As the seasons turn, and dusk arrives earlier, Matt Gaw tunes into native bats at Knettishall Heath
THE barn owl ghosts over the riverside path just after we set out, flying low and silent over the long grass of the riverside meadow. I follow her with my binoculars, the gold of her wing and the chalky whiteness of her body bright against the darkening sky.
Along with about 20 other people, I’m here at Knettishall Heath to look and listen for bats. The only mammal capable of true flight, bats are creatures of myth and magic, both maligned and revered as a symbol of life and death, a token of good fortune and impending doom. The superstition of the old world may have largely gone, chased into the light by science, and made cartoonish by Halloween and Hollywood, but the fascination for this winged creature of the night remains.
The owl disappears. Sam Norris, the ranger for Knettishall, and Mark Smith, from Suffolk Bat Group, lead us on, a swishing conga train of cagoules and Goretex now walking away from the river and towards the heath. All of us are carrying bat detectors, a hand-sized gizmo that transforms the inaudible ultrasonic calls of bats into a sound the human ear can hear. I walk with it at head height to keep it away from the interference of my phone, listening to its white noise crackle. A lightning rod for bat contact.
The detectors are set to 50kHz, a frequency that will capture two of the most regularly heard bat calls, those of the common and soprano pipistrelle. Back in the car park, Mark had told us that many bats will sweep through this frequency, it is then a question of tuning up or down to identify the peak frequency at which the bat is calling.
The range covered by bat calls can overlap,
but the peak frequency can be crucial to species identification, from the low castanet smacks of the barbastelle at around 36kHZ to the bubbling call of the incredibly rare lesser horseshoe bat at 109kHz. The tone, repetition and rhythm are all key too.
Mark also played us some recordings of calls to demonstrate and help us identify in the field – the pipistrelle that he described as having a fast clip-clop-clip-clip-clop rhythm, while the noctule, Britain’s largest bat, is louder and slower. Clip. Clop. Clip. Clop.
As we move from heath to wood pasture, one or two of the detectors pick something up. A wet galloping sound, like a high-speed heartbeat or the pulse of electricity on rail tracks. A bat. But it’s gone almost as soon as it arrives, leaving us with static and low conversation.
We keep walking. The light is really going now. Sucked into leaves, the gloom of the heath replaced by night time tunnels created by corridors of birch and pine. It is in these sheltered areas that bats hunt. If the air hangs still so do the midges and flies, clustering into smoky clumps that are easy targets for the bats’ echolocation.
Our group fans out, picking their way through the trees, each person finding their own spot, the dark only broken by a few wobbling torch flashes.
The calls come gradually; first quiet and then growing louder. There is more than one bat here. Mark thinks three, possibly four pipistrelle. They aren’t flying past but circling around, picking off moths and flies.
I retune my detector slightly, the clip-clop rhythm of the common pipistrelle is there, a jowelly wobble board of slurpy wet smacks. The calls get closer and closer together as the bat gains on its prey. The chase ends in an electronic raspberry – bat, ultrasonic call and insect coming together. Supper and death.
We walk from the wood back across the heath, the light from torches catching the backs of the eyes of Exmoor ponies that graze the heath. Asleep with their eyes open. Mark stops us one more time. He has picked up another bat. A soprano pipistrelle. Even smaller than its common namesake, its wing span between just 192mm and 232mm, its call is higher, the ultrasonic cries, sharper and closer together to catch its tiny prey. For the first time tonight I can actually see it too. Its flight is fast and erratic, dropping out of the darkness like a shadow made solid. Flitting, disappearing. Another little bat joins it and they fly fearlessly close to our heads.
To them we are no more than obstacles to be avoided, moving non-foody lumps in a world of darkness and echoes.
‘A wet galloping sound, like a high-speed heartbeat or the pulse of electricity on rail tracks’
Pipistrelle bat in flight. Photo: BCTRJ Brookes
Britain’s largest bat, the noctule. Photo: Belizer adobestock