Bats in Caithness

Before we discuss bats in a local context, I have included some information about bats in general.

 

Species

Eighteen species of bat call the UK home, seventeen of which are known to breed here; remarkably, bats account for almost a quarter of our mammal species.

​

Habitats

Depending upon the species, bats frequent a wide range of habitats for feeding, ranging from Broadleaf & Conifer Woodland, Parkland, Hedgerows, Water Bodies such as rivers and lochs as well as meadows etc.

They also need good commuting habitats to help them travel safely between their roosts and feeding grounds. Because bats use echolocation to navigate, hedgerows, tree lines or rivers can help them find their way more accurately – as well as sheltering them from predators.

​

Roosts

A bat refuge is called a roost; bats need different roosting conditions at different times of the year, and they will often move around to find a roost that meets their needs. In summer, female bats form maternity roosts while in winter, bats will use roosts which are suitable for hibernation.

Most British species evolved to live in trees, crevices within rock faces and caves etc., but many have now adapted to roost in buildings including barns, houses, tunnels, and bridges etc. Bats can show a high degree of fidelity to their roost sites, particularly those used for hibernation and maternity, returning to the same locations year after year.

​

Echolocation

Blind as a bat; actually, bats have quite good eyesight and will use their vision for locating their roosts or for spotting predators. However, it is echolocation that enables bats to take advantage of foraging for nocturnal insects, avoid predation i.e., birds of prey, and the ability to navigate in the dark.

​

Echolocation, the active use of sonar along with special physical features and functional adaptations that basically allows bats to see in the dark with sound. Most bats produce echolocation sounds by contracting their larynx (voice box). Though a few species, click their tongues.

These sounds are generally emitted through the mouth, but Horseshoe bats emit their echolocation calls through their nostrils: where they have basal fleshy horseshoe or leaf-like structures that are well-adapted to function as megaphones.

​

Diet

All UK bats eat insects, so they need to find roosts near good foraging habitats such as pasture, woodlands, or water.

Each species has their favourite prey types and hunts them in its own specific way, they feed on a range of insects from midges, flies, beetles, and moths etc.

Most insects are caught and eaten in mid-air, though bats sometimes find it easier to hang up to eat larger prey.

Flying is energy demanding, an individual common pipistrelle bat can eat over 3,000 tiny insects in a single night.

​

Hibernation

Bats living in the UK and elsewhere in northern Europe etc., have evolved to cope with the variability of the weather, firstly during November to March, bats go into hibernation. Basically, hibernation is an extended period of deep sleep that allows animals to survive during harsh weather. A bat’s body temperature lowers, and their metabolic rate slows, meaning they use less energy and can survive on the fat they have stored up instead of trying to forage for food. During hibernation, bats need roosts that are cool and remain at a constant temperature. They often move into buildings and underground sites, such as cellars and caves, though they are exceptions. I’ll return to that later.

​

If during the spring or summer the weather is poor for several consecutive days i.e., wind and rain with a drop in temperature, this would be accompanied by a significant reduction in insect prey activity. Hunting in such conditions would be counterproductive in terms of energy loss, so during periods of poor weather bats will remain within their roosts, and go into a condition known as torpor, which is a less acute version of hibernation that still saves them energy, by reducing their heart rate and respiration.   

​

Threats to Bats – Habitat Loss

In the UK, bat populations have declined considerably over the last century, the decrease in bat numbers reflects the changes which have taken place in our countryside, particularly with the change to intensive agricultural practises. Natural habitats (foraging areas) such as hedgerows, woodlands and ponds have been declining and fragmenting.

​

The use of pesticides has led to a reduction in the abundance of insects which the bats rely on as their only food source.

It is important that we create new suitable habitats and manage and enhance existing ones, in-order to help bats recover and survive.

​

Threats to Bats - Development

Bats that roost in buildings are extremely vulnerable to the activities of humans, they can be directly threatened by building works, if they are present while the work is underway or if a demolition is taking place.

If bats are disturbed at a particularly sensitive time of year (during hibernation in winter or during breeding), it can have hugely detrimental impacts upon local bat populations.

​

Tree felling can also result in the loss of roost sites.

 

Caithness

Within Scotland in general ten species of bats can be found; however, in Caithness only three species have been recorded breeding; namely:

​

• Common Pipistrelle (Pipistrellus pipistrellus). 

• Daubenton’s bat (Myotis daubentonii), and

• Brown Long Eared bat (Plectus auritus).

​

​

​

​

​

​

​

​

​

Vagrant bats have also been recorded within the county, even from latitudes further north, such as the aptly named Northern bat (Eptesicus nilssonii).

I have been surveying bats in Caithness both on a professional basis and for my own interest for some fifteen years now. Though I have undertaken surveys from as far afield as the south and north of England, Northern Ireland, and all over Scotland for approximately twenty-five years. What I’m about to discuss is based upon my observations in Caithness in comparison to the other areas I have been.  

​

Local Observations

To be honest my observations in Caithness, relating to two of our three breeding species; namely, the Daubenton’s bat, and Brown Long Eared bat, range from infrequent to non-existent. Though not from the lack of trying, I’ve surveyed places where further south you would almost be guaranteed to record Daubenton’s bats but not here.

​

Brown Long Eared bats are basically a woodland species, their distribution being largely confined to parts of the county with greater amounts of long-established woodland i.e., the southeast. Additionally, this species is very difficult to record on detectors due to the short range of its echolocation.    

Subsequently, the observations I’m about to discuss relate to the Common pipistrelle; however, certain aspects will also be applicable to the other two species.  

​

I have recorded more than forty Common pipistrelle roost sites within the north of the county, in all but one the number of bats present did not exceed six individuals. Whilst some have turned out to be summer day roosts, used by one or two male or none breeding female bats, others possess characteristics associated with breeding (maternity) roosts. It is worth noting at this stage that all the roost sites that I’m referring to were found in buildings, ranging from cavities within stonework or beneath roof and ridge tiles. Both in occupied and unoccupied buildings.

​

Even elsewhere in Scotland, Common pipistrelle bats are not known for their large maternity roosts, they can range from 50 to several hundred individual females, not like their close relative the Soprano pipistrelle (Pipistrellus pygmaeus) where larger roosts have been recorded, some consisting of several thousand bats, personal observation. 

 

The largest Common pipistrelle maternity roost I have recorded here, consisted of 23 individuals; however, I have recorded at least a dozen roosts that possess attributes of maternity roosts with only 5 or 6 individuals present i.e., bats return to the roost sites in May and vacate the roost by late August / early September. All such roosts have been recorded within fractures / voids within stonework on south facing walls of structures, so that the maximum solar irradiation (warmest microclimate) is achieved within the roost.

​

My conjecture is that in Caithness, not only do we have a smaller number of breeding species (three), but the populations are also smaller and more widely distributed.  That said, much more research is required before this hypothesis can be accepted; however, my speculation is based upon the following thinking.

​

Climatic Influences

The extremities of the climate at such a northerly latitude, where the summers are shorter, temperatures drop, nights are shorter too, especially during the high summer months, with reduced hours of darkness available, for bats to hunt their insect prey.

​

The combination of reduced foraging time during the critical summer months, particularly for lactating females and the rigours of the weather may impact upon their breeding productivity.

​

It is speculated that bats have evolved to forage during the hours of darkness for two principal reasons; firstly, to take advantage of a food resource (insects), as feeding at night avoids competition with insectivorous birds, such as Swallows etc. Secondly, to avoid predation, particularly from birds such as owls, though some researchers disagree with this theory.  

​

Also, there is evidence relating to several negative physiological costs associated with female bats that go into torpor during pregnancy or whilst lactating, due to prolonged poor weather.  Given the requirement to conserve energy during such periods there are consequential reductions in foetal development and milk production.

​

Therefore, the longer or more frequent use of torpor by pregnant or lactating females, the greater the potential of a reduction in their breeding productivity.

​

Suitable Habitat

A study of bats in western Norway which explored the spatial distribution of bats around lakes in valleys. Stationary ultrasound detectors were used to record ultrasound at various distances from the shores of lakes during the bat’s late pregnancy and lactation period. The researchers noted that there was a clear non-linear distribution pattern, with a steep increase in bat activity close to the shores.

​

They concluded that freshwater is of high importance to bats, combined with the presence of woodland near lakes and other water courses, due to the presence of hatching insects upon which the bats feed, particularly during the summer months when females are lactating.

 

Similar habitats are present in Caithness, we do not have any shortage of freshwater lochs and other watercourses, but suitable woodland habitat is not commonplace and what exists is well dispersed.

 

It is therefore no surprise, that the largest maternity roost I have recorded in Caithness, occurs some 30 metres from a watercourse with well wooded banks.

   

Breeding

Just like birds (the dawn chorus) breed during the spring and early summer, bats have a breeding season too, only bats mate during the autumn and sometimes into the winter when they hibernate, after which, the females then store the sperm and do not become pregnant until springtime, delayed implantation. 

​

Also, as male birds sing to define their territory and to attract a mate, male bats do too. 

​

Common pipistrelle social calls for example are generally used for courtship, to attract a mate and to protect a territory. Between late August and September/October males will create a courtship territory approximately 200 meters in diameter, into which they will try and attract females and which they will also defend. Males make repeated song flights around their territories making social calls or can sing from a song post.

 

Social calls consist of a series of between 2 and 6 pulses: normal navigation, and foraging echolocation calls peak between 45 to 47 kHz. However, social calls are emitted at significantly lower frequencies with the lowest around 20 kHz.

​

I have included a sonogram depicting the recorded echolocations made by a Common pipistrelle bat, I recorded recently. The lower markings are the social calls, you can also listen to a shortened version via the audio player, the louder chirping noises are the social calls. 

​

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Hibernation

As stated earlier the Common pipistrelle is our most widespread species; however, we don’t know where they all go in winter! As the number of recorded hibernation roosts do not account for the numbers of bats seen during the summer months.

Common pipistrelle’s courtship behavioural period (late August to October), which usually occurs as a prelude to hibernation, as in common with other bat species, pipistrelle bats are known to hibernate near where they perform their courtship routine.

​

Common pipistrelles are also known to hibernate in relatively exposed locations, such as between roof tiles, as unlike other species which select places such as caves and cellars with suitably stable air temperature regimes etc.

​

Another study of how bats used roosts during late autumn and winter in Norway, discovered them using rock crevices or scree within which to hibernate, but also used man-made structures where available.  Putting such findings in to a local context, it is feasible that bats will use such natural roosting sites here too. Additionally, buildings with suitable fractures and voids within the stonework and roof tiles, features that replicate natural roost sites, and are common characteristics of buildings that I have surveyed, are also highly likely to be used by hibernating bats.    

This trait in terms of exposed roost locations has been previously observed by the author and reported by researchers such as Sendor and Simon (2003).

​

Other researchers have recorded hibernal roost switching and concluded it to be an important aspect in the Common pipistrelle’s mortality avoidance regime, developed to avoid the risks associated with a fluctuating environment. Basically, roost switching is a trade‐off between the risk of freezing to death in more exposed (less buffered) roosts when cold weather occurs, and the risk of starving due to a high energy demand associated with stable but warmer hibernacula.

​

Such an approach appears to be favoured by natural selection due to the pipistrelle’s small body size and their apparent inability to accumulate sufficient fat reserves to survive the winter in a stable hibernaculum. Subsequently, pipistrelles selected hibernaculum sites that tend to be more exposed to the elements but facilitates the need to easily exit such roosts to feed during the winter when warmer conditions allow.

 

Therefore, the microclimate of a hibernacula is less important with regards to the Common pipistrelle than with other bat species.

 

Conservation

My hypothesis if correct has important conservation ramifications, firstly roost classification, in bat conservation terms both maternity and hibernation sites are the most important, as they provide the right conditions to raise young and facilitate hibernation during the winter months.

​

Let’s say hypothetically, an old croft building, or barn contains a maternity roost, and is scheduled for redevelopment, which is not an uncommon scenario in these parts. As part of the planning application process, the council specifies that a bat survey is required, now thankfully a common practise. A bat surveyor from outside the area, (further south) who is not familiar with local bat ecology is called in to carry out the survey, during which they record five Common pipistrelle bats emerging from a void within the stonework of the building.

​

Given that their expertise is based upon surveys in other areas, where Common pipistrelle maternity roosts contain a greater number of individuals, it is therefore highly likely that the roost would be given a lower conservation classification than it should, i.e., a day roost used by a small number of male and or none breeding females.

​

Subsequently, the mitigation measures put forward so the redevelopment can proceed will not be as stringent as they should. Potentially, resulting in the loss of a locally important maternity roost. I have encountered similar situations over the years I have been surveying in Caithness, and I dare say that there are plenty of other times this situation has occurred, that I’m unaware of.  

​

Habitat Creation

Woodland creation around our lochs and watercourses would overtime increase the amount of foraging habitat for bats; however, other species would benefit too, including birds and invertebrates.

 

I hope you have found this subject matter informative; I intend to update this page as my surveys and research progresses.

 

References

Bat Conservation Trust

Hull G. Unpublished Bat Survey Report Reports and Observations. 2006 – 2023.

Jenkins. E.V. Laine. T. Morgan S.E. Cole. K.R. & Speakman. J.R. (1998) Roost selection in the pipistrelle bat, Pipistrellus pipistrellus (Chiroptera: Vespertilionidae), in northeast Scotland. Animal Behaviour, 56, 909–917.

Michaelsen T.C. Jensen K. H., and Högstedt H. Roost Site Selection in Pregnant and Lactating Soprano Pipistrelles (Pipistrellus pygmaeus Leach, 1825) at the Species Northern Extreme: The Importance of Warm and Safe Roosts, Acta Chiropterologica 16(2), 349-357, (1 December 2014).

Michaelsen T. C. Olsen O. Grimstad K. J. Roosts used by bats in late autumn and winter at northern latitudes in Norway. Folia Zoologica, 62(4):297-303 (2013).

Odysseus. Pipistrelle Social Calls. 2021.

Common pipistrelle (Pipistrellus pipistrellus) makes several different types of call e.g., for navigation, hunting, bow calls (often in roost and mother-child communication) and social calls.

Sachteleben J. and Helversen O. Songflight behaviour and mating system of the pipistrelle bat (Pipistrellus pipistrellus) in an urban habitat, Acta Chiropterologica 8(2), 391-401, (1 December 2006).

Sendor. T. and Simon. M. Population dynamics of the pipistrelle bat: effects of sex, age, and winter weather on seasonal survival. Journal of Animal Ecology 2003 72, 308– 320© 2003 British Ecological Society. Blackwell Publishing Ltd.