Chirps in the noise - the hunt for a missing bat

“Chirp…chirp…chirp.” I’m sitting on a clump of grass, with the farms and villages of the Dead burn valley laid out in front of me, one or two lights beginning to come on as the light begins to fail. “Chirp…chirp…chirp.” I’m holding a directional yagi antenna, pointed towards some farm buildings below me and on my lap sits a strongly-built radio receiver. The gain is set to maximum and the signal strength meter bounces to the right, in time with each sound. 

This is a pleasantly relaxing moment, watching the world winding down and waiting for my target to show its face. Before long I will leap into action, but for now I can just wait and relax.

A walk-in-talkie clipped to my jacket crackles. “Nothing yet.” says a remote voice. 400 feet below me and about half a kilometer as the crow flies, a couple of young ecologists with similar equipment are standing in the farmyard, waiting for a Common pipistrelle to emerge from her roost. We’ve been following this bat for several nights and are starting to build a picture of her nightly behavior. But a couple of times she has simply disappeared from the area where she spends most of her time and we haven’t yet succeeded in finding her before she returns. But tonight we’re ready for her.

Glued to the fur on her back with a special rubbery glue is a minuscule radio transmitter, half the size of my little fingernail, it’s hair-thin wire antenna trailing over her tail. It’s tiny battery produces a tiny radio signal - a steady series of chirps we can only hear if there is nothing between us and her. Not too hard perhaps in the flatlands of East Anglia, but here in the southern uplands of Scotland it’s an exciting challenge - every fold in the land is capable of blocking the signal, so that we hear nothing.

 

The radio crackles again. “She’s out.” The chase is on. Almost immediately the sound changes and the strength of the chirps vary, as she flies round the farm buildings, the old sandstone walls attenuating the signal. I slowly move the antenna to the left a few degrees and check the signal. It’s louder. To the right again. It’s quieter. Constant adjustments and checks allow me to keep track of her. The team below are doing the same and we constantly share compass bearings. She is roughly where they intersect on the map. They give chase, but my tack is to monitor from my hill-top eyrie and help them fill the gap.

True to form, after forty minutes of foraging around the tree-lined margins of a large pig-field she suddenly makes a move. I’m alerted by the fleet-footed team below and a moment later “we’ve lost her.” But I still have a faint signal for another few seconds before she disappears and I get that all-important bearing. A check of the contours on the Ordnance survey map reveal a fold  in the land in that direction, with a tree-lined pond. I vector the team onto it and off they go. Ten minutes later the walk-in-talkie crackles with a triumphant shout. “Got her!”

 

I sit back onto my comfy clump of grass, satisfied with another piece fitted into the jigsaw of this bat’s behaviour. With a sigh of satisfaction I change the frequency of the detector, to see what’s happening with the bat that team two are following.

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