Crackle and Pop

The thing about bat walks is that there are no photos. They take place after sunset, and torches and flashes are discouraged because they ruin peoples' night vision. But they are wonderful.

The walk on 16th April was in Kelsey Park, Beckenham. It was organised by the Friends of the park and led by Ishpi Blatchley, a local lichen and bat expert. The park is closed at night, so we were locked in for the duration of the walk.

Ishpi gave her usual talk about bats, their nature and habits, and we wandered around listening on heterodyning bat detectors for their supersonic echolocating calls. At first we heard the commonest British bats, common pipistrelles, and saw some against the sky flitting across clearings. When we came to an outlook over one of the lakes, we saw and heard a soprano pipistrelle, a little scarcer than common pips. Pipistrelles make a call that sounds like a fast series of hollow pops, very distinctive when you are familiar with it; at around 45 kilohertz for common pips, and 55 for the sopranos.

The real treat came when we approached the big lake. Not only was the air like bat soup, with pipistrelles flocking around the lake margin, but we heard one of the bigger bats, possibly a Leisler's or a Serotine. It was calling at about 27 khz, slower and louder than the pips. It circled around several times, but no-one got a reliable sighting. Ishpi recorded it and should be able to identify it later from an analysis of the recording.

This lake is a great place to see Daubenton's bats. These creatures are specialised to feed over water. They zoomed around the lake just above water level, and as they turned, followed by a guide's torch beam (strictly pointed away from us at low level) we could see their white bellies. They have big hairy feet with which they can catch insects, but they were too far away for us to see this detail.

Daubenton's bats call at about 40 kilohertz and make more of a crackling sound on the detector, almost sounding like radio interference with the pipistrelles, which of course were calling at the same time. Tuning into bat calls is very like tuning an old-fashioned long wave radio; they are both heterodyning devices.

Since first going on a bat walk last year I have become much more aware of their presence. And pipistrelles are easy to find. Last week I was standing on my balcony at night watching my cat wander around my tiny garden, and three of them flew less than two metres over my head; first one, then two more. Magic.