Goliath - the Daddy of harp traps

Working with bats can be a technical business, with bat detectors, recording equipment, passive monitoring equipment, traps, nets, kit for weighing and measuring etc. I am fairly practical and enjoy the challenge of building equipment where commercial solutions aren't available or if I feel they could be improved on.

One type of equipment I have worked on quite a bit is the harp trap, also known as  the Tuttle trap. In 2008 I built a fixed frame harp trap, for use at roost entrances, which proved to be a great success and since then it has been used regularly, safely catching many bats (see "The kitchen table harp trap", January 2008 and "Happy harp trapping", September 2008).

Since then I have turned my attention to methods of catching bats in free flight. Traditionally the two ways of achieving this are to use mist nets, in a very similar way to bird ringers or to use larger harp traps. As with most things in life, both techniques have drawbacks. Mist nets allow a much bigger catching area, are less likely to be identified by bats in flight and result in a better catch rate. However, the potential for distressing bats is greater than with harp traps. Avoiding this requires skill and experience, whereas harp traps are relatively easy to use and cause relatively little stress. On the other hand they tend to have a smaller catching area and are easier for bats to identify in flight, making the catch success rate lower.

I like a challenge and wondered if it might be possible to find a way of building a larger harp trap, which could give the size benefits of mist netting with greater ease of use. After all, the very first Tuttle trap (see picture), used to catch Mexican Free-tailed Bats was positively enormous A giant double/string trap seemed to be worth investigating. Enter the Goliath harp trap!

The original Tuttle or harp trap - a development on the earlier and simpler Constantine trap.

I dabbled with Goliath on and off for several years, trying various dead ends and attempting to resolve them. I wanted to build a harp trap 5 metres high, almost 1.5 times higher than any of the commercial traps. One key reason harp traps aren't normally this high is the problem of rolling up the strings when the trap is not in use. Anyone who uses commercially made harp traps will tell you this can be one a nightmare, with strings becoming entangled with one another and snapping regularly due to the stresses of being tightened and loosened frequently.

With my smaller roost-type harp trap the strings were permanently stretched onto a fixed frame. I recently broke a string for the first time (through carelessness), despite the trap having been used somewhere between 50 and 100 times over a period of six years. Would it be possible to build a 5m high fixed frame harp trap? I set out to find out!

Goliath in place in the car roof-bars - a bit of a handful!

Using lightweight aluminium alloy as a frame made the prototype trap surprisingly easy for two people to carry, despite the size. A bigger issue was of course transport. I calculated that 5 metres was the maximum length that could be transported on the roof bars of my Subaru Forester (with tie-downs to the front and rear bumpers), but what about width? There's no point in having a very high trap if its so narrow that bats can simply fly round it! The trap needed to be sectional and I hoped to have two segments, each 1.3m  wide, which would plug together to give a total catch area of 2.6m x 5m. Sadly, when I strung the two sections (each of which were open-sided) I found that I had under-estimated the power exerted by a total of 520m of nylon monofilament line, pulled taught. Unsurprisingly the frames were pulled out of shape, but what I hadn't anticipated was that it was simply impossible to manually pull them square in order to connect the two halves of the trap! Back to the drawing board...

Goliath's first outing. The relatively narrow width and central pillar were unavoidable but reduced the effectiveness of the trap.

To allow the two halves of the trap to connect when strung there would need to be a central pillar. I wasn't happy about the impact this would have on the "visibility" of the trap to bats, but this appeared to be unavoidable. So the final Goliath prototype was effectively two traps side by side, each 5m tall and 1.3m wide. To facilitate raising and lowering of the trap the legs acted as pivots and the guys ropes (essential for stability) could be used to pull it up into position. A bag at the base completed the trap. I followed the successful bag design from the kitchen table harp trap, with the bag surrounding the trap on all sides, avoiding the drawstrings used on some commercial traps, as these often allow bats to escape.

In practice Goliath proved to be too unwieldy for regular use. It did catch bats, but required a team of 4-5 people to erect and there were problems with the strings being damaged in transit. It had been s useful test-bed, allowing me to try out some ideas, but after using it three times a year ago I decided to retire Goliath and start looking at a design for an especially large harp trap with rolled up strings.

Goliath in use with an acoustic lure.

A year on and the intervening time has been put to good use: Goliath II is almost ready for use. When erected it will sit 2.5m wide by 3.8m high, giving a total catch area more than twice that of the commonly-used Austbat harp trap. I've included a number of interesting innovations, both to make it more effective than other roll-up harp traps and to make it relatively easy for others to replicate. I'll post about Goliath II once it has been tested, but it's looking promising. Watch this space!

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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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Two steps up for bat monitoring?

I've written before about my hope that the rescue and revitalisation of Titley Electronics (makers of Anabat) would result in a shake-up of the market for unattended monitoring equipment. With Titley and Wildlife Acoustics (makers of the Songmeter) battling for the marketplace the winners will be those of us who use their equipment. With remarkable symmetry both companies have looked at their existing offer, listened to customer feedback and both are launching ground-breaking new machines. Interestingly they are also swapping places in the cost stakes.

Wildlife Acoustics appear to have addressed a lot of the issues with their SM2BAT with their new SM3BAT. With a die-cast aluminium case it will be more robust than the previous machine. They have also addressed problems with cumbersome access and risk of letting water inside the unit. The control and screen are now on the outside of the case, making them more accessible and the batteries now slide into slots in the side of the machine. 

The new Songmeter SM3BAT from Wildlife Acoustics

(Photo copyright Wildlife Acoustics)

The Wildlife Acoustics approach of recording in ZCA (Analook) format as well as in compressed WAV (audio) format is strengthened by a massive increase in memory to a Terrabyte, allowing those large files plenty of room for a change. Another problem with the old SM2BAT was the vulnerability of the microphones and W.A. have a new and more robust microphone for the SM3BAT. They also claim an improvement in power consumption and there is no longer a need for an adaptor to use an external battery.

It all sounds very promising. The downside is that they have abandoned the £1000 including VAT price point to pay for the improvements and I understand the SM3BAT will market at £1500. The new microphones are also double the cost of the old ones at £200 each. A big increase, but maybe worth it?

Titley are countering by moving downmarket with their new Anabat Express. At just £500 each you could buy three of these machines for one SM3BAT (though the SM3BAT can record from two microphones concurrently, if required). 

The new Anabat Express

(Photo copyright Titley Electronics)

Compared with the older SD1 and SD2 Anabats the Express is very different and, like W.A. Titley have listened carefully to customer feedback. A key reason why people started buying SM2BATs in preference to Anabats was the the Anabat's lack of integral weatherproofing. The Express is built into a stout weatherproof case, screen-printed with camouflage. The case is little bigger than the original Anabat machines, making it rather smaller and neater than the SM3BAT. The older chunky Anabat microphones have given way to a new and smaller mic. Happily Titley have finally given up on CF memory cards, moving to the ubiquitous and smaller SD card (in fairness they had already done this with the Roostlogger).

Titley also claim a significant improvement on power consumption and the Express uses 4 AA batteries. Oddly in the SM3BAT W.A. have chosen to carry on using 4 large and heavy D cells.

This interior view of the Anabat Express, with AA batteries in place shows how small this low-cost machine is. 

(Photo copyright Titley Electronics)

The Express also has built in GPS for the first time. It's not really a hand-held unit, so the purpose of the GPS receiver is to allow the machine to calculate sunset and dawn times, making it possible to programme the machine to start and stop recording at times relative to these, rather than fixed times. This was always a strength of the SM2BAT, compared to the older Anabats, which require regular reprogramming through the season.

It's great to see both companies listening and innovating and it will be interesting to see what these two new machines are like in practice. W.A. have a habit of making equipment complex to use in the field, which is fine if you're a bit of a techno-geek, but more of a problem if you're just an average bat worker. Titley's previous machines have always been relatively user-friendly, though the manuals for their previous models have been unintelligible! Whether all these innovations deliver the goods remains to be seen. Happily both manufacturers have been kind enough to offer me machines to try out.

On paper it looks like W.A. are aiming for the gold standard with the SM3BAT, though it's costly and more visible, therefore potentially vulnerable when left in the field. The Anabat Express, with its low price-point could be bought in quantity and its small size and camouflaged appearance makes it potentially less vulnerable. If both machines live up to their promise it could be a case of "horses for courses". For long-term placement on a wind farm Met mast the SM3BAT could be ideal. For large scale deployment over a site the Anabat Express could fit the bill. Time will tell...

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"I know there aren't any bats..."

In my work I meet many bat roost owners, varying in attitude towards "their" bats from delighted to appalled and all points in between. I also encounter many people who are commissioning a bat survey because they have been obliged to do so. Often in the UK this occurs when someone applies to their local authority for planning consent to carry out work on a building. The law requires the local authority to consider protected species, so if there is any possibility for a bat roost to be present a bat survey must be carried out. "I know there aren't any bats there, but we have to have a survey" is often the client's opening gambit. It's not an unreasonable attitude: what little most people know about bats leads them to expect them to be highly visible, extremely rare and likely to roost in caves or gothic castles. Against that background it seems insane for a local authority to request a bat survey of a three bedroom suburban house, a 1970's primary school or a city centre council flat. Yet I and my colleagues frequently come across bat roosts in buildings like these.

This modern primary school is home to 280 Soprano Pipistrelles. They roost alongside the main entrance and most people think the noise is made by nesting birds.

Part of the problem is one of perception - bats are rare, that's why they're protected. Yet some bat species are very common. Here in central Scotland Soprano and Common Pipistrelles and Daubenton's Bats are all common, but have the same level of protection as rarities such as Whiskered Bats or Nathusius' Pipistrelles. Rarity isn't the sole reason for protection - vulnerability is a key issue. Our commonest species (in central Scotland), the Soprano Pipistrelle gather in their hundreds and even thousands to breed. The destruction of one of these roosts could harm the species over a large area. Bats reproduce very slowly - a single female will often only have one offspring in a good year and the need to gain weight swiftly in order to survive hibernation means that many do not survive. Thus bats are usually slow to recover from set-backs. Reducing prey availability caused by pesticides, illegal roost destruction through prejudice and stupidity and a host of other factors all combine to make legal protection essential.

Another factor in the misconception is the expectation that bats are visible in their roosts. We have Hollywood to thank for that. People expect to glance inside an attic and see a host of bats hanging from the ceiling, with their wings wrapped around their bodies. In the case of Horseshoe Bats that genuinely is the case, though bats tend to be a great deal smaller than they appear to be when in flight. Most other bat species in the UK roost in holes or crevices of one type or another, meaning that when they roost in buildings they are less likely to be visible to the uninitiated. Not long ago I surveyed a farm, the owner of which insisted there were no bats around his farm. Not only were there three species of bats roosting in his farm, there was a colony of about 80 Common Pipistrelles roosting alongside his bedroom window.

This colony of Brown Long-eared Bats are tucked into a crevice between two roof beams. If disturbed they tend to move deeper into the crevice, out of sight.

I'm not sure where the popular conception that bats only roost in old buildings comes from. It's certainly true that large country mansions and castles usually do contain bat roosts, but this often has as much to do with the presence of undisturbed spaces and the maturity of surrounding habitat as anything else. Modern house builders are surprisingly good at including loose-fitting fascia boards, poorly attached flashing and many other features which bats find to their liking and it is usually in modern buildings that people are particularly surprised to find bats.

Not a building which fits with the popular vision of a building used by roosting bats, but 800 Soprano Pipistrelles roost under the slates of this house every summer.

I have no doubt that there are many buildings around the country containing bat roosts that the owners are unaware of. Either they don't notice the signs or don't recognise them for what they are. Two years ago I spent a couple of weeks flying (when I'm not working with bats I fly gliders) from an airfield in Leicestershire. The club house there is a single-storey, L-shaped building about five years old. In the shelter of the L-shape are comfy tables and chairs, accessible from the bar. It's a nice place to sit on a summer evening and is well-used by club members. I had only been there a couple of days when I realised that a colony of Common Pipistrelles were roosting  at the wall-head alongside these tables. Each evening as the sun started to go down around ten bats would emerge and fly through the group of people who were sitting drinking and chatting. Nobody ever noticed them! I suspect people occasionally glimpsed a bat and assumed it was a songbird. The club management were entirely unaware of the presence of a bat roost in their recently-build clubhouse. If they applied for planning permission to change it and were asked for a bat survey I expect they would be astonished and annoyed at being asked to commission a bat survey...

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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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