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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Happy Harp Trapping!

A couple of times this year I have described my Tuttle (or harp) trap, built with the intention of creating an effective trap for harmlessly catching live bats, but smaller and more versatile than the commercially-available traps. Cheaper too! (see The Kitchen Table Harp Trap, February 2008).

Back in May I described the trap's first outing, when it was unfortunately placed over a roost entrance which wasn't in use. (See A Bat in the Hand, May 2008). As the bat maternity season started soon after that I had to wait with gritted teeth for the breeding season to end, so I could find out whether the trap worked or not.

A couple of weeks ago Lothians Bat Group had a trip to a site in the Borders and I set the harp trap up at the entrance to a Daubenton's Bat (Myotis daubentonii) roost and waited with baited breath. With 8 or 10 people waiting to get a close-up view of the bats, I couldn't help dwelling on the fact that the wretched gadget had yet to catch a bat - talk about pressure!

The first 2 or 3 bats flew past the trap, finding ways around it or even through it, causing a sweaty brow on my part. However, this is always a problem with harp trapping at a roost, so after a little work closing the gaps with coats, twigs and anything else that came to hand my fingers were re-crossed.

I got involved in a conversation and missed seeing the first bat to be caught, but yes, a bat was actually caught. My week's hard work back in the winter wasn't in vain after all! It quickly became apparent that the plastic lining of the catching bag was too long, which slowed the bat in moving into the sides of the bag, where it could not get out. In all, twelve Daubies were caught and I was able to check several of them for parasites, watched by some Group members. At the same time, other members of the Group were able to gain some valuable bat handling training.

The trap in place at the Natterer's roost (the roost is in a crevice in the wall behind)

Now that I knew the bat worked I followed up with an evening at a newly-discovered Natterer's Bat (Myotis nattereri) roost in the wall of an old farm steading, where I needed to confirm how the roost was being used. Again, the trap did it's thing and I was able to catch five bats (although a sixth was caught as I was taking the trap down). With three adult females, a juvenile male and a juvenile female, I was able to conclude that this had been a maternity colony, with the young bats now flying and the trap had earned it's place.

A juvenile Natterer's Bat

Before I next use the trap I'll trim back the plastic lining. The only other problem that has come to light is that the legs are a bit spindly and wobbly. The trap is light in weight, so the legs don't need to be too strong, so I'll delay making any changes to that for now. But at last, eight months after building it, I know the trap is fully functional and effective.

Please note, everything described above was carried out under appropriate S.N.H. licences. Trapping and handling bats can be harmful to them if not done with the correct equipment and skills. To do so without a licence is a criminal offence.

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The kitchen table harp trap

For some time I'e been thinking about building a harp trap for my own use and I'm delighted to say that this winter I have finally found the time to do so. What is a harp trap? Also called a Tuttle trap (after Merlin Tuttle, whose invention it is), it's an ingenious trap which catches bats in flight, without harming them.

It works like this: a bat flies towards the trap and recognises a harp-like collection of vertical nylon lines about 2.5cm apart. With great agility it successfully flies between the lines, but what it doesn't expect is a second set of lines offset from the first, so that it can't avoid hitting them. It then slides unharmed down the lines, into a canvas bag below. The bat will try to climb the side of the bag, so it can take off again, but the sides of the bag are lined with thick, slippery polythene. The bat can climb the canvas, but soon reaches the seam where it is joined to the polythene and can go no further until it's removed.

Harp traps can be marvellously effective, when used properly. They are particularly good at trapping bats leaving roosts, as the bats have less time to see and avoid the trap than they would if they were free flying. More importantly, the bats are entirely unharmed and in my view suffer less stress than if caught in a mist-net. The problem is the cost: a single harp trap costs upwards of £800. See http://www.faunatech.com.au/products/harptrap.html for pictures of a commercial trap made by Austbat.

I reckoned that I could reduce that cost to under£200 by building my own trap, and add a few ideas of my own. Firstly, I wanted a smaller trap. With a catching area 1.8m wide and 2.4m high, the Austbat trap is unwieldy in some circumstances. It is also difficult to erect, as it is necessary to unroll and stretch the lines every time the trap is used. This process is fraught with difficulty at the best of times, let alone in darkness and pouring rain, when you're tired and fed up at 2am! My thinking was that a smaller catching area could allow for the harp section to be permanently erected within a frame. When not in use, the frame could be bagged , with the legs and catching bag removable for transport. A smaller trap would also be lighter and could be lifted on a couple of poles to cover the entrance to a high tree roost or the eaves of a roof. You can do that with an Austbat trap, but you need ropes and a team of stout people singing sea shanties, to haul it up!

And here it is...

The catching section is slightly over 1m square, with the frame, legs etc, made from lightweight square-section aluminium, which also has the advantage of being an easy material to work. The legs are easy to remove, without affecting the rest of the structure, so that the frame can be slung between two poles and lifted up to a roost entrance.

I took the opportunity to address two weaknesses I've noticed with the Austbat and similar traps. They have the nylon monofilament tied around the frame top and bottom. This has the effect of making the nylon vulnerable to scraping against rough surfaces and fraying or breaking. I tried to address this by tying the nylon lines to tiny hooks screwed into the frame, so that the frame itself protects the nyon.

The Austbat traps are also vulnerable to escapes at each end of the bag, as the plastic material is only attached to the two long sides. I have made a point of attaching plastic to the two short sides, in the hope of preventing that. The base of the harp frame sits slightly inside the bag, to allow this to be possible.

The hardest part of building the trap was sewing the canvas and plastic. Dressmaking isn't a skill of mine anyway, and it takes a lot of effort to push a sailmakers needle through several layers of the thick canvas. My fingers may never be the same again!

Where does the kitchen table come in? That's where I built it (it's too damn cold in the garage at this time of year!) The table still bears the scars.

Of course, I'm rather proud of the trap, but success isn't realy measured by appearance, or sound engineering or even my pride - it's measured by whether it actually works: will it catch bats?

Watch this space...

Please note that the use of a harp trap to catch bats in the UK is heavily restricted. I am able to use one because I have a roost visitor's licence, endorsed for the use of a harp trap, I am also licenced to catch a number of bats for a specific scientific study. Trapping bats without he necessary skills and licence is illegal and likely to be harmful to the bats. If you're interested in getting involved in this kind of work, the best starting point is to join your local bat group.

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