Wednesday, 27 February 2008

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.

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