can do without any special equipment; just go out in the twilight and look up (obviously this works best in the mid-summer, when bats are most active, and is pointless in winter). Bats’ fluttery flight is quite distinct from the more direct flight of birds. I love watching the aerobatics of bats in the dusk light, and it’s often possible to identify if bats are close to a roost – they sometimes fly around the entrance – or are feeding, characterised by sudden, rapid changes in direction to intercept prey on the wing. On my transect at Hawk Cliff, twilight activity was mostly made up of big brown bats, the species I encounter most in Ontario, with a few eastern red bats bringing variety.

As the darkness draws in it becomes impossible to see bats even when they’re flying over my head. But with my bat detectors in hand I can still tell when they are there. Activity can become less intense as the night goes on. Many species gorge themselves at sunset and then take things easier for the rest of the night. Activity is also diluted as individuals disperse through the landscape.

A closer look at my bat detecting setup. In my hand is my heterodyne detector, with my ultrasound microphone attached to the side with highly technical elastic bands. I usually like to have my laptop running in my backpack so I’m not distracted by the screen, but here I have it out to check the microphone is functioning correctly. It is, as demonstrated by the big brown bat echolocation calls visible in the spectrogram on the screen!
A closer look at my bat detecting setup. In my hand is my heterodyne detector, with my ultrasound microphone attached to the side with highly technical elastic bands. I usually like to have my laptop running in my backpack so I’m not distracted by the screen, but here I have it out to check the microphone is functioning correctly. It is, as demonstrated by the big brown bat echolocation calls visible in the spectrogram on the screen!

Finding my way at night, even at sites I am familiar with, often leads to interesting times. At Hawk Cliff I had planned part of my transect to pass through the forest. For the first half of this section I was able to follow a trail, and things were easy. I then crossed a ravine, luckily without too much difficulty. Finding my way out of the forest was more difficult as there was no trail, but I made it with the minimum of bushwhacking. However, things were more complicated a few weeks later when I returned to repeat my survey. This time the ravine was filled with goldenrod that had grown up above my head, and it took me almost forty minutes to find a safe route to cross. The effort was worthwhile though, as in and around the forest I heard several rarer species, such as the threatened little brown bat.

Another picture of a big brown bat, showing the wing extended and the general size of the bat. This bat can give a painful bite, which is why I am wearing a thick glove.
Another picture of a big brown bat, showing the wing extended and the general size of the bat. This bat can give a painful bite, which is why I am wearing a thick glove.

I usually wrap things up a few hours after sunset. Bat activity is usually lower through the middle of the night, so it’s a case of diminishing returns – and I still need to sleep. After a survey it’s nice to head home to bed – although at Hawk Cliff, that wasn’t to be. To maximise my coverage I had decided to stay the night and repeat the transect at dawn, so I bedded down in a sleeping bag in the back of the car instead…

Even when I did get home, there was still work to do. I have to go through the recordings from my microphone to confirm the identifications of bats I heard and then cross reference them with my GPS file to get a map of bat activity. I like to do this step as soon as I can, while observations are still fresh in my mind, and then write my final report.

A picture of a little brown bat along with a spectrogram of its echolocation call. Spectrograms are visual representations of the frequency, temporal and amplitude characteristics that are used by bat biologists to try and identify species based on echolocation calls. Again, the bat’s mouth is open to facilitate echolocation, rather than in aggression.
A picture of a little brown bat along with a spectrogram of its echolocation call. Spectrograms are visual representations of the frequency, temporal and amplitude characteristics that are used by bat biologists to try and identify species based on echolocation calls. Again, the bat’s mouth is open to facilitate echolocation, rather than in aggression.

Learning to listen to bats is a great way to experience wildlife in a rather novel way. Of course there is a limit to what we can learn just by listening and to answer some questions we have to catch bats, and I enjoy doing that too. However, I’ll always love seeing bats in their element – even if sometimes my ears have to do the seeing.

Thanks again to ‘Dispatches from the Field‘, for inviting me to write about my fieldwork experiences.

Night Rider

As I cycled along the dark gravel lane with only my dimming head torch for illumination, aboard a borrowed bike with a single semi-functioning brake, I had to face the possibility that this probably wasn’t normal behaviour.

Night Bike
My rusty steed.

The lane I was travelling circumnavigates the tiny landmass of Amherst Island, which I was visiting in an unsuccessful attempt to catch bats. Having spent the past two nights watching bats flying over, around but never into my nets, I decided my time would be better spent just watching bats and not worrying about trying to trap them. I borrowed a bike from my hosts and headed out. I took my Batbox Duet, which is still going strong after 10 years in the field. This is a heterodyne type detector that allows me to listen to bats in real time. While there have been a number of exciting developments in the world of bat detectors there is something about the immediacy and musical quality of heterodyne that I enjoy.

Watching bats in flight and listening to their calls is always a worthwhile exercise. The rapid twisting and turning of a bat catching airborne insects, combined with the frenetic sound of its echolocation calls never fails to return me to the initial fascination I experienced the first time I saw it.

Big brown bat
A big brown bat I photographed earlier in the year.

As I headed around, with one hand on the functioning brake lever and my other holding on to my bat detector, I listened to the bats of the island – and there were many! It was rare for more than a couple of minutes to pass without hearing one and much of the time the calls were continuous. Mostly I heard big-brown bats. Sadly my confidence with identifying North American bats on a heterodyne detector extends only to distinguishing calls as ‘big-brown’ or ‘something else (unless it’s a strange big-brown)’, however I did also hear a number of bats in the something else category.

Night bike bars
50% of these levers do things.

Much of my enjoyment from observing the bats of the world comes from knowing that most people forget that they are there. How many people give thought to the acrobats flying above them, as they drive along at night or lie in bed? Learning to find bats is like being let into a secret, or finding another world – except rather than being at the back of a wardrobe this one is just a few meters above our heads.

Being out at night also brings one into the realms of other creatures, unlikely to be encountered during the day. Pursuing bats mean that I saw much other nocturnal wildlife in the UK, including deer, badgers, foxes, hedgehogs and glis glis. Of course there must have been much more that I missed as I stomped noisily through the woods checking bat traps. On my ride around Amherst, I saw several groups of skittish deer, what I think was a mink and a number of domestic cats. I did not see any of the coyotes, perpetual enemies of the islands sheep farmers, which was a little disappointing if not unexpected.

Night drive

To bring things to a close, I would encourage anyone who normally wastes their nights sleeping to head out and explore the new world on your doorstep. There is much to see, but if nothing else it’s the best time to watch bats. Doing so from a mechanically questionable bicycle is entirely optional.

The Beginning

I have given a number of talks recently, both academic to the general public, and have frequently used the story of the beginning of my interest in bats as an introduction to get people’s interest. Having recounted the tale an unusual number of times recently I was struck, as I came to think of a topic for the first post of my revived blog, that this could be a good opportunity to put things down in detail. It runs the risk of sounding somewhat self-obsessed, but I think my experience can be used to illustrate a wider point, and I’ll aim to do so.

Young me
Me around the age I first became interested in bats.

My interest in the natural world has existed as long as I can remember, but the point at which it became focused on bats is well defined. It wasn’t, however, on the first occasion that I encountered a bat. Around the age of five I went on a birthday trip to a railway museum and as we walked to the entrance we encountered a bat lying on the ground. Clearly that is not a healthy position for the bat, but I was excited by the prospect of trains and paid little heed – my father moved it from the path to some bushes and we carried on*.

Several years later bats got another chance. On a summers evening, at the age of eleven, my mother took me along to a ‘bat walk’ in our local park. There I listened to a talk about bats by the fledgling North Buckinghamshire Bat Group, followed by a walk around the park with bat detectors to listen to the bats as they flew above us. This clearly made some sort of impression on me, and so the next week we travelled across town to attend another bat walk and listen to bats in a different park. A week later we headed out once again, this time joining the bat group to capture bats as they flew from a roost inside a church. I was fascinated by the opportunity to see these strange animals, of which I had previously known almost nothing, up close and personal. To top it all I was allowed, with gloves, to hold a bat and examine it for myself.

Bedgrove Spinney
The park where I first watched bats.

I am always inclined to consider the time I first held a bat as the key moment that sparked my interest, however it would be wrong to ignore another factor. Following that first contact, the infatuation with bats that I had gained grew into a full on love affair (if you’ll excuse the crummy metaphor), as I learned to survey for, identify and catch them. The credit for most of that growth belongs to the myriad individuals who educated and encouraged me, or if nothing else went out of their ways to give me lifts to and from bat events around the county. If any of those people should read this they will doubtless know who they are, and I am grateful.

Natterer's bat
A natterer’s bat, the same species as the first bat I held.

This brings me to what I think is the important point. The interest in bats that has grown to consume much of my time, indeed has become my day job, resulted from a chance counter reinforced by a wealth of encouragement. If the people who provided that encouragement hadn’t, I might have had a much less interesting life. This is reason that I am always keen to give and arrange public talks about bats and my work with them, and I would encourage others (many already do) to do the same. Bats and the natural world are more in need of champions than ever – and you never know how far the next person you share your enthusiasm with will take it!

*If anyone in the UK encounters a grounded or injured bat I would encourage you to call the Bat Conservation Trust helpline, and they will endeavour to find someone to come and look after it. You should avoid touching the bat yourself.