I must be batty!

 

End of 2020 National Bat Monitoring Programme. (NBMP)

 

That’s another season done! The last of the NBMP surveys is the Waterways Survey in the first and second halves of August each year.

This survey counts the number of Daubenton’s Bat passes, as heard on a bat detector, as one stands beside the water, for each of 10 four-minute periods along a kilometre of water, be it lake or stream or river. One of the characters of Daubenton’s Bats is that they feed by skimming the surface of water, picking off insects that are emerging from the water or just hovering over it. (When seen on an infra-red night-sight they look like ice skaters.) By counting the number of passes at each station along the bank one gets an idea of how many bats are feeding in the area. So year on year one builds up a picture of the population changes for this bat.

 

 

A typical section of the Tamar where I record.

I do three sites for this survey; two on the River Tamar which separates Cornwall from Devon, and one on the River Lynher, which drains the eastern side of Bodmin Moor and joins the Tamar opposite Devonport. The sites are randomly selected nationally by the kilometre square, and we have to devise a route which allows us to set up 10 stations, at least 100m apart along the bank so that we have a reasonable view of the water. Initially the view of the water was thought to be important to allow the surveyor(s) to verify what they heard on the detector as Daubenton’s by shining a torch across the water to see the bats skimming the water. (The rich riverine habitat also attracts a lot of other bat species, especially Soprano Pipistrelles which fly higher and sound different on the detector.) But we soon found that a bright light very quickly frightens off the bats. So now, with experience, we rely entirely on the sound we hear from the detectors

The Lynher is a more intimate river.

I’ve been surveying the Lynher site, upstream from Callington Newbridge twice every August since 1997. The other two on the Tamar have been going since 2000. So these surveys now have a good long data-set, and I’m beginning to get the idea. People frown on me doing them alone, so I usually advertise when I’m going among my batty followers and the local wildlife recording group. But it is usually only me and one other. (It’s a bit difficult, stumbling around in the dark on a river bank, and keeping quiet for four minutes while one counts. All in the wilds of Cornwall, and effectively a 2 kilometre trudge.)

Numbers vary so much that it is hard to see any definite trend. We note the weather (but not the state of the flow – which I think is a mistake) and we record some basic habitat data (width of river, amount of over-hanging vegetation, whether the surface is riffled or calm, etc.) But for our local rivers we ought to be recording what the water state is at each station. And that varies with almost every count depending on the flow at the time. But over the years I would say that my results show a very steady increase in numbers.

I’ve not yet had anyone fall in, and no unpleasant surprises. On one occasion, while we were counting I heard some whistling calls. First upstream, then downstream, which made me think of Dippers. I said to James that Dippers should not be flying at night, but when we’d finished counting we would turn the lights on to see what it was. Then I heard a rustle at my feet, so switched my dim light on to look. A teenage Otter cub had crept out of the water, sniffed at my welly and shot back down the bank. So now I know what Otters sound like too. We often switch the torches on after counting to look for bats, and several times we’ve seen Otters, but usually at some distance, when the reflection from their eyes gives them away. But on one occasion an adult swam right up past us, passing only 3 m. away. It just adds to the joy of watching nature do its own thing.

See also: https://maryatkinsonwildonline.blogspot.com/