Time Team vs Bat Team

My favourite TV programme is Channel 4's "Time Team", in which a team of archaeologists dig a historic site and attempt to make sense of it in just 3 days. Although the practical archaeology is interesting, for me the best part is when Stuart Ainsworth and Mick Aston take to the air and attempt to tie their site into the surrounding area. A Roman villa or iron age settlement is all very well, but when you can identify how people travelled to and from it, where they fished or went to market and why they might have chosen that particular place, it all makes so much more sense.

Surprisingly, there are strong parallels with bat work. We tend to concentrate on roosts and hibernation sites. And why not? They are the focus of the bats' activity: where you can conduct a roost census or watch bats swarming. If you have a reason (and licence) to trap and examine them, this is the best place to do it. Roosts are so much more exciting. Roost sites can be hard to find and of course they are where a population of bats may be at its' most vulnerable and in need of protection. Anyone can go for a walk with a bat detector and find a bat in flight, can't they?

Possibly they can, but with a bit of planning and focus, finding bats in flight can help to build up an understanding of how bats use the landscape, in the same way that "Time Team" try to work out how Romans or celts did.

Here's a simple example. Last year I was conducting a series of sunset surveys in some farmland, where a water pipeline was due to pass through. In two places I noted large groups of Soprano Pipistrelles (Pipistrellus pygmaeus) feeding soon after sunset. This can be an indication that a roost may not be far away. An even better indicator is a commuting bat. Bats commuting to and from their roost at dawn and sunset tend fly fast in a straight line: they're on their way somewhere, so why waste energy flitting about, as they might when foraging? A bat flying fast, and straight at dusk, is therefore a good indication that the roost lies in the direction it's coming from. When it's followed by other bats doing the same there is a good chance it isn't far away.

The next survey was in a path running through a strip of woodland. Over twenty Sopranos passed through at sunset, all going in the same direction, the direction of one of the foraging sites I had previously noticed. Soprano Pipistrelles usually roost in buildings and the large-scale map showed a Victorian lodge half a kilometre away, right in line with where they were coming from. I dragooned some help and the next morning we were lying in wait for them near the lodge. But no bats arrived. Spreading the net a little wider, we came across about twenty bats, swarming in the corner of a field. Just to confuse us, there were no buildings close to where they were swarming )they would normally swarm in front of the roost). It turned out they were roosting in a tree, which is quite unusual for Pipistrelles in Scotland. It took another couple of visits to work out which tree the bats they were using: a mature oak.

It's always nice to find a new roost, but the icing on the cake was to also have an understanding of how they were using their habitat. I hope to find time this year to look at some of the other hedgerows and woodland edges which radiate out from the site, and try to work out what other commuting routes are used, adding to the picture of how these bats are using their landscape.

Of course, it'll take more than the three days that "Time Team" get....

The field where we found the Soprano Pipistrelles swarming at dawn. No, the oak tree in the middle isn't the roost: that would be too easy, it's to the left, out of sight.

My website: plecotus.co.uk

Hibernating Pipistrelles

Today I spent a very pleasant couple of hours wandering through a semi-ruined castle with members of Lothians Bat group. We were there to look for hibernating bats, which occupy crevices and cracks in the decaying stonework. The most popular spot is in the deep, barrel-vaulted ceilings, which in other old buildings are often used for summer or autumn roosting.


To understand why they are so popular with bats, look at the schematic below. In a typical medieval building, arches and ceilings are built from stone blocks arranged into a curve, so that each is supported by the one outside of it. Above this, the infill is usually made of rubble. Over the years the mortar tends to fall out of the gaps between the stone blocks. As long misguided perfectionists don't repoint the stonework, this creates crevices, some of which may extend into spaces within the rubble fill, creating sometimes quite large bat havens. The downside for us is that surveying these buildings quickly causes a sore neck, from peering upwards, pointing a light into the crevices!


What is particularly interesting about the site we visited today is that, not only is it used by hibernating Brown Long-eared Bats (Plecotus auritus), two of which we saw, it is also used by Pipistrelles (Pipistrellus sp.). Remarkably little is known about the hibernating behaviour of Pipistrelle bats: they are rarely found in the mines and caves where larger British bat species are found hibernating. It is generally assumed that they hibernate individually in small crevices in trees or buildings and are probably more tolerant of fluctutations in temperature and humidity. So it was nice to find at least twelve of them here.

Unfortunately, it isn't possible to say whether they were Soprano Pipistrelles (Pipistrellus pygmaeus) or Common Pipistrelles (Pipistrellus pipistrellus): the physical distinctions are difficult enough with a bat in the hand! However, a braver bat-worker than I might note the dark muzzles and wonder if they might be commons.

Click on the picture below to see a larger version of it and you'll see the brown splodge in the centre resolve into a Pipistrelle's face.

The following two pictures show two groups of bats, sharing crevices. I have no idea how many are hiding behind the mortar in the first picture, but the crevice in the second contained seven bats.

My website: plecotus.co.uk

Lime kilns, mines and souterrains

During the recent spell of unseasonably pleasant weather (back to normal now - there's rain going sideways past my window) I and a bat group colleague spent an interesting afternoon in an upland area of Midlothian, looking at various man-made structures with potential to be either hibernacula or summer roosts.

Information on the sites had come from a mixture of word of mouth and references on the CANMORE archeological database (see Armchair roost-searching, February 2008). Some CANMORE references can be little more than a name and grid reference, others include detailed archeological descriptions, which can be fascinating.

The first site was a limestone mine. This is a known hibernaculum, which has been grilled in the past, partly to protect hibernating bats from disturbance and partly to prevent adventurous youngsters from getting themselves into difficulties. I wanted to take a look, as I hadn't previously seen the site and it apparently used to have an underground link with another site, which Lothians Bat Group survey every winter for hibernating bats.

Hibernaculum grilles have quite widely-spaced, horizontal bars, to prevent human access, but make it easy for bat to fly through. It seemed some local wit had been there before us, and left his thoughts for all to see.

The next plan was to look at several lime-kilns, which are prolific in the limestone areas of Midlothian. These are large stone-built structures, usually built into a hillside. Inside is a tall, brick-lined charging-column, into which limestone and fuel was placed. At the base are draw-holes, usually with a barrel-vaulted roof, which are used to control the air-flow, and to draw the completed lime out, ready to be mixed with water and used as fertiliser or building mortar.

Barrel-vaulting can lose it's mortar over time, creating deep crevices, perfect for hibernating bats. Also, the stonework of the kiln can decay, creating roosting opportunities. Unfortunately, none of the three kilns looked at had many crevices in the draw-holes, but all had decaying stone-work to a greater or lesser extent.

One of the kilns was a massive structure, with two charging columns and six draw-holes, all in excellent condition, making me wonder if it may have been rebuilt in the 1930s, when the government offered incentives to farmers to burn lime, due to concerns about deteriorating land fertility.

At the other end of the scale was a badly-decayed kiln within dense woodland. Landslips had partially buried it and we found a solitary bat dropping in a huge crack in the stonework. A single dropping doesn't make a roost, but it may be worth a return visit in the summer.

The third site had a lovely Barn owl (Tyto alba) roost: a deep hole, high up on the side of the kiln, with long white streaks, left by the owl's characteristic runny droppings. Inside and on the ground below were plenty of large, dark-coloured owl pellets.

There was no sign of the owl (or owls), though it could have been deep inside it's roost-hole, out of sight. Not a bat roost, but it was very nice to see signs of this scarce bird, all the same.

Having seen enough lime kilns to last a while, the last site of the day was a soutterain. These are iron age (about 700BC-500AD) structures, believed to be underground storage facilities. In an age before refridgeration, building a shallow, stone-lined tunnel was a good way to keep food dry and cool, to last through the winter months.

This particular soutterain lies in a small, fenced-off area in the centre of a field. Accessed via a low roofed doorway (and when I say low, I'm talking about crawling on hands and knees), and short entrance tunnel, it is almost 16 metres long and up to 2m wide and high. The walls are made of unmortared stone, full of very deep crevices.

No hibernating bats were visible, but there could have been legions of them, out of sight! With a low, stable temperature and high humidity (yes, it's mud you have to crawl through...), the conditions are ideal for a hibernaculum, especially as it is in area full of excellent bat habitat.

The soutterain is out of sight of the nearest road and probably not known to many people, so is unlikely to suffer much disturbance, especially in winter. That said, a team of Powergen workers, dangling from nearby power lines looked fascinated by what we were up to.

More information about barn owls: barnowltrust.org.uk

Keep up to date with the latest posts Facebook.com/Davidsbatblog

Confessions of a bat surveyor

I sometimes find working with bats a little humbling. Not only is there a vast amount we don't understand about the bats here in the UK, let alone worldwide; there are many big bear-traps, waiting to catch the unwary bat surveyor. I thought I knew most of the worst ones, but today I discovered I had fallen into a trap I actually knew about, a really obvious one. Oh bum!

I think Brown Long-eared Bats (Plecotus auritus) must be my favourite bat species (hence my choice of web address). They are very attractive (by bat standards), with enormous rabbit-ears, dwarfing their bodies and a face that makes them look faintly bemused by life. Better still, they occupy a very specialised and fascinating niche: emerging late, flying slow and using their ears to listen for their prey. Not called the "whispering bat" for nothing, their echolocation is very faint, allowing them to catch those moth species which have developed rudimentary ears to help them take avoiding action when an echolocating bat is near. In short, they are nothing short of amazing.

Brown Long-eareds are generally easy to identify in Scotland: nothing else has ears anything like that size. In the South of England, things are harder as there is another long-eared bat: the Grey Long-eared Bat (Plecotus austriacus), which is extremely difficult to separate from it's brown cousin. Bat-workers there have to resort to measuring tiny parts of the bat's body to separate the two. Unfortunately, I don't have that excuse!

The trap I fell into was to identify a Brown Long-eared Bat as a Daubenton's Bat (Myotis daubentonii). How is this possible? The Daubie has tiny ears by comparison! Long-eared bats have an endearing habit of folding their ears under their wings when they go into torpor, to help them retain warmth. This leaves the tragus (the spear-like middle-part of the ear) sticking up, looking for all the world like a small ear...

In my defence, I must say that I wasn't alone, there were several (nameless) people with me on a hibernaculum survey last month, and they share in my crime! I even took a photograph of the bat in question, which, when enlarged, clearly shows the roots of the ears folded over the flanks, but it was a bit fuzzy, so I didn't look closely at it until this evening. Today we did a second survey of the hibernaculum (the National Bat Monitoring Programme requires two winter surveys, a month apart). We found a bat in a similar place, in the same attitude, yet seeming to be a different species. It seemed too much of a coincidence, so on arriving home, I checked the photographs. It's a fundamental error, but I suppose we were peering in torchlight at a bat on the roof of a mine...

Anyway, in the spirit of public humilition and restitition I hereby present my guide to not making the same mistake! The three pictures below tell the story.

Firstly, the Brown Long-eared in question. Note the wing roots folded over the flanks, the shape of the tragi (masquerading as ears) and their pale colour. If the picture looks a bit odd, it's because the camera is pointing up and zooming into a bat hanging from the roof of the mine.



Next, a Daubenton's Bat for comparison.

Finally, it's close relative and another species commonly found in our hibernacula: the Natterer's Bat (Myotis nattereri)

Now go forth and learn from my error! Mea culpa, mea culpa, mea maxima culpa.

Please remember that handling bats, disturbing them or their roosts, photographing them or surveying their hibernacula, all requires a licence. If you're interested in getting involved, join your local bat group. See here for a list of contacts: http://www.bats.org.uk/batgroups/batgroups_list.asp

For information on the National Bat Monitoring Programme (NBMP) see: http://www.bats.org.uk/nbmp

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

The kitchen table harp trap

For some time I'e been thinking about building a harp trap for my own use and I'm delighted to say that this winter I have finally found the time to do so. What is a harp trap? Also called a Tuttle trap (after Merlin Tuttle, whose invention it is), it's an ingenious trap which catches bats in flight, without harming them.

It works like this: a bat flies towards the trap and recognises a harp-like collection of vertical nylon lines about 2.5cm apart. With great agility it successfully flies between the lines, but what it doesn't expect is a second set of lines offset from the first, so that it can't avoid hitting them. It then slides unharmed down the lines, into a canvas bag below. The bat will try to climb the side of the bag, so it can take off again, but the sides of the bag are lined with thick, slippery polythene. The bat can climb the canvas, but soon reaches the seam where it is joined to the polythene and can go no further until it's removed.

Harp traps can be marvellously effective, when used properly. They are particularly good at trapping bats leaving roosts, as the bats have less time to see and avoid the trap than they would if they were free flying. More importantly, the bats are entirely unharmed and in my view suffer less stress than if caught in a mist-net. The problem is the cost: a single harp trap costs upwards of £800. See http://www.faunatech.com.au/products/harptrap.html for pictures of a commercial trap made by Austbat.

I reckoned that I could reduce that cost to under£200 by building my own trap, and add a few ideas of my own. Firstly, I wanted a smaller trap. With a catching area 1.8m wide and 2.4m high, the Austbat trap is unwieldy in some circumstances. It is also difficult to erect, as it is necessary to unroll and stretch the lines every time the trap is used. This process is fraught with difficulty at the best of times, let alone in darkness and pouring rain, when you're tired and fed up at 2am! My thinking was that a smaller catching area could allow for the harp section to be permanently erected within a frame. When not in use, the frame could be bagged , with the legs and catching bag removable for transport. A smaller trap would also be lighter and could be lifted on a couple of poles to cover the entrance to a high tree roost or the eaves of a roof. You can do that with an Austbat trap, but you need ropes and a team of stout people singing sea shanties, to haul it up!

And here it is...

The catching section is slightly over 1m square, with the frame, legs etc, made from lightweight square-section aluminium, which also has the advantage of being an easy material to work. The legs are easy to remove, without affecting the rest of the structure, so that the frame can be slung between two poles and lifted up to a roost entrance.

I took the opportunity to address two weaknesses I've noticed with the Austbat and similar traps. They have the nylon monofilament tied around the frame top and bottom. This has the effect of making the nylon vulnerable to scraping against rough surfaces and fraying or breaking. I tried to address this by tying the nylon lines to tiny hooks screwed into the frame, so that the frame itself protects the nyon.

The Austbat traps are also vulnerable to escapes at each end of the bag, as the plastic material is only attached to the two long sides. I have made a point of attaching plastic to the two short sides, in the hope of preventing that. The base of the harp frame sits slightly inside the bag, to allow this to be possible.

The hardest part of building the trap was sewing the canvas and plastic. Dressmaking isn't a skill of mine anyway, and it takes a lot of effort to push a sailmakers needle through several layers of the thick canvas. My fingers may never be the same again!

Where does the kitchen table come in? That's where I built it (it's too damn cold in the garage at this time of year!) The table still bears the scars.

Of course, I'm rather proud of the trap, but success isn't realy measured by appearance, or sound engineering or even my pride - it's measured by whether it actually works: will it catch bats?

Watch this space...

Please note that the use of a harp trap to catch bats in the UK is heavily restricted. I am able to use one because I have a roost visitor's licence, endorsed for the use of a harp trap, I am also licenced to catch a number of bats for a specific scientific study. Trapping bats without he necessary skills and licence is illegal and likely to be harmful to the bats. If you're interested in getting involved in this kind of work, the best starting point is to join your local bat group.

Keep up to date with the latest posts Facebook.com/Davidsbatblog