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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