I’m learning a lot as I get out there to try and survey bats. I’ve had no formal training. I’ve got a bat detector, and a sheet of notes about how different bats sound. It talks about wet slaps, metallic clicks, castanets and whether the bat was arrhythmic.
It’s a curious business, translating a stranger’s words into an understanding of a sound. We spent quite a lot of time huddled round the notes, reading them by torchlight, discussing what we’d heard.
A bat can be somewhat identified by the frequency at which it makes sound. However, many share ranges of frequency, so to tell them apart, you need to consider the kinds of sounds they make. That sound changes if they are flying at you, right over you, or flying away. Some bats can only be picked up if your detector is facing the right way in relation to them. Sometimes the detectors pick up just a few sounds – a more distant bat going the wrong way for you, perhaps. Tantalising possibilities that defy translation.
It was really exciting making sense of how sounds change as the bats move so I can hear when they are flying towards me. Sometimes that means seeing them as they fly over, sometimes it means scanning about, mystified as to where the bat I can hear actually is. Once it’s darker, it means knowing the bat is close even though I have no scope for seeing it. Sometimes they get really close. One of the ones I didn’t see apparently went right under my chin!
The most exciting moment of the latest batting night came when we picked up something at 110 kHz. Only one native British bat makes sound at that frequency. It was a distinctive sound, too, totally unlike anything else we’d picked up. A lesser horseshoe – generally a rare bat. There are known horseshoe bat roosts locally (maybe more, but definitely two) so it wasn’t entirely surprising to find one, but still, really exciting.