Dawn Swarming

Someone once said that bats are hard to study "...because you can't see them and you can't hear them." It's certainly true that they don't give away their secrets easily, though bat detectors have allowed us to start the long hard job of understanding them.

A behaviour exhibited by most British bats which tips the balance a little on our side is dawn swarming, in whcih bats returning to a roost fly around the roost entrance for a period of time at dawn, often making false landings at the roost entrance. In doing so they give away the roost location. This is in stark contrast to evening emergence, when they dive out of the roost, flying hell for leather to avoid any possible predators, making it hard to spot where they have emerged from!

Of course the downside is that dawn is a horribly antisocial time of day to be about, something I was reminded of when my alarm clock went off at 2.45 am this morning. With dawn in Edinburgh at 5 am I wanted to be alongside a roost site at a suburban house well beforehand, to watch swarming develop. It wasn't just for fun: I had a suite of things I needed to know about these bats. All I knew was where the roost entrance was: at the end of a flat roof bargeboard, giving access to a hollow wall; and that they were Pipistrelles. I needed to find out which species they were, whether they were roosting in any other parts of the building and approximately how may bats roosted there.

For the first half hour there were steady comings and goings, with Soprano Pipistrelles (Pipistrellus pygmaeus) entering and leaving the roost in small groups and occasional bats flying close to the entrance in twos and threes. At 4.25 am swarming started, with numbers gradually building up to a peak five minutes before dawn, with around 60-70 bats zooming round the adjacent garden. You can see a brief snippet of the action here:

Soon after this bats started to enter the roost. Up until this point, everything was as I expected: no other part of the house roof was involved, I knew the species of bat and could see an approximate number. All I had to do was count them as they entered the roost, giving an accurate count and the job was done...or was it?

Only 30 bats actually entered the roost. The remainder gradually gained height until they were around 5-10m higher then the house, then one by one they flew off to the north-east. I have never heard of this behaviour before and was quite astonished to see it. Stuart Smith, of Lothians Bat group was later able to tell me of another roost site about half a kilometre away in that direction, so perhaps that is where they were heading. It is quite common for a colony to move around between several roost sites, but I have never before heard of them swarming at one site and then splitting up, to roost in mor than one location.

Whatever the cause, it was worth being up early to see the spectacle: the video doesn't really do it justice!

My website: plecotus.co.uk

"Untold riches" of swarming bats...

I am officially conferenced out, having spent the last three days in York for the
Mammal Society Conference. It was definitely worth going: I'm buzzing with thoughts about Irish hares, Beaver management in Bavaria, small mammal survey trials and much else besides. It's a little disappointing that there were no papers on bats, but that was more than made up for by Professor John Altringham's Cranbrook lecture.

John's team from Leeds University have carried out a large and complex study of bat swarming behaviour at caves in the Yorkshire Dales and the North Yorkshire Moors. Autumn swarming is a behaviour which is increasingly seen as an important part of the annual cycle of bats, especially Myotis and Long-eared species. Bats gather at hibernation sites, usually caves and mines, to check out the site, to help young bats find it and for courtship and mating. Swarming is typified by chasing behaviour late into the night, often peaking three or four hours after sunset and takes place during August to October.

They looked at Natterer's Bats (Myotis nattereri) and, by ringing bats at known summer maternity roosts and then harp trapping at swarming sites, were able to demonstrate that bats wer flying up to 60km to a swarming site. They were also able to demonstrate that huge numbers of bats were using these sites: up to 400 per night, with considerable turnover between nights. Furthermore, by comparing data year on year they were able to demonstrate high fidelity: bats tend to return to the same site each year. The conservation implications are huge: these sites can provide a key part of the lifecycle of very large numbers of bats, from a wide geographical area.

What struck a chord with me was the fact than John described the importance of these sites and went on to mention that it was then usual to find only a handful of hibernating bats within the caves, as many were not visible. In other words, some of the hibernation sites we survey in the Scotland could be swarming sites for similarly large numbers of bats. Not for nothing was one of John's conclusions the possibility of "untold riches" for bat-workers.

This tiny hole, high in the hills in South-west Scotland, gives access to a tunnel over half a kilometre long, which contained 9 Natterer's Bats and 1 Daubenton's. Could this be a swarming site on a similar scale to the ones in the Yorkshire Dales?

Another intriguing part of the lecture related to a study in which John's team compared the numbers of hibernating bats to the physical features of a cave or mine. They found that the size of entrance was irrelevant ("if you can get in, they will"), as was the altitude, the orientation of the entrance and the habitat nearby. What makes a good hibernaculum is large spaces inside, a reasonable depth, some cover near the entrance and not too wet. This struck a chord with me, as we've done several surveys in the past winter of a limestone mine, which seemed to have all the right features of a hibernaculum, but we never found any bats. John's conclusion was "the more water, the fewer bats", which seems to fit.

I must finish by plugging the Mammal Society's new edition of "The Mammals of the British Isles". It's not cheap (and you may need to reinforce your bookshelf to take the weight), but it is worth every penny. If you buy a copy via the Mammal Society website, all ofthe profit will go to the society.

John Altringham's Leeds University web-page: http://www.fbs.leeds.ac.uk/staff/profile.php?

The Mammal Society: http://www.abdn.ac.uk/mammal/

My website: plecotus.co.uk

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