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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A very unfortunate hat-trick

For those outside the UK and a handful of Commonwealth countries I should explain that the term hat trick is derived from cricket, a game inexplicable to anyone who doesn't love it. A hat trick occurs when a bowler takes three wickets in succession, an unusual and impressive achievement for a bowler. The hat-trick I am referring is certainly not impressive. I would also prefer it if it were unusual, but I fear it may be getting less so. Three times this month I have been asked by clients to help them resolve problems caused to them by sub-standard bat surveys.

Sadly this appears to be a growing problem in this country. Local planning authorities (LPAs) are obliged to consider protected species such as bats before issuing permissions such as planning consent, listed building consent or demolition warrants. LPAs are becoming increasingly aware of their responsibilities and as a result their requests for bat surveys to be carried out are becoming more commonplace. This is all well and good, as the intended result is that bat roosts are discovered in time and appropriate licensing, mitigation and compensation measures included in a project. The result is the protection of species which already face a wide variety of threats.

The problem is that there is an ever-growing band of people who have spotted this niche in the market and are attempting to fill it. There's nothing wrong with that, after all I'm one of them and my company David Dodds Associates Ltd carries out hundreds of successful bat surveys every year, as do many other companies. But bat surveys aren't cheap - they involve expensive equipment and  teams of trained professionals, working unsociable hours. And where there's gold there will always be gold-diggers.

It's hard for someone to establish the bona fides of someone offering to carry out bat surveys. Does having a bat license mean you have the required skills, equipment, experience etc? Not necessarily. How about membership of CIEEM, the professional body for ecologists? Sadly no, it means very little (though CIEEM are working on that).  The result is that people occasionally find themselves hiring someone who looks the part, but turns out to be a disaster in the making.

Timing is all in bat surveying. Night time surveys cannot be carried out during at least five months of the year (sometimes as much as seven months, depending on latitude and prevailing weather). Where a client gets caught out with a sub-standard surveyor sometimes their project is held up for half a year before the damage can be resolved, often causing great expense and a negative view of bat conservation.

So how does the problem arise? In my experience there are a number of types of sub-standard bat survey, and each seems to stem from a different source:

1. The well-meaning amateur. This is often a friend, relative or neighbour who knows something about bats and owns a bat detector. They are flattered to be asked to do a survey and possibly excited at the prospect of earning some money from bat conservation, which is normally a spare time enthusiasm. Their naivety means they either don't know about the BCT Bat Survey Guidelines or their lack of resources means they cut corners. They often lack the breadth of experience to interpret what they find. Their reports reflect their naivety and often don't provide the information an LPA requires to determine a planning application, causing the report to be rejected.
2. The over-worked junior ecologist. Big ecology companies face a problem: professional ecology is bottom heavy and experience is at a premium. The result is that junior ecologists are often sent out to do bat surveys with a low level of experience, sometimes assisted by random office staff who have even less experience. Bat surveying is not just about putting a standard process into effect - it's about being able to understand what you see, often cryptic and fragmentary data. Back at the office the sub-standard data is written up into a highly professional report, adopting the advice and reviews of experienced seniors. But if the core field-work wasn't up to scratch the result is often embarrassment when a project is halted because a bat roost is discovered which was missed or misinterpreted as something less significant than it actually was.
3. The cowboy ecologist. Sadly these exist in professional ecology as in any other field: people who cut corners, do half a job, or invent survey results (really - it happens!). They then tell their client whatever they want to hear and melt away. Often they get away with it, but sometimes their work results in a project being halted when a bat roost is found, causing embarrassment, expense and delay for the unfortunate client.
4. The desperate one-man band. The last ten years has seen a proliferation of small ecology companies. Most are excellent free-lance ecologists, working within their limits to deliver great professional standards. But from time to time someone gets desperate to pay the mortgage and takes on a bat survey job they are not geared up for. Their report will stand out like a sore thumb as the night-time surveys will have been carried out by one person, rather than a proper survey team covering all elevations of a building and bat roosts get missed.
5. The complete barking nonsense. A few years ago I was shown a bat survey report submitted to an LPA in support of a planning application. It was a single sheet of letter-headed paper from a pest control company, stating that their operative had sat in the attic for half an hour with an infra-red camera and hadn't seen any bats (Note - if this seems reasonable you really need to read the BCT Bat Survey Guidelines!). The best part was that the council ecologist had a fight on his hands over this, as one of the local councillors decided to weigh in and back to pest control company! 

Unfortunately in this era of austerity and cut-backs fewer and fewer LPAs can afford to have in-house ecologists reviewing reports. Increasingly bat survey reports are being accepted without being read by someone who has the knowledge and experience to be able to interpret them. This means that sub-standard surveys don't always get picked up, meaning those who produce them don't learn from their mistakes (or less charitably, they get away with their actions) and they go on to repeat them. 

The result of all this is that from time to time people who commission a bat survey in good faith find themselves with a rejected survey report and a long wait until next summer to commission a new one. Others find their projects halted because a bat roost has not been identified by sub-standard surveys and is then discovered or worse destroyed during work. Others still apply for a derogation license, based on the survey report and discover that either the survey was inadequate or the report doesn't provide a suitable mitigation and compensation scheme and their application is refused.

I don't mean this post to suggest that professional ecology in the UK isn't fit for purpose. We are fortunate to have well-developed conservation law and due process to maximise it's effectiveness. And we have many, many excellent professional ecologists. None of us is perfect and anyone is entitled to make a mistake and learn from it. But bat conservation is not well served when people hire apparent professionals in good faith and are let down. And that happens too often.