Protecting and monitoring Scotland's bat hibernacula

This article was first published in the April 2015 edition of Recorder News,  the magazine of BRISC - Biological Recording in Scotland (www.brisc.org.uk)

As a bat specialist I am regularly asked why bats are protected when they seem to be quite common. To the uninitiated a bat is just a bat, but we have at least ten species in Scotland and whilst some are relatively common several are much rarer. 

Bat populations today are a fraction of what they were a few decades ago and, whilst the decline of some species appears to have slowed, recovery to previous population levels is a long way off. Even the Soprano Pipistrelle (Pipistrellus pygmaeus)and Common Pipistrelle (P. pipistrellus), our two commonest Scottish bat species and most often seen due to its habit of emerging before full darkness, face a plethora of threats.

What often isn’t recognised is the vulnerability of bats in our temperate climate. The females of most Scottish bat species are only capable of giving birth to one juvenile per year (Noctules are an exception, occasionally having twins). From the moment of birth the clock is ticking and time is against each tiny and utterly dependent baby bat. They have to grow at a prodigious rate from birth in June to be ready to fly around two months later. They then have to rapidly climb a massive learning: in around three months not only do they have to learn to fly with sufficient skill and agility to outwit and capture their insect prey but they need to do so with sufficient proficiency to rapidly build fat reserves in readiness for hibernation.  Insufficient fat will result in a failure to survive hibernation. 

It’s no easier for the adults. Females spent the summer devoting all their energy to hunting and feeding their young and now they too have a limited time to build fat reserves. Males have an easier summer but must work hard through autumn, attracting females to mate before they hibernate too.

Scottish bats live on a knife-edge at the best of times, so the negative effects of human activity are especially pronounced. Agricultural pesticides and development have reduced hunting habitat and prey availability. Timber treatment of older buildings has been harmful to attic-roosting bats (though the worst chemicals are now banned). Conversion of old agricultural and industrial buildings has removed many roosting opportunities and though legislation protects roosts in buildings from disturbance or destruction, implementation of the law varies from local authority to local authority. Fragmentation of habitat is especially problematic for bats: a single bat colony may use dozens of roosts for different purposes through the year and they need safe commuting routes to link these with each other and with suitable foraging habitat. Removal of hedgerows and tree-lines reduces their ability to commute freely between these locations. Sadly, deliberate destruction of bat roosts by indifferent or ill-informed people is far from unknown.

 A hibernating Brown Long-eared Bat (Plecotus auritus)

During hibernation bats enter a condition of deep torpor, reducing their body temperature to between 2 and 8 degrees Celsius, slowing their heart rate to as little at 10 beats per hour and breathing perhaps once per hour. This reduces their use of stored energy to the minimum that supports life. But hibernation is not continuous and bats regularly wake, sometimes moving location. They may even hunt if the weather is warmer and Pipistrelles are occasionally seen in daytime, hunting for winter-flying insects in the midday sun. However arousal from deep torpor is expensive in energy and being forced to arouse by disturbance can reduce a bat’s ability to survive hibernation.

Conditions within hibernacula are critically important. Temperature must be steady, usually between 2 and 8 degrees Celsius. High humidity minimises water loss, reducing the need for bats to arouse to drink. It can take up to half an hour for a bat to arouse from deep torpor, so safety from predators is important, as is a lack of human disturbance. Hibernation tales place in differing locations, depending on the bat species. Noctules (Nyctalus noctula) and Leisler’s Bats (N. leisleri) tend to hibernate in deep tree holes, Pipistrelles (Pipistrellus sp.) often use crevices in buildings, cliffs or under loose tree bark which, whilst appearing relatively exposed, contain a suitable microclimate. Underground hibernacula such as caves, mines and tunnels tend to be used by bats of the Myotis genus - Daubenton’s (M. daubentonii), Natterer’s (M. nattereri), Whiskered (M. mystacinus) and Brandt’s (M. brandtii) - and Brown Long-eared Bats (Plecotus auritus).  

Man-made or natural underground sites with suitable conditions for hibernation are uncommon and increasingly under threat. In the Lothians disused limestone mines are well-used and are usually located at the base of quarries. Out of six mines known to be used by hibernating bats one is regularly disturbed by members of the public, one is unsafe due to vibration from an adjacent land-fill site and the landowner at another site recently had to be warned by SEPA to cease illegal landfill activity.

The quarry at Hope Mine, near Pathhead was filled in during the 1990’s and 2000’s. Access for bats was maintained via a grilled access hole. Airflow is critical in underground hibernacula: warm, stale air needs to be continually vented to maintain suitable hibernation conditions. At Hope a subsequent underground rock-fall and lack of maintenance of the air vent installed in the 1980s has caused the temperature underground to reach levels of over 14 degrees, rendering the mine unusable by hibernating bats.

There is a happier story at the remaining two mines. Middleton Upper Quarry near Gorebridge has recently been filled with over 600,000 tonnes of spoil from the Borders Railway. My company (David Dodds Associates Ltd.) worked closely with NWH Group, the owners and operators of the site. We used acoustic monitoring to assess which access tunnels were favoured by bats entering and leaving the disused mine workings. Under a Scottish Natural Heritage derogation license NWH staff used gabion baskets to create a safe access route for bats to continue accessing the mine after the quarry was filled in. Although the appearance of the site has changed considerably, a section of cliff face above the favoured entrance has been retained and stabilised, acting as a sign-post towards the entrance favoured by the bats.  The position of this entrance within the mine allows warm air to vent naturally, but an additional ventilation pipe has also been installed to ensure that temperature conditions remain suitable should that change. I’m happy to say that the first underground survey, during January 2015 showed that the mine continues to be used by Natterer’s, Daubenton’s and Brown Long-eared Bats.  We hope to use a similar approach to the adjacent Middleton Lower Quarry in due course.

Middleton Upper Quarry - an underground hibernaculum successfully safeguarded 

(photo courtesy of Birch Tree Images www.birchtreeimagesphotography.co.uk)

The National Bat Monitoring Programme (NBMP) is managed by the Bat Conservation Trust (BCT) on behalf of a partnership including BCT and the Joint Nature Conservation Committee (JNCC). This programme is an excellent example of citizen science at its very best. Throughout each year volunteers all over the UK carry out a variety of different bat surveys, from walked transects in open country or along watercourses, to counts of bats emerging at known roosts and searches for bats swarming at roosts at dawn. 

The slow and difficult search for hibernating bats

(photo courtesy of Birch Tree Images www.birchtreeimagesphotography.co.uk)

The surveys are designed to allow anyone to make a contribution, from those with virtually no experience of bats to skilled, licensed bat-workers. The NBMP website includes training materials and bat detector training courses are regularly run, to ensure as many people as possible take part. The NBMP data is collated and used to provide a statistically robust assessment of how bat populations are faring in the UK and in Scotland where sufficient data is available (more volunteer surveyors are urgently needed by the NBMP in Scotland), published each year as “The State of the UK’s Bats” and is widely used to inform and target bat conservation effort.  It also forms on of the UK Government’s biodiversity indicators.

One NBMP survey method which requires especial skill and experience is hibernaculum counts. These must be carried out by bat-workers who are specifically licensed for hibernaculum work, usually assisted by small teams of dedicated volunteers. This is not easy work. Underground hibernacula are often muddy, wet and physically demanding to access and to move around in. Safety equipment is paramount and surveyors need to be suitably trained and equipped to cope with conditions underground. Underground hibernacula may be natural caves or man-made structures, such as tunnels, mine-workings etc. The latter are usually disused and a continual assessment must be made as to whether a hibernaculum is safe to survey. Bat conservation is important, but human health and safety is always the priority.

Bats in Scottish underground hibernacula are usually found on their own, or very occasionally in small groups. Often they are dispersed throughout a large area and it is not unusual for a team of four surveyors to spend several hours searching to find only a handful of bats. Depending on the temperature and humidity conditions in a particular hibernaculum bats may be found on roofs or walls but often they are concealed within cracks and crevices, where they can find security from disturbance and a suitable microclimate. It is rarely possible to fully census bats within a hibernaculum, as many individuals may be invisible in deeper crevices or in areas unsafe to survey or in some cases mine-workings are simply too large to survey comprehensively. To ensure that data used by the NBMP is as robust as possible a repeatable survey method is used: as far as possible each year’s surveys will be done by the same number of people, with a similar mix of experience, spending a similar amount of time on the survey and following the same route through the hibernaculum. Although a long, hard survey may yield only a handful of records of bats, when combined with dozens of other surveys around the country and compared year-on-year useful population data starts to emerge.

Great effort is taken to ensure that disturbance of bats during a hibernaculum survey is kept to an absolute minimum.  Surveys are normally carried out twice each winter, with several weeks gap. Noise is kept as low as possible and bats are illuminated with torches only for as long as it takes to identify them. Bats are never normally touched or handled and in more confined spaces care is taken to avoid standing below hibernating bats or breathing on them, to avoid the surveyor’s body temperature from having an impact.

A hibernating Natterer's Bat (Myotis nattereri)

Identifying bats within hibernacula is challenging and requires a good deal of experience. Critical identification characteristics are often invisible without handling a bat or hidden by the crevice the bat is in. Surveyors are forced to use secondary identification characteristics such as fur colour, face and ear shape, size of feet etc., all of which are difficult to measure except on the basis of experience. Often it is only possible to identify a bat to genus.

It isn’t usually possible to identify individual bats, especially in hibernation when wings are tightly folded so that scars to the wing membrane cannot be seen. It is possible to ring bats using loose, horseshoe-shaped aluminium rings around the forearm, but this technique is used more sparingly than for birds due to potential impacts on the bats, so it is rare to see a ringed bat within a hibernaculum. In January 2010 I found a Daubenton’s Bat hibernating in a tunnel high in the Lowther Hills. This bat not only had a ring but also had most of one ear missing. “one-ear”, as she became known had been rung by researchers working on behalf of Scottish Natural Heritage, testing for rabies at Falls of Clyde nature reserve 35km away in August 2009.  Her injury seems not to have prejudiced her ability to hunt successfully, as I have regularly recorded her hibernating in the same place in the five years since then.

"One-ear" in hibernation. Her missing right ear and the ring on her forearm are clearly visible, as is condensation on her pelage.

It cannot be stressed highly enough how important hibernacula are for bats in Scotland. The need for sites with stable, low temperature and high humidity, combined with long-term security and lack of disturbance means that suitable sites are not common and it is common for bats to travel long distances to reach them. Finding and monitoring these sites is essential if we are to protect them and to measure variation in bat populations. Destruction of hibernacula is just one of many threats faced by our bats and protecting their ability to hibernate safely is critical for the long-term survival of these sensitive, vulnerable and oft-maligned animals.

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The new generation of unattended bat detectors

It's been a little while since I wrote a blog post, which has irritated me as I have a host of subjects I want to write about, but so little time to do it. However, the inevitable bat worker calendar effect has happened and I am looking out of the window at deep snow and reflecting on the fact that my "to do" list is the shortest it has been for about a year. This wonderful situation won't last long so I'd better get blogging!

Back in 2011 I wrote about the rivalry between Titley Electronics and Wildlife Acoustics, whose Anabat SD1/2 and Songmeter SM2BAT were at that time the only realistic options for unattended monitoring of bat calls. Both machines had strengths and weaknesses, both were (and still are) very widely used by ecologists and bat researchers.

In the past year both companies have launched new unattended bat detectors. In both cases they seem to have listened to customers in an effort to improve on their previous models. It is interesting that the result of their market research has been very different. Wildlife Acoustics moved up-market with a signficantly more expensive machine, the SM3BAT and Titley went for a smaller and cheaper machine, the Anabat Express. I am fortunate enough to have had my hands on both machines for the past year and have had an opportunity to put them through their paces.

Wildlife Acoustics' Songmeter SM2BAT and the slightly improved SM2BAT+ have achieved much popularity in the UK and elsewhere in recent years. Their price tag of around a thousand pounds made them cheaper than the Anabat SD2, though still expensive for non-commercial users. Their ability to record from two microphones at once was attractive, especially for those wanting to record at height and at ground level simultaneously and the fact that they recorded to SD cards, rather than the dated CF cards used by Anabats was also attractive. However they were complex to use, requiring careful setting up and were easy for the inexperienced user to get wrong.

Perhaps Songmeter's most attractive feature compared to the Anabat was that they came in a waterproof case, whereas Anabats needed an additional weatherproof box - Pelicases for the well-heeled, sandwich boxes for others! The  Songmeter's big selling point was that it arrived oven-ready, without the need to fabricate a waterproofing solution. However they suffered from condensation at times and the internal memory card holders were vulnerable to damage when inserting the massive D cells the machines use. Exposing the internal workings of the machine to change batteries or memory cards in inclement weather was hardly ideal, though the SM2BAT+ model had it's internals sprayed with a water-repelling coating to improve this.

Anabat SD1 and SD2 models need additional waterproofing for unattended use. This sandwich box is the low-tech approach!

An SM2BAT mounted at the base of a mast, with one microphone attached and another at the top of the mast, connected by a cable.

Unlike Songmeters, Anabats (this is an SD1) can also be used for transect work.

The Anabat SD2 (and the previous SD1 model) by comparison, though about 35% more expensive was more robustly built (waterproofing aside) and much easier to use, reducing the risk of expensive repeat surveys. They also use standard AA cells, rather than the larger D cells used by Songmeters (though both can also be connected to external 12v batteries for longer term use). They also have the advantage of being excellent for hand-held transect surveys, whereas the Songmeter can only be used passively.

So given all these pro's and cons I was intrigued to see what the two new machines are like and how the two manufacturers have responded to the feedback of their customers.

The Songmeter SM3BAT was the first of the new machines to reach me. One of the criticisms of its predecessors was that they were a bit "plasticky" and some parts were easy to damage. My first impression of the SM3BAT was that it had been cast in a furnace, rather than built! It's taller than the old machine and the case appears to be capable of withstanding having cattle tap-dance on it (a more common issue with this type of equipment than you might think), though I wouldn't recommend experimenting with that. It's also heavy at 2.5Kg without batteries. The solid metal casing includes holes for securing the machine, either by bolting it to a wall or by attaching it to something solid with a bicycle-lock style cable.

The SM3BAT in use. The microphone is at the end of the cable on the right, allowing the machine to be put out of sight if necessary.

The controls and screen are now on the outside of the machine, with robust plastic to protect them. Battery compartments and memory card compartments are accessed by removing waterproof plugs on the side, so that there is no longer any reason (or indeed any ability) to access the interior of the case. Thus several criticisms of the SM2BAT are removed - the new machine feels solid and robust, it gives confidence that it can cope with whatever is flung at it.

In use the SM3BAT is similar to the SM2BAT. It can record in ZCA (Analook) format, or in WAC or WAV formats. As before there are four memory card slots to allow you to provide the machine with plenty of space for the latter two memory-hungry modes. Wildlife Acoustics are proud of the fact that it is compatible with the latest generation of 256GB SD cards, providing potentially a Terrabyte of memory. A big criticism of the SM2BAT was the memory-hungry nature of Wildlife Acoustics' preferred WAC format. With advances in computer technology the ability to store and process large amounts of data has become steadily easier since then, though that burden should still not be underestimated if you plan to use these modes.

The SM3BAT comes with an upgraded microphone, which like the SM2BAT one is omnidirectional (a key difference to the Anabat SD1/2 microphones, which are unidirectional). However it is significantly bigger and can no longer be plugged directly into the side of the casing. Wildlife Acoustics advise it is more sensitive than their previous microphone.

Titley Electronics' new machine is the Anabat Express. They have chosen to produce something much more compact and with less visual impact than their competitors machine. The Express is a similar size to the old Anabat SD1/2 but is now in a camouflaged waterproof case. The case, though made of plastic is strongly made and similar to those used for camera traps. It now has an omnidirectional microphone, which is stored safely in a slot within the case and then screwed into the exterior for use. Unlike the SM3BAT the Express has a built-in GPS, which it uses to calculate sunset times (the Songmeters need to be programmed with their location to calculate this). This is a big step forward for Anabat. Titley's previous models could only be programmed to start and end recordings at fixed times, with no allowance for day-by-day changes in sunset and dawn times. Songmeters have the useful facility to start and finish at a set time in relation to sunset or dawn. Now the Express can do this, with the added bonus that it doesn't need to have latitude and longitude pre-programmed, as with Songmeters.

The diminutive Anabat Express, with a £2 coin for comparison.

The interior of the Anabat Express, including built-in instructions.

In use the Express is virtually idiot-proof (believe me, I'm an excellent measure of this). When we're out in the field, setting up equipment and it's cold or wet and we've been working all day even the best ecologist or bat-worker is liable to do something daft - we're only human after all! The Express even has step-by-step instructions pasted to the inside of the lid. All you have to do is connect the microphone to the outside, switch it on and wait for the GPS to get a signal. Then you cycle through three options for recording time (continuous, sunset to dawn or pre-programmed), close the case and you're ready for action. A cord attached to the case ends in a magnet, which can be used to check the machine is functioning correctly. A friendly blue light winks to confirm all is well when the magnet is placed in the right position on the case.

The Express uses standard SD cards, with no prior set-up, so formatting CF cards is a thing of the past. Of course, unlike the Anabat SD1/SD2 this machine isn't suitable for transect or other hand-held work - it's only effective as an unattended detector. It only records in ZCA format, so if you prefer to use audio recordings for analysis this is something to think about. Personally I prefer ZCA for most purposes. I'm not thrilled about the fact that you have to open it up to operate it - that was one of my criticisms of the Songmeter SM2BAT. However, the interior of this machine is far more robust than the SM2BAT and, like the SM2BAT+ the interior has been sprayed with a water-repelling coating.

So Titley have chosen to go for a low-profile, simple-to-use machine, whereas Wildlife Acoustics have gone for a big, solid, bomb-proof detector. Which would I buy? As ever it's a case of horses for courses. The SM3BAT suffers from high visibility but balances it by being massively robust. It would be hard to install it in a situation where theft or vandalism is a concern. Not only is it big and obvious it looks expensive. Your average thieving ned couldn't help but wonder what it will fetch on eBay. Although it can be securely attached to something, that wouldn't protect the controls or the battery and SD card compartments from interference. At over 2.5Kg before you put batteries in, you wouldn't want to carry a rucksack of these machines around a site for temporary installation. However you might choose it as the ideal machine for long-term installation on a met mast, or somewhere where unauthorised interference can be prevented.

The Anabat Express on the other hand doesn't suffer from these security issues. It's small and camouflaged and therefore suitable for installation with low visual impact, perhaps in places with regular visitors. It comes in a protective zip-up case and you could happily carry a dozen of them with you all day. So for short-term, high risk installation it couldn't be better and I like many others already have several of them for that purpose. If you do want to install one in a higher-risk situation Titley sell a steel case and steel python cable, allowing you to attach it securely to something solid. The Express is also very easy to set up in the field, so the risk of mistakes is low. Unlike the older Anabats you don't need to worry about where the microphone is pointing and it lends itself to attachment to a tree trunk or similar.

An Anabat Express in use with Titley's optional security system

There are a couple of other issues to consider in comparing the two machines. One is power consumption, Wildlife Acoustics claim that the SM3BAT is less power-hungry than the SM2 models were and claim up to 20 nights on a set of four Alkaline D cells. It can also be used with an external 12 volt battery or power supply. The Express can last for up to 30 nights if you use Lithium AA batteries (high capacity Lithium batteries are not an option for D cells, so far as I know). you cannot connect an external 12v battery to the Express, so it's not a machine for long-term use, though 30 nights is a long monitoring period!

The other thing to consider is price. Wildlife Acoustics have abandoned their £1000 price point with the SM3BAT, which sells for over £1400. This compares with £660 for the Anabat Express. In other words you can have two Expresses for the cost of an SM3BAT and still have money left over. I supsect this is the clincher. Both are good machines, both appear to work extremely well and I have had no problems with reliability of either machine. But two machines for the price of one is hard to ignore, especially when the Express is also less likely to be stolen or vandalised and easier to deploy in numbers across a site. For bat groups and those working on a budget the decision seems like a no-brainer. But if you want a long-term installation or if you want something with a well-engineered and professional appearance perhaps the SM3BAT has something to offer.

It's particularly interesting that Wildlife Acoustics have now launched a smaller, lower-cost, ZCA mode only model called the SMZC. From the little I have seen it seems extraordinarily like an attempt to emulate the Anabat Express, though it still has the look of something made in a foundry and weighs 1Kg (the Express weighs 385g). It also lacks the GPS functionality of the Express, but is priced about 10% cheaper, so it may turn out to have something to offer.

Please note that all prices quoted include VAT and appear to be correct at the time of writing. All weights exclude batteries. 

See our website: David Dodds Associates Ltd