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 art of radio-tagging a bat

People sometimes ask me when I'm going to write my next blog post. The honest answer is that I'd love to write posts more often, but much of my time is taken up by doing bats surveys, writing reports, analysing bat calls and all the other myriad tasks that keep me busy whenever bats are active!

Here, to keep you going, is an example of what's keeping me from blogging. It's a great set of photos taken by Mike Beard, showing my colleague Rebecca Brassey and I (with help from Isla, Richard and Mike) preparing some Noctule bats for a radio tracking project this spring.

First find your bat. Here it's Rebecca's turn to check a bat box and see if our target Noctules are there. Typically, we found a group of them in the very last box we looked in.

Before attaching the radio tag, biometric data was gathered about each bat. First the bat is sexed, the forearm is measured with a pair of callipers and then the bat is weighed.

Weight is especially important as I need to be sure that the radio tag will weigh less than 5% of the animal's body mass. The ratio between forearm length (which doesn't vary once a bat is adult) and weight is also a useful indicator that the bat is healthy and therefore a suitable candidate for radio-tracking.

Next I wanted to age each bat. This is hard to do with non-juveniles, but with larger bat species it is sometimes possible to get a hint of age by looking at wear on the teeth.

Another feature which can help with understanding the age of a bat is the amount of scarring on its wing membranes, though again this is an imprecise science!

I hadn't examined this colony for ectoparasites previously, so each bat was examined and with Rebecca's help some specimens of Spinturnix acuminatus were taken. This is a species of wing-dwelling mite normally found on Noctules.

The specimens are preserved in a 70% solution of Isopropyl alcohol and labelled in pencil on small slips of paper, which are inserted in the vial with the specimen. This prevents the specimen and label from becoming separated.

Next I check the manufacturer's label on the tag, so that we can check the radio frequency of it's "chirp" transmissions. I am also double-checking the weight of it, to ensure it is below 5% of the bat's weight.

A small patch of fur between the bats shoulder-blades is snipped short. This location is used for the tag, as it is a hard location for a bat to reach to groom the tag off. That won't stop it's roost-mates from having a go.

A thin layer of mastic glue is applied to the trimmed patch of fur and allowed to cure until it is sticky.

A similarly thin layer is applied to the tag....

...and the tag is pushed firmly in place.

Next another layer of glue is painted over the top of the tag...

...and the adjoining fur pulled over it.

In this picture you can see the tag's antenna, the thickness of a human hair, trailing behind it.

Voila, one Noctule bat, with radio tag attached and ready to be released at sunset.

The tags of course do not harm the bat: after a short period the fur will grow and either the tag will fall off naturally or it will be groomed off. But for a few days or weeks the bat will give us a windows into it's life:when and where it hunts, where it roosts, whether it associates with other bats and so on.

My thanks to Rebecca, Isla and the various trainees and volunteers who helped out with this particular project and of course to Mike for the photography.

Please note: all bats are protected from disturbance and harm in the UK and EU. This type of work should only be undertaken by experienced personnel under a specific license, issued by the relevant statutory nature conservation organisation (Scottish Natural heritage, Natural England etc). Attempting to handle bats without an appropriate license is a criminal offence and carries risks. If you are interested in getting involved in working with bats on a voluntary basis contact your local bat group (The Bat Conservation Trust will put you in touch with them). 

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Memories of Summer Bats

This post has absolutely no meaning or purpose, except that I was tidying up my hard drive and realised I had recorded several video sequences of bats in the hand over the past year or so. It's minus four outside and the only bats I have seen for a couple of months have been deep in torpor (hibernation). So, to relieve the winter blues, here are some lively, wriggling bats to enjoy...

First, a Brown Long-eared Bat (Plecotus auritus). This was one of a group of 12 in a bat box on a golf course near Edinburgh. I filmed several sequences, trying to get the bat to extend it's massive ears, but the best I could get from it was the wiggle of its ears you see here. Long-eared Bats use their ears like parabolic dishes, concentrating sounds made by prey such as harvestmen, spiders and larger moths, allowing them to minimise the intensity of their echolocation calls. Some of their prey have rudimentary hearing and this prevents them from hearing the bat's approach.

Next up is a Noctule (Nyctalus noctula). This is the biggest bat species found in the northern U.K. This female demonstrates nicely the importance of using the right hold and the correct amount of pressure when holding a bat (see the note as the end). To begin with she struggles to free herself, but she quickly realises that she is secure and relaxes.

Next is a Soprano Pipistrelle (Pipistrellus pygmaeus). It's not a particularly good video segment, but it sits quite nicely beside the previous segment as the Soprano is our smallest bat and looks tiny by comparison with the bruiser in the previous video!

Lastly, here is a longer segment of a male Whiskered Bat (Myotis mystacinus). This species is quite rare in this region, so I took the opportunity to record a video showing some of its morphological features. Whiskereds are hard to tell apart from Brandts Bat (Myotis brandtii) or Alcathoe's Bat (Myotis alcathoe). One of the best characters for telling them apart is their dentition, though examining the teeth of a small and annoyed animal isn't always easy! Sadly the camera I had available on that day wasn't capable of showing that level of detail, so you can't see the teeth very clearly.

Important note - disturbing bats in the United Kingdom or anywhere in the European Union is an offence unless you hold an appropriate license. In the UK these are issued by one of the statutory nature conservation organisations (Scottish Natural Heritage, Natural England, Countryside Council for Wales, etc). If you would like to learn to work with bats your best starting point is to contact your local bat group for help, advice and training.

See the Bat Conservation Trust Website for details of your local bat group: Local Bat Groups

My website: www.plecotus.co.uk

Noctules, Boxes and dangling bat-workers

During 2007 and 2008 I spent some time on investigating the distribution of Noctules (Nyctalus noctula) in south-east Scotland, a region where the species was traditionally considered scarce and probably beyond it's natural range. I gathered together incidental records from many bat-workers (thanks!), some focussed car surveys and records from commercial bat surveys carried out by a number of consultants, including me.

The result was a clear indication that Noctules are widespread in the Borders and Lothians regions and probably at a greater density than was previously realised. The results were presented in a poster at the B.C.T. Scottish bat Conference in November 2008. If you're interested you can download a copy as a .PDF file here: http://www.plecotus.net/Noctule-poster.pdf (You are free to distribute this information, in the interests of bat conservation, but please attribute it)

One evening in September I was relaxing in the bar at Preston Montford Field Centre, near Shrewsbury during a training course. My phone made one of the irritating noises that mobiles make, to tell me I had received a text from Carol and Nigel, both active members of Lothians Bat Group. They attached a photo, with the tongue-in-cheek message "do you know what this is?!" You're probably ahead of me here - it was a Noctule, of course. To rub my nose in it, they had found it in a bat box at a country park 2 miles from my home. If I hadn't been on the course I would probably have been there when the boxes were checked. Grumpy wasn't the word for my response....

To their credit, Carol and Nigel had found the first ever Noctule in a bat box in Scotland and it resulted in some positive press coverage for bat conservation. Fast forward two weeks when I was checking bat boxes at another country park, also in Midlothian and believe it or not, there was another Noctule. Like Nigel and Carol's this was an adult female, squeezed into a slim bat box. She was a delight to handle: very relaxed, even when I placed her on a log to photograph her, before returning her to her box. She was also kind enough to give up some Spinturnix acuminatus mites (well, I was happy about it, even if my companions were a little taken aback).

To put this in context, these two animals were both found in boxes in a county where I can count previous records of Noctules in flight on my fingers. Was it coincidence (or was it the same bat, with a sense of humour?) or does this tie in with our new knowledge of the spread of Noctules? It was certainly a surprise in an area with many bat box schemes and two decades history of only ever finding Pipistrelles in them. By coincidence, in 2007 and 2008 a small colony of Natterer's Bats were found in a West Lothian bat box, so these Noctules seemed too good to be true.

Stuart Smith, chairman of Lothians Bat Group (and one of the grandees of Scottish bat-work), came up with an unusual plan to respond to these new records by installing some new bat boxes, higher than normal, something he had seen at a Vincent Wildlife Trust site in Dorset. Most bat boxes in the Lothians are between 12 and 20 feet (3.7-6.1m ) above ground. Noctules are tree-roosting bats, with a tendency to roost in tree holes, often high up, so higher boxes makes sense. The problem is that 20 feet is the maximum height for access by ladder, without extra safety measures and skills.

The solution? Suspended bat boxes! We used Schwegler woodcrete bat boxes, suspended from pulleys attached to high branches. Wire rope is used to pull the boxes up to full height -around 40-45 feet above ground (12.3-13.8m) - and tied off on bolts mounted on the tree trunk. These are at ladder height, to prevent the local "yoof" from reaching them. When we need to check and clean the boxes we will simply climb a ladder to normal height, unwind the wire rope from the bolts and lower the box to ground level. Once finished, we simply haul the box back to the top of the tree and tie off the wire rope once more. Ingenious!

Now, there is a chicken and egg problem here: how to get 40 feet up to mount the pulley? The Bat Group is lucky to have access to the services of a professional tree-climber, who also has an affinity for bats. George used his skills to climb the trees and install pulleys for us, making it look easy, as tree climbers always do. However, it's very hard work: you need to combine a high level of physical fitness with some somewhat counter-intuitive skills. Come to think of it, strolling about at the top of a tall tree as about as counter-intuitive as it's possible to be!

Watching a bat-box being hauled 40 feet into the top of a tree was quite exciting. Whether they will attract Noctules remains to be seen...

With work over for the day, George offered a couple of group members the opportunity to try out tree climbing techniques (at very low height) and so, trussed up and roped to the tree, we took our turns at making fools of ourselves. I had tried my hand at this in the past, so probably should have known better, but didn't.

An advantage of this being my blog is that you won't get to see my feeble attempt at tree climbing. However, this seems a good moment to wreak revenge on Carol for last year's Noctule text message. You have to hand it to her: she's enjoying herself, even if she isn't making much progress up the tree!

Any comments: blog@plecotus.co.uk

My website: http://plecotus.co.uk/

The Hunting of the Noctule

For some time I have been rather fascinated by Noctule bats (Nyctalus noctula). One of Britian's largest bats, they emerge from their roosts earlier than other species and fly high abve pastures and river valleys, dipping down to catch larger insects. With a very loud call (reputedly louder than the legal safe limit for audible sound), their distinctive "CHIP CHOP" can often be heard from several hundred metres away.

You can hear a Noctule here: http://www.plecotus.net/noctule.mp3
(It was recorded in the Scottish Borders, using a hetereodyne bat detector set to 22kHz).

Traditionally they have been assumed to be largely absent from Scotland, apart from Dumfries and Galloway. It has been known for a while that the occasional Noctule turns up in the Scottish Borders and that there is a small population in the Tweed valley. More recently, there have a handful of records from reputable sources of Noctules in the Lothians, which led some bat-workers to query whether they are more widely spread than previously thought.

I set out to try to clarify this, by gathering together records from many sources across the Lothians and Borders area. I eventually gathered 41 records, from across the region, ranging from audio recordings validated by reputable ecologists to visual records of "large bats". That sounds like a lot of records, but they range over a period of eight years and twenty of these records relate to just 4 sites.


You can see a map of these sites here: http://www.plecotus.net/noctuleatlas.jpg


Hoping to locate more of these bats, I decided to use a variation on the car survey method, used for the Bat Conservation Trust Bats and Roadside Mammals Survey. This very successful technique involves driving at twenty miles an hour along country roads, with a bat detector poking out of the window, allowing many records to made in areas where bat-workers are thin on the ground (See below for the detailed method).


My theory was that, by doubling the speed to 40mph, a lot of ground could be covered in one session: typically 80miles in a session. The Noctule is a loud, high-flying bat, so mounting the detector on the roof and travelling faster should be feasible. The detector could be linked to the car stero with a wireless link, so that it's output would be clearly heard and we could stop and investigate when we heard anything suspiciously Noctule-ish. Thus, two nights ago, with Nigel acting as navigator, we set off in a vehicle decorated with rotating amber beacon and reflective warning sign to drive down the Tweed Valley and test the theory.


It was a perfect evening for bats: warm, dry and with plenty of flying insects around. We encountered an initial problem with air moving over the detector's microphone, causing unacceptable levels of background noise. This necessitated a speed reduction down to 30mph, after which the system seemed to work well and we soon started to hear Pipistrelles.

As a slight cheat we paused by the Dryburgh Suspension Bridge at Newtown St Boswells, a spot where there is usually a Noctule or two. Sure enough, a Noctule flew past and we could clearly hear it. Further on we crossed the Tweed at Coldstream and, after negotiating an unexpected diversion we found another bat near Wark. This was certainly a Nyctalus bat, but it is just possible it could have been a Leisler's Bat (Nyctalus leislerii). We heard only a couple of brief passes and the Leisler's has a similar call. Typically, we had two hand-held detectors linked to minidisk recorders and both failed at the same time! Without a recording to analyse and verify the bat's call we have to simply call it Nyctalus sp. Nonetheless, it proved that this variation on the BCT method is effective and worth persevering with.

You may ask, why not use the standard method? Firstly, 20mph is simply too slow to cover the vast amount of ground we need to survey and 30mph appears to work with these louder, high-flying bats. Secondly, the standard method requires some very expensive equipment and teams of four people, both factors which could be limiting. That in no way negates the amazing work that has been achieved by people in the UK, Ireland and elswehere using the BCT method.


The plan is now to carry out more surveys this year, focussing particularly on habitat that seems likely to be suitable for Noctules. To avoid making the same mistake twice, the audio from the roof-mounted detector will be fed via a lap-top, so that a permanent record can be made.


Anyone fancy a drive in the country?....


The standard method: http://www.ibats.org.uk/page.aspx?tabid=256


My web-site: plecotus.co.uk