A little before nightfall, we enter the river gorge equipped with fast-failing eyes and a box that will let us listen to what we cannot hear. As long as the backdrop to the crescent moon is still blue, we can almost outperform this bat detector. We start to see shapes flittering around the treetops on the opposite side of the gorge, but it’s not until they draw nearer and swoop directly overhead that the machine buzzes and spits at around 55kHz. That means these are soprano pipistrelles.
Our aural targets tonight are the lesser and greater horseshoe bats that hang up by day in the limestone caves and the tunnels and flues of the long-abandoned ironworks. Rather tidily, horseshoes’ echo-locating calls register at between 80kHz and 110kHz, far higher than other species. They also have a sonic signature that is worth waiting for.
When twilight comes, we are lost in the grey and engulfed by pipistrelles. The detector has dialled up a victory. There must be lots of bats crisscrossing in front of us, for although we catch only half-glimpses, the detector is on overdrive, recording the spatters of clicks from feeding buzzes as the bats close in on their prey. Somehow, above this electronic mush, I catch a loud splash from the river below and we turn the detector off.
It sounds like a toddlers’ paddling pool party, a full-on splashing session, except that instead of infant shrieks, there are little “huff-puffs” from the water. A mother otter with two, maybe three cubs. One submerges and swims in front of us, and we see its tail and sleeked body arc and turn away. We can barely see the otters; they can feel fish underwater. The little family slides into a back channel and there is silence.
The detector is on again and now horseshoe bats have arrived, hunting all around us. Their calls express as a messy symphony of alien bleeps and burbly warbles, better suited to a 1960s sci-fi movie. Our device has rendered this high-frequency chatter into outlandish sounds to our ears. But what do the bats hear?
• Under the Changing Skies: The Best of the Guardian’s Country Diary, 2018-2024 is published by Guardian Faber; order at guardianbookshop.com and get a 15% discount
