Occasional posts on subjects including field recording, London history and literature, other websites worth looking at, articles in the press, and news of sound-related events.

24 September 2010

Adventures in ultrasound: recording with the London Bat Group

IF YOU PUT yourself on the email list for the London Bat Group, you’ll get regular updates on the training sessions and bat surveys they’re running. I’d strongly recommend going along to one of them.

Even if you’re not (yet) much interested in wildlife, it’s still satisfying to explore in a group those nocturnal London places which usually only appear in newspaper reports: wooded area, dense undergrowth, unlit footpath.

Last Wednesday’s survey objective was to see if any Nathusius’s Pipistrelles were visiting the Leg o’ Mutton reservoir in Barnes, south-west London. To identify bat species during a survey, you need to listen to their ultrasonic squeaks with a bat detector. Here’s the cheapish model I use, a Magenta Bat 5:

Nearly all bat detectors have line-out sockets so you can record what they pick up. Bats usually mate during the autumn, and around this time several species in Britain start to form swarms and emit mating calls. Early in the evening we encountered a swarm of perhaps a dozen Soprano Pipistrelles, hurtling and wheeling only a few feet above our heads:

One of the pleasures of using a bat detector lies in the roughness of the signal which comes out of it, a quality of sound which more and more signifies the past. The bats’ sonar is mysterious when emerging from a background of hiss and crackle, like the cryptic radio messages in Jean Cocteau’s 1950 film Orphée.

Other things beside bats produce ultrasound: clothes snagged by brambles, the clink of coins or keys in a pocket, and insects. A harsh, monotonously regular signal had us wondering, until Philip Briggs, the survey’s leader, explained that it was the chirp or stridulation of a Dark Bush Cricket, here recorded at 25 kHz:

No Nathusius’s Pipistrelles were heard that evening, but by the reservoir’s edge was the sonar rattle of two or more Daubenton’s bats that flew fast to shock the vague clouds of insects an inch or so above the water.

Hello and welcome to Joe the biologist and anyone else who was there that evening, and has stopped by to read this.