Bat

Here’s a novel way of listening to bats over the Summer. Put the receiver — powered by four AA (R6) cells — on a window-ledge, for example, preferably aiming the ultrasonic detector towards an outdoor light or some trees. Run out a few metres of cable so as to install the small loudspeaker inside the house. Wait for nightfall and, if there are bats around, you’ll hear a sound like bursts of crackling from the loudspeaker.