Country diary: Into the twilight zone with horseshoe bats | Derek Niemann

Frome, Somerset: When they finally join the party, their calls are a messy symphony better suited to an old sci-fi movieA 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. Continue reading...