Thursday, 8 May 2008

The Hunting of the Noctule

For some time I have been rather fascinated by Noctule bats (Nyctalus noctula). One of Britian's largest bats, they emerge from their roosts earlier than other species and fly high abve pastures and river valleys, dipping down to catch larger insects. With a very loud call (reputedly louder than the legal safe limit for audible sound), their distinctive "CHIP CHOP" can often be heard from several hundred metres away.

You can hear a Noctule here:
(It was recorded in the Scottish Borders, using a hetereodyne bat detector set to 22kHz).

Traditionally they have been assumed to be largely absent from Scotland, apart from Dumfries and Galloway. It has been known for a while that the occasional Noctule turns up in the Scottish Borders and that there is a small population in the Tweed valley. More recently, there have a handful of records from reputable sources of Noctules in the Lothians, which led some bat-workers to query whether they are more widely spread than previously thought.

I set out to try to clarify this, by gathering together records from many sources across the Lothians and Borders area. I eventually gathered 41 records, from across the region, ranging from audio recordings validated by reputable ecologists to visual records of "large bats". That sounds like a lot of records, but they range over a period of eight years and twenty of these records relate to just 4 sites.

You can see a map of these sites here:

Hoping to locate more of these bats, I decided to use a variation on the car survey method, used for the Bat Conservation Trust Bats and Roadside Mammals Survey. This very successful technique involves driving at twenty miles an hour along country roads, with a bat detector poking out of the window, allowing many records to made in areas where bat-workers are thin on the ground (See below for the detailed method).

My theory was that, by doubling the speed to 40mph, a lot of ground could be covered in one session: typically 80miles in a session. The Noctule is a loud, high-flying bat, so mounting the detector on the roof and travelling faster should be feasible. The detector could be linked to the car stero with a wireless link, so that it's output would be clearly heard and we could stop and investigate when we heard anything suspiciously Noctule-ish. Thus, two nights ago, with Nigel acting as navigator, we set off in a vehicle decorated with rotating amber beacon and reflective warning sign to drive down the Tweed Valley and test the theory.

It was a perfect evening for bats: warm, dry and with plenty of flying insects around. We encountered an initial problem with air moving over the detector's microphone, causing unacceptable levels of background noise. This necessitated a speed reduction down to 30mph, after which the system seemed to work well and we soon started to hear Pipistrelles.

As a slight cheat we paused by the Dryburgh Suspension Bridge at Newtown St Boswells, a spot where there is usually a Noctule or two. Sure enough, a Noctule flew past and we could clearly hear it. Further on we crossed the Tweed at Coldstream and, after negotiating an unexpected diversion we found another bat near Wark. This was certainly a Nyctalus bat, but it is just possible it could have been a Leisler's Bat (Nyctalus leislerii). We heard only a couple of brief passes and the Leisler's has a similar call. Typically, we had two hand-held detectors linked to minidisk recorders and both failed at the same time! Without a recording to analyse and verify the bat's call we have to simply call it Nyctalus sp. Nonetheless, it proved that this variation on the BCT method is effective and worth persevering with.

You may ask, why not use the standard method? Firstly, 20mph is simply too slow to cover the vast amount of ground we need to survey and 30mph appears to work with these louder, high-flying bats. Secondly, the standard method requires some very expensive equipment and teams of four people, both factors which could be limiting. That in no way negates the amazing work that has been achieved by people in the UK, Ireland and elswehere using the BCT method.

The plan is now to carry out more surveys this year, focussing particularly on habitat that seems likely to be suitable for Noctules. To avoid making the same mistake twice, the audio from the roof-mounted detector will be fed via a lap-top, so that a permanent record can be made.

Anyone fancy a drive in the country?....

The standard method:

My web-site:

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