To hell with birdies, I want BATS!

Spring has sprung, the grass is riz,
I wonder where the birdies is?
Cancel that - I don’t care,
But when will bats take to the air?

I’ve hugged the fire and shivered lots,
I’ve had enough - I want some hot.

And when it comes, what I want most

Is wakeful bats - not one, a host!

All this tweeting is quite nice,

But I’ll ignore it in a trice.

It’s chips and chops I really like,

Hollow claps, clicks and suchlike.

My detectors polished til it gleams,

Batteries are charged, ready it seems.

‘Tis patience I am really short of,

I need some bats, to adore and love.

So, don’t keep me waiting, little bat,

‘Tis time to make your belly fat.

I’ll wait no more, I can’t, I vow,

So shift your butt and fly right now. 

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The thirty-seven quid bat detector

When I was first involved with bats, there weren't many models of bat detectors available. The Bat Box III was king. A couple of types of time expansion detector were available for those with deep pockets and the low-cost option was the basic Magenta II heterodyne.

The Magenta was also available as a kit, but required some soldering skills. Luckily I'm a dab hand with a soldering iron, so it was my first bat detector, and it did a good job for me, whilst I learned some basic skills.

Today of course there are a plethora of types of detector on the market and the advances in technology over the past couple of decades have been simply amazing. I like to monitor what bat detectors appear for sale on eBay, mostly out of interest and partly in case any stolen ones turn up - I've lost a couple over the years. One machine that keeps appearing is the Haynes bat detector kit. At about £25 this seems and looks thoroughly cheap and nasty, so I've had no experience of it. However, I got to thinking about public bat walks...

To be honest, I'm no enthusiast for doing bat walks - there are much better people than me for talking enthusiastically about bats to groups of the public (TV is fine - they camera looks much friendlier to my mind). But occasionally there's nobody else and we shouldn't squander chances to influence people about bat conservation! 

To give everyone an enjoyable experience you can never have too many bat detectors, but there are two problems: an armful of bat detectors costs a lot of cash. This isn't a problem for me, as I have access to the armoury of a well-equipped bat consultancy, but I'm not keen on handing valuable kit to random people who may, or may not look after it.

So I wondered if I could do something useful with one of these Haynes kits at minimal cost and so £25, including postage saw one landing through my letterbox. It's a very basic heterodyne bat detector and electronically very similar to the old-school Magenta II I first learned on, giving me a pleasant sense of 'coming home'.

It's a very simple kit - a well-made printed circuit board, with plug-in battery connector, microphone and speaker, plus knobs for the two controls It all goes together into the supplied case in about ten minutes (longer perhaps, if you read the instructions properly). And it works perfectly well as a simple heterodyne detector, ideal for bat-walks or kids. 

The microphone is one of the old security ones, which has a pronounced peak in sensitivity around 40kHz. It works ok for Pipistrelles or Myotis bats, but is a bit deaf at the lower frequencies and the chances of picking up a horseshoe bat are probably negligible. But this will never be a serious piece of survey equipment, so why worry? Will they hear bats and be happy? Yes, they will.

It has draw-backs though. First, the case is cardboard. It's coated in some sort of polythene film, but robust, it is not. Secondly, the microphone protrudes from the top, in a manner almost designed to ensure it gets broken off. So, low-cost it may be, but it won't last long. Or will it?

A quick ferret about on Amazon secured me a solid plastic case for £12 (possibly a trawl through the local pound shop might produce a suitable bit of tupperware for less than that). I drilled a hole in the top for the microphone and two in the front for the two control knobs. Finally, a group of small holes allow the speaker to be heard. The nuts on the control shafts hold the circuit board in place and some heavy-duty, double-sided sticky pads hold the battery, speaker and microphone securely in position. I mounted the speaker inside, so that it peers through the hole in the case, thus protecting it. This probably narrows the field in which it picks up bats a little, but it seems ok.

To finish off, I dusted off the label printer from the back of my desk drawer and there we have it. For thirty-seven quid and less than an hour's light work I have a nice, solid bat detector, which should see many year's service, helping people to hear their first bats. It can probably withstand being dropped regularly and smeared in whatever sticky confection junior is munching. And if it gets lost or dropped in a river, at that cost I won't cry about it. 

Given that nearest alternatives cost two or three times as much, I reckon it's worth making a few of them for your local bat group.

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An under-rated bat detector - the Peersonic RPA3

I first bought a Peersonic bat detector in 2014. Having tried it out I rapidly bought a further seven and I’ve just ordered another four. So what is it about this detector that makes me part with money so willingly, in a world which is increasingly cluttered with bat detector models and (in the case of at least one manufacturer) some quite vocal marketing?

Peersonic RPA3 bat detector

When you first pick up a Peersonic RPA3 (the current model) it doesn’t scream “Buy me, use me, I’m a really good detector.” In fact, it looks unprepossessing and, if appearance is a factor in your choice of machine you’ll probably walk away and look at some of the many shiny, well-marketed options available from Anabat, Wildlife Acoustics or Elekon.

Bats, as well as occupying a lot of my spare time are also my profession, so, whilst professional image is important, functionality is what really floats my boat when we’re buying sets of detectors for our team to use. Do we want fancy colours or gadgets? No. Do we want visible sonograms to distract our team members from where we really need them to be focussing - the building they’re surveying? Of course not. So for our purposes the bland cream case of the RPA3 is absolutely fine. (By the way, our supervising ecologists are mobile around our survey sites with machines which DO show live sonograms).

So what does it do? Well, it records full-spectrum WAV sound, whilst allowing you to listen to frequency division or tunable heterodyne output. The optional date and timestamp module does exactly what it says on the tin. Recording is to an SD card, mounted internally and the unit is powered by three AA batteries and we’ve seen no evidence of it being power-hungry.

The screen is small, but displays sufficient information to be useful. It displays the menu items as you scroll through using a rotary knob on the side (which also doubles as the heterodyne tuning control) and a button on each side for enter and back. When in use it displays the peak frequency of the sounds currently heard – useful for on-the-hoof Pipistrelle species ID, it also gives an amplitude reading in dB, which I don’t find too useful, though the matching bar-graph is handy. You can choose to record bat passes manually or automatically and in the latter mode the screen is blocked for a moment or two whilst the machine finishes the recording, which can be mildly irritating.

You’ve got to be tough to do bat surveys north of the border – an RPA3 in action, with hundreds of hungry Scottish midges.

In use we’ve found the detector to be as sensitive as most other detectors currently available and more so than some. It fits nicely into the hand, depending I guess, on the size of your hands. The case has a wrist-strap (yet ours still get dropped occasionally. Go figure.) and a threaded hole on the rear, which allows tripod mounting, if you’re so inclined (Peersonic also make a static monitoring version, which utilises this).

The ability to switch in use between FD and Het. is useful, especially when supported by the peak frequency indication. We tend to use FD mostly, but when Pipistrelle foraging activity is high, the ability to switch to Het, and listen for later-emerging species by tuning down to around 35-40kHz is handy (possibly less so in areas with a broader species assemblage than Scotland). As the machine records in full-spectrum, regardless of what mode you are listening in, this gives a measure of security from missing anything inportant.

One possible weakness, depending on how you use your detector is that the RPA3 lacks a loudspeaker, so you use headphones. Purists will tell you (rightly) that this is the best way to listen to bat calls, if you want to hear them clearly. However, I’m not a purist and I’m not keen on random, possibly dodgy people being able to sneak up behind our surveyors at night. We also like them to be able to hear their two-way radios, so headphones don’t work well for us. And this is where another factor comes into play. Peersonic is a UK-based company, run by engineer, Peter Flory, who isn’t just willing to help with random requests, he positively seems to relish them. So our machines were modified by Peter to work with mono earpieces, rather than stereo headphones.

We have now racked up several thousand surveyor hours using these machines and we’ve really only had three problems, all of which were easily rectified. Firstly, if a machine is dropped (which you really shouldn’t do, but hey, it’s dark, it’s cold and we’re tired…) it will sometimes stop working. However, the fix for this is easy, as it is simply caused by the internal SD card being dislodged. Undo two screws, pop it back in and you’re back in action. We’ve had a couple of machines ‘skipping’, when moving through the menus. This turns out to be caused by the nut on the rotary knob being too tight and slackening it slightly resolved the problem.

One of our survey kits, ready for use with personal radios, torches, batteries and four RPA3 detectors

The only other issue we’ve had was a case cracking slightly around the battery compartment. The problem here was that the battery holder grips the batteries tightly, so we tend to lever out the AA batteries, putting a strain on the case. One of our cases developed some cracks after this had been done a few hundred times and had to be replaced. It’s the only one though and at about £40 it didn’t exactly break the bank, which raises another angle on Peersonic. In an age of built-in redundancy and frequent upgrades Peter assures me that the RPA3 has been designed to be future-proofed and repairable.

So to sum up, the Peersonic RPA3 is a sensitive and effective bat detector, doing everything you need for commercial bat surveys and, but I’ve saved one factor to the end. The price: At the time of writing, Peersonic retail the standard RPA3 at just £245 + VAT and carriage. I’ve told Peter he could and should double that price, but he’s committed to making the machine freely available - for bat group or casual use this is a lot of bat detector for an affordable price. So whatever you do, don’t overlook Peersonic, just because there are shinier machines out there, made by companies with bigger marketing budgets.

Peersonic website: www.peersonic.co.uk/

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"Are heterodyne bat detectors crap?"

One of the interesting things about writing this blog is that the imps who run these things (any Terry Pratchett fan can tell you that all technology is actually run by tiny imps, who are usually very efficient, but can develop "attitude problems") generate clever reports that tell you about the people who look at your site: what countries they come from, what links they followed to get here and so on. An especially interesting part is what people type into Google before being directed here and it's intriguing how many people Google "bat mite" and "bat bug"!

One person recently Googled "are heterodyne bat detectors crap?" and ended up here. It's actually quite a good question and I thought I'd try to provide a relatively straightforward answer. In a word the answer is "no". However that doesn't mean they are ideal for every application. For example, for commercial bat surveys I wouldn't dream of being without a heterodyne detector, but it would be a very poor standard of survey if that was all we used. So let's consider why.

Steph Cope, a former member of our bat survey team, modelling a hand-held bat detector and digital recorder, plus the obligatory Scottish midge net! 

(Steph is now wildlife ranger at the stunning Glengorm Castle Estate on Mull)

A wise man once said that the problem with watching bats is you can't see them and you can't hear them. That's not entirely accurate, but it is basically true. Most European bat calls are ultrasonic, pitched between about 15kHz and 120kHz, depending on the bat species. For most of us 15kHz is close to the highest frequency we can hear. Our maximum varies, depending with age and gender - younger people can hear higher pitched sounds, and women usually have a higher range than men (so I get the short straw on both fronts!). So, if we want to hear bat calls we have to employ electronic means to reduce the frequency of the calls to something we can hear.

If you think back to high school physics classes you'll remember that sounds comprise a flow of waves. The more waves in a given time, the higher the frequency. So what we need is a machine that can reduce the number of waves so that we can hear the bat's call, whilst making it sound as much like the original bat call as possible. There are essentially three ways of doing this. Our unknown Googler's underlying question was probably "what is the best type of bat detector to use?" and sadly there is no perfect detector. Each of the three methods has strengths and weaknesses.

Time expansion  is the purest method and devotees of time expansion bat detectors can be more than a little evangelical about them. The bat's call is replayed at a slower rate (often ten times slower), thus reducing it's frequency. Time expanded calls tend to sound a little like bird chirps and are a perfect representation of the original bat call, making them excellent for computer-based call analysis.

Frequency Division is the third method and reduces frequency by removing a proportion of the waves, often leaving one in eight or one in ten. What is left is a "broad brush" picture of the bat call.

Heterodyne bat detectors mix the bat's call with the product of an oscillator and the two sounds together create a sound we can both hear and make sense of. They have a tuning knob, which allows you to select a frequency band to listen to. However the mixing process renders the output useless for analysis. Most better FD and TE detectors have a heterodyne detector built in as well, so you can have the ease of listening to heterodyne, whilst recording FD or TE for later analysis. A good example of this is the Bat Box Duet, which outputs heterodyne to the loudspeaker/headphones, whilst sending FD to the tape socket.

The Bat Box Duet - an excellent Heterodyne/Frequency Division bat detector.

So, are heterodyne detectors crap? No, but when selecting a bat detector you need to think about what you're going to use it for and select the type of detector that best meets your needs and budget.

Listening to bats - The cheapest bat detectors are simple FD ones, which don't need tuning and thus will allow you to hear all the bats around you without fiddling with controls. Ideal for public bat walks.

Identifying bat calls in the field- For this it is often important to identify the frequency of different parts of the bat's call and for this a heterodyne detector is best.

Confirmation of species identification - To record bat calls for later analysis you'll need either a TE or FD detector. As a general rule TE give much clearer sonograms (a graph of the bat's call, plotting frequency against time) but are much more expensive, whereas FD results in muddier-looking sonograms but are far more affordable. (TE machines are often more fiddly to use as well).

Critical species identification - I always carry a TE detector, which I don't often use, but it's ready if I come across a bat which I think might be something unusual. The high quality of sonograms produced by a TE detector mean that I'll have the best chance of identifying the calls of that special bat!

So to return to the original question "are heterodyne bat detectors crap" the simple answer is no, they're a fantastic tool but depending on what you are trying to achieve you might want a higher level of equipment.

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10 Pet hates of an ecological consultant

I'm a fairly laid back person and I learned long ago that its not worth getting wound up about things. But in my work as a Consultant Ecologist and bat specialist there are certain irritants that crop up time and again. I thought it might be fun to create a top ten, so here it is.

10 - People who think all bats are the same. We have 18 species of bat in the UK and at least nine here in Scotland. Each occupies a distinct niche, with behaviour, habitat, roosting habits etc entirely different from the rest. Why then do people (including some of the poorer consultant ecologists) insist on talking about bats as though they were one unvarying species?

9 - Jokes about "bat man". I like a joke as much as the next person. But I mean, really? Bat man? Satirical wit of the highest calibre it ain't! And guess what? I've heard it before! Just once or thrice.

8 - Rigid adherence to the BCT Bat Survey Guidelines. The word "guideline" gives it away. This document is an excellent starting point in planning a bat survey, but to rigidly adhere to its content, without consideration or understanding is just plain silly. You should be able to explain why you're deviating from the guidelines, but an ecologist who lacks the knowledge and experience to do so also lacks sufficient knowledge and experience to be carrying out bat surveys in the first place. Enough said!

7 - Sub-standard equipment used in bat surveys. It's simple: a good quality broadband bat detector and a means of recording calls during a survey for later confirmation of species if necessary. Yet often commercial bat surveys are done with heterodyne bat detectors, often cheap and nasty ones too. One large Scottish consultancy regularly sends out bat survey teams with only enough bat detectors for half of them to be equipped at all. And that verges on criminal negligence.

6 - Clients who ask for bat surveys at the end of September. Here in Scotland the bat survey season ends at the end of September. We told you that months ago. We wrote it in a report for you. We reminded you in an email. We even explained why. Next time should we tattoo it on your forehead? Would that help?

5 - Attention-seekers who pretend to be scared of bats. A tiny number of people have a genuine phobia about bats. Often they are also phobic about birds. It's hard for them and I'm sympathetic. But I have no time for the much greater number of people who pretend to be phobic because they want to be the centre of attention. I've seen genuine hysteria and it's very different.

4 - Clients who pretend they don't know about a bat roost in the hope we won't notice it. Really? You have 500 Soprano Pipistrelles in your roof and you seriously hoped I wouldn't notice? Not only is it very silly but if you need our professional assistance we need to work together and that involves trust. So stop playing silly buggers...

3 - People calling me a bat expert. The more I work with bats the more I realise how much there is we still don't know about   these amazing and enigmatic creatures. Yes I know some stuff, but I can't echolocate, I can't turn in high speed flight a whisker away from a wall and I can't  slow my heartbeat to once a minute. If you want a real bat expert ask a bat. 

2 - Garden centre bat boxes. There are many designs of bat box around. Some are more successful than others. Some work better for this species or that one or in certain types of location. What is never going to work is the type which is on sale in garden centres and pet shops with a horizontal plywood perch at the base, at a right angle to the entrance slot. The bats need to emerge from the entrance and dive to rapidly gain airspeed. If they do so with one of those bat boxes they'll get a sudden headache! Which is why they are a waste of money.

1 - Overly complicated bat detectors. I'm male, therefore I like gadgets. Big shiny gadgets with lots of buttons and knobs make my eyes go wide with excitement. Some bat detector manufacturers have seen me coming and are making ever more clever and complex machines so I can spend my pennies on shiny toys. But bat detectors aren't like other gadgets. We use them in the dark. At night. When we're cold. And tired. And fed up by the lack of bat activity. And when we keep tripping over things. And we're trying to juggle a detector, a recorder, a torch and a notebook and pen. By all means make them clever and fill them with whistles and bangs. But for heavens sake make them so I can work them in the field without needing a masters in geekiness and a 100 page manual!

So there's my annual rant. Now I can return to being calm, collected and relaxed about my work.

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Ten Things Every Duet Owner Should Know

Last night I was surveying for emerging bats that didn't emerge, so I had plenty of time to think about life, the bats and everything. It occurred to me that the detector I was using, a Bat Box Duet is of the most popular detectors on the market, and rightly so. It's sensitive, selective, ergonomic, robust and sensibly priced.

As with any piece of equipment, there are hints and tips that help the user, some of which I've picked up from other users and some I've worked out myself. So here are my ten top tips for using a Bat Box Duet:

1. Try plugging your headphones into the tape socket, instead of the headphones socket. You will hear the heterodyne detector in your right ear and the frequency division one in your left. Whatever frequency the detector is set to, you will still be aware of bats on other frequencies. The audio level is fixed, so the volume control has no effect: if you're beside a road or river you may struggle to hear over the background noise.

2. If you want to try this technique somewhere where there is some background noise, try plugging the detector into a minidisc machine and plugging your headphones into the minidisc. You should hear the audio from the bat detector and the minidisc volume control will allow you to hear it louder. This is good practice when recording, as it allows you to be confident that the detector output is being recorded properly. It's very frustrating to get to the end of a survey and find you've recorded nothing because you accidentally knocked the stop button or a plug has come out.

3. When the low battery BAT warning appears on the display, don't panic if you haven't brought a spare battery: the detector will continue to function for a short while. Eventually the frequency indication will go haywire and then the detector will switch off the display to save power, but the detector will still operate for a little while longer.

4. Even in that situation you can by without the frequency display. Although you don't know what frequency you're tuned to, if you rub your fingers together, this sound will be loudest at 40kHz, giving you a very rough frequency indication. Of course, its a lot easier just to carry a spare battery!

5. Have you ever wondered why a seemingly random digit appears when you first switch the detector on? This is the software version.

6. If you have a childish sense of humour (like me) you can convince gullible people you have a very clever detector. Using a worn battery, turn the volume high, without headphones. The loudspeaker is the most power-hungry part of the detector so, when a bat is picked up, the extra current consumption will cause the word BAT to appear. When the call ends, the word disappears again. Fun for all the family!

7. Fed up tuning up and down, to ensure you don't miss any bats? Try tuning to 42kHz. You'll hear Myotis calls as a regular machine-gun type call, Common Pipistrelles as an irregular, thudding and Sopranos as an irregular squeaky sound. Noctules can be hear when close, as this frequency is close to the first harmonic of their call. For heaven's sake don't try this for anything important: you will lose a lot of sensitivity, and you may miss something important (especially Horseshoe bats) but it's handy if you just want a general idea of what's happening. I sometimes do this when I set up a Duet on a tripod to record the frequency division output. In that situation I know everything is being safely recorded, so missing the odd call is less important.

8. Did you know there have been some subtle changes in the design of the Duet since it first came out? Earlier versions had a less sensitive microphone, which looks like a small black plastic grille. the later, more sensitive microphone looks like a tiny metal disc, surrounded by a rubber grommet.

9. Another change is the function of the REF button. In earlier versions this produced a steady reference tone. With more recent Duets, this button shunts the microphone to normal audio. This is handy for taking field notes, as anything you say into the detector will be picked up on the left-hand stereo track and recorded (assuming you are recording the survey). Be sparing in it's use: whilst you are talking the frequency division detector is inactive.

10. I only had nine things, so for number ten I'll mention that the Bat Box III has now been revamped into the same style of case as the Duet. I have yet to get my hands on one, but I suspect it will be a very good heterodyne detector. The original Bat Box III was excellent, but it's only weaknesses were poor ergonomics and difficult frequency indication. The new version (Bat Box IIID) fixes both of those issues and will probably be a really good little brother for the Duet.

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Spring Has Sprung....

Three weeks ago I asked whether spring was springing yet, at least in terms of bat behaviour. We can safely say that Spring has now sprung. The chart below shows the night-time temperatures at Gogarbank meteorological station, near Edinburgh, since 1 April. It clearly shows that, after a brief blip at the start o last month, followed by a colder period, the warmer weather is now settling in.

(Met Office Data)

Last night Lothians Bat Group members returned to Blackford Pond and we found a big difference to the events of three weeks ago. For a start, we didn't have to tiptoe through hordes of amorous toads! Of more interest to us was the big increase in bat activity. At least 4 or 5 Daubenton's Bats (Myotis daubetonii) were foraging over the pond and finding plenty of invertebrates to eat. Above the footpaths were many Soprano Pipistrelles (Pipistrellus pygmaeus) and one or two Common Pipistrelles (Pipistrellus pipistrellus).

Whilst we were there, I was able to hook a frequency division bat detector up to a lap-top running Batsound software alongside the pond. Newer members of the group were able to see sonograms of the common species and compare them with the sounds, to help get a handle on the basics of bat call identification.

To give a flavour of the evening, here's a brief video clip showing two of the Daubenton's Bats over the pond last night. You can hear a heterodyne bat detector in the background.

We mustn't leave out the Pipistrelles: the most charming and delightful to watch of our native bat species. Here's a very short clip of a Soprano Pipistrelle feeding just before dawn this week, at another site in Edinburgh.

My website: plecotus.co.uk

Connecting Bat Detectors to Audio Equipment

This is the time of year when bat survey equipment starts to get dusted off and checked over, ready for the new survey season. Top of the list of course is the bat detector ( I can't even begin to understand what it must have been like to survey for bats before portable detectors was available).

Many bat-workers make recordings of bat calls they hear in the field, either so they can have a second attempt at identifying a bat at home, with a glass of something warming or, in the case of frequency division or time expansion detectors, to look at calls using Batsound or other call analysis software. I thought this would be an opportune moment to look at the connections between the detector, the recording equipment and the computer. It seems simple enough, but there are a few easy errors, which are less obvious than might be expected.

To understand how to connect things properly we first need to understand a couple of techy things about jack plugs and sockets:

Jack Plugs
There are several types of audio jack plugs, but virtually everything we are interested in uses 3.5mm jacks. There are two types: mono and stereo. The stereo plug is connected to three wires: one common and one for each of two audio channels (normally used to create that "close your eyes and visualise the orchestra" effect by producing slightly different versions of a music track in each ear). The mono plug has only two wires and, you've guessed it, only one audio channel.

Where it gets slightly tricky is that the mono and stereo jacks are the same size and fit into sockets intended for one another.

Sockets
There are four types of audio socket found on bat detectors and recording equipment:

1. Audio out (also labelled as headphone or speakers) is the socket on the detector or recorder which squirts out sound. The level is controlled by the volume control.
2. Line out (often labelled record) is the same as audio out, but with a steady volume, unaffected by the volume control.
3. Line in (also called audio in or sound input) is the socket on the recorder or PC through which the equipment receives audio.
4. Microphone in is similar to audio input, but is easily overloaded, as microphones produce very low level audio.

That's the geek stuff over. So, why does it matter? Most bat detectors are mono. As the two types of jack plug are interchangeable it doesn't really matter which we use. In fact it's helpful that they're interchangeable as it means stereo headphones can be used. A bog-standard stereo jack to stereo jack lead can be bought at Comet to connect the detector to a digital audio recorder, minidisc recorder or cassette recorder. Incidentally, avoid cassette recorders: the tape speed falls as the battery runs down, turning Common Pipistrelle calls into Nathusius' ones.

It's always advisable to connect the line out soocket on the detector to the line in socket on the recording equipment. That way, the machines take care of the audio levels and we can get on with looking at bats. Life being the way it is, some machines don't have these sockets, so we have to get a bit sneaky.

If there's no line out (or record) socket on the detector, it's necessary to use the headphone socket (and plug the headphones into the recorder's headphone socket, so we can hear what's happening), but it's necessary to make a few experimental recordings to work out what level the detector volume needs to be set to, to produce the clearest recordings.

If there's no line in on the recorder then it's necessary to use the microphone in socket, taking care not to overload it by having the detector volume set too high. The recorder's automatic level control will cause the background white noise to increase when there;s no sound, but it should drop the moment a bat is picked up.

The Bat-Box Duet
Things get more complex when using a Duet. These are excellent detectors, allowing the user to listen to a heterodyne detector, whilst recording what the user hears and the sound from a frequency division detector, one on each stereo channel. This permits later computer analysis. Admittedly, the sonograms produced are not quite as perfect as those made with recordings from a time expansion detector, but they're usually good enough for most purposes and it's much easier to use (anyway, how many hobby bat-workers can afford to pay a grand for a time expansion detector?)


The Duet achieves it's cleverness by using the two stereo channels: left for the frequency division audio and right for the heterodyne audio. As long as the recording equipment is also stereo then a standard stereo jack to stereo jack lead can be used. However, some digital audio recorders only have a mono microphone input. If you put a stereo jack in here the machine will record a weird amalgam of the output of both detectors. A cable which splits the stereo into the two mono outputs is required - see below. You need to go to a specialist shop like Tandy or Maplin, or if you're handy with a soldering iron, make one yourself.

The other difficulty comes when you replay the bat calls into a computer. If you don't have a line in socket on your computer the microphone socket may work, but it will be mono, rather than stereo, so the same cable is required.

Incidentally, if you're buying a minidisk recorder for use with a bat detector, check the line in socket carefully. Most have some sort of audio input, even if there's no line in socket, but a generation of minidisk players exists where the line in socket is a weird digital thing which looks like a 3.5mm jack socket, but simply doesn't work, except to record from other digital equipment.

Of course, all this should soon be academic, as we move towards bat detectors with built-in memory cards. Hopefully that will make redundant the geeky knowledge I acquired from my youth, spent making dodgy cassette recordings of LPs......

The BCT have published a useful guide to recording with digital equipment: http://www.bats.org.uk/helpline/documents/Digitalrecordingwithbatdetectors_005.pdf

More about the Bat-Box Duet: http://www.batbox.com/duet.html

My website: plecotus.co.uk

Finally, if you follow any advice here, you do so at your own risk.

Bats, bombers and acoustic mirrors

One reason why I first started working with bats was that bat work combines my love of wildlife with my longstanding interest in electronics. Fortunately people who do surveys with me tend to tolerate my tendency to come up with strange new ideas or pieces of equipment!

Several years ago I found myself wondering whether parabolic dishes might have a place in bat work. These are used in the recording of bird calls and other wildlife sounds and look similar to TV satellite dishes. In essence, the dish reflects sounds into a focal point, where a microphone is placed. This has two effects:

1. Sounds are concentrated by the dish, making them louder.

2. Sounds are picked up from a narrow direction, filtering out background sounds from either side.

Incidentally, not just any dish will work: a parabola is a specific curve, calculated using a mathematical formula.

I experimented with an old Sky TV dish and found that when I put a bat detector at the focal point of the dish, bat calls became louder and so could be heard from further away. However, it was clear that this would have limited practical uses, as it only worked when the dish was pointed directly at a distant bat. People recording bird calls can see a bird and point a dish at it. Bats don't allow that luxury!

More recently I became aware of something called a parabolic plane. This is sometimes used in microwave radio applications. It is like a dish, but only curved in one plane, a bit like Rolf Harris's wobble board. Between the wars gigantic parabolic planes up to 200 feet long by 26 feet high were built at strategic sites around Britain's coast. These "acoustic mirrors" were used to listen for approaching bombers: operators could hear an aircraft from 20 miles away. Unfortunately they were never used for their intended purpose: by the time World War II broke out, radar had taken over.

Image (c) English Heritage

The beauty of a parabolic plane is that it only concentrates sound from the horizontal plane, so that a bat flying in front of one at any height should be heard. It could be possible to create a kind of listening curtain: in theory any bat passing through it would be detected. To test this I made up a prototype reflector, using foam board, card and sticky-backed plastic (eat your heart out, Blue Peter) and tried it, using a low-level ultrasound source as an artificial bat.



The prototype is only 60cm by 20cm, so I wasn't expecting miracles, but tests at varying ultrasound levels showed that, using the reflector a Duet bat detector could pick up the artificial bat between 30% and 70% further away than it could on it's own. Positioning of the detector microphone is critical, as it needs to be at the precise focal point of the reflector for best results. The half-brick will need to be replaced by something with a bit more finesse!

There is some interesting potential for parabolic planes, but there are some potential draw-backs too. In theory, the bigger the reflector, the further away you can hear bats. But a bigger reflector also means the bat would be audible for a shorter time, passing through a narrower curtain. (The acoustic mirrors could hear aircraft from 20 miles away and place them to within 1.5 degrees.) Ideally a bat needs to be audible for 1-2 seconds, to be sure enough of it's call is
recorded, to allow for identification, so this may limit possible size..

Another problem is the practicality of lugging a big reflector around. Even using it in a fixed position, more than 2-3 metres in width would probably be impractical, unless as a permanent fixture. Any fixed-location survey involves predicting where the bats will fly, and they don't alway read the same books as us!

More about acoustic mirrors: http://www.ajg41.clara.co.uk/mirrors/

My website: Plecotus.co.uk

The Versatile Bat Detector

The most basic tool of the trade for a bat worker must be the heterodyne bat detector: the pocket-caculator look-alike that allows us to listen in to the sounds made by bats. Not only does it help us find bats, it often allows us to identify them to genus or species, to tell when they are feeding and to hear their social calls. It would be easy to be satisfied with that little lot, but the humble detector has other uses too.

Being pedantic, "bat detector" is the wrong name for the machine, as it implies that all you will hear on it are bats. This may stem from our tendency to talk about ultrasound as though it were something special and magical. In reality, ultrasound is simply sound above about 20kHz in frequency. The only thing that sets it apart from sounds below that frequency is the fact that one rather arrogant species with an abysmally poor hearing range can't hear it!

Many everyday things produce ultrasound: pouring water creates a loud noise around 40kHz. The standard test of whether a detector is switched on is to rub your fingers together in front of the microphone, a sound which is much louder around 45kHz than it is at frequencies within our hearing range. "Dry", rustling sounds tend to be much louder at higher frequencies: when our alsatian jumped into a huge pile of dead leaves behind me whilst I was using a detector with headphones, I though I was going to be deaf for life...

Bats aren't the only species which produce ultrasound. A common summer hedgerow sound is the high-pitched, furious squeaking of squabbling shrews. The lowest frequency of that call fall just withing our hearing range (older people may struggle to hear it at all), but on a bat detector tuned to about 20-40kHz they are very clear. In 2005 I and some colleagues from the BATML project were conducting a series of surveys along the towpath of the Union Canal. We kept hearing shrew sounds from one particular place by the water's edge. It was noticeable because these calls were extremely loud and unexpected. When you've had cause to swear loudly at something, it sticks in your mind! The Mammal Society were running their nationwide Water Shrew (Neomys fodiens) survey at the time, so I placed some tubes, baited with casters (blowfly pupae) and sure enough, the scats left in them were confirmed as having been left by a Water Shrew. This was particularly nice as it was the first record of this species from the Lothians for almost 20 years.

Another branch of natural history in which bat detectors are increasingly finding a place is in the study of Orthoptera: grasshoppers and crickets. It is possible to accurately identify them to species by listening to their sounds. Although many can be heard with the naked ear, they are clearer and louder when heard on a bat detector. Unlike bat (or shrew) calls, these sounds have no frequency variation, as they are stridulations, rather than calls, i.e. the sounds are made by rubbing legs together. Identification is made by listening to the rhythm of the sounds. As the insects tend to stay in one place whilst stridulating you can use the detector as a direction-finder to home in on them.

There's a good introduction to Orthoptera id using a bat detector on the web-site of the Environmental Records Centre for Cornwall and the Isles of Scilly (ERCCIS), along with a lot of other useful resources: http://www.erccis.co.uk/species/orthopteraaudio.htm

The Mammal Society's water shrew survey page: http://www.abdn.ac.uk/mammal/water_shrew_survey.shtml

The Bat Conservation Trust (BCT) website has a guide to choosing a bat detector. The models mentioned are mostly out of date now, but the technical advice is very sound: http://www.bats.org.uk/helpline/documents/Whichbatdetector.pdf

For what it's worth, I rate Bat-Box detectors as the best. They have the ideal balance of cost, effectiveness and sturdiness in the field. They're slightly cheaper if you buy direct: http://www.batbox.com/

My website: plecotus.co.uk