Bats!
Back in 2022, while introducing a family to Weird Cave during a late-afternoon coasteer, one of our guides spotted something unexpected circling the roof of the outer chamber. Too big for a pipistrelle, a strong steady wingbeat – almost certainly one of the larger UK species, and our first suspicion was the now-rare Greater Horseshoe.
Not wanting to disturb whatever it was, we quietly left. A couple of our team have some background in ecology and bat work, so we knew enough to know what we didn’t know – that a roost in a sea cave on this coast could be a big deal, and that deciding for ourselves was neither our place nor, strictly, legal. We briefed all our guides to give the cave a wide berth and went looking for help.
Why a horseshoe bat would be a big deal
Pembrokeshire is internationally important for the Greater Horseshoe. The UK population is down to roughly 13,000 individuals – fewer people than live in Tenby in August – and the species has lost over 90% of its UK range since 1900. Around a tenth of what’s left lives in our county, alongside Devon and the Mendips as one of only three regional strongholds. The nursery roosts at Stackpole and Slebech are designated as a Special Area of Conservation.
They’re unusual animals even by bat standards. Body 5-7 cm, wingspan 35-40 cm, weight 17-34 g – and they can live for up to 30 years, the longest-lived bat in Britain. A horseshoe-shaped nose-leaf focuses their echolocation at a constant 82 kHz, well above human hearing. Unlike the pipistrelles most people see zipping around at dusk, horseshoes hunt by perching and pouncing – waiting on a ledge for a moth to pass and snatching it on a short flight. When they roost, they wrap their wings around themselves like a cloak, where most UK bats tuck into crevices.
Calling in Ecoleg
We got in touch with Ecoleg, a Cardiff-based ecology consultancy run by Elliot and Ben – bat specialists with the licences and experience we don’t have. In the UK, every bat species and every roost is protected under the Wildlife and Countryside Act 1981 and the Conservation of Habitats and Species Regulations 2017. You can’t just pop back for another look. Roost surveys require a Natural Resources Wales licence, the right kit, and people who know what they’re doing.
Within a few weeks we had a plan for Phase 1: get the ecologists into the cave by sea, document the habitat, look for signs of use, and run acoustic detection to confirm species.
Getting them there
This is where Tenby Adventure’s usefulness started. Weird Cave – our in-house name, and we’ll keep it that way – is accessible only by water, and only on the right tide. Wrong state, and you can’t get into the outer chamber at all; right state, and you’ve still got surge, exposure, and a commitment you can’t easily back out of. For a team carrying expensive survey kit and zero coasteering experience, that’s a problem.
So TA supplied what Ecoleg couldn’t: access, route-finding and water safety cover. Two of our guides led them out on SUPs from a nearby beach, timed to a slack high tide with a kind forecast. We handled the route, the tide and daylight windows, the kit transport, and the call on whether to go at all. Elliot and Ben handled everything inside the cave. We carried their bags and kept our mouths shut.
How bat detectors actually work
A bat detector is not a microphone in the ordinary sense. Most bat calls sit in the ultrasonic range – 20 kHz to over 100 kHz – well above the 20 kHz ceiling of human hearing. A detector picks those calls up on an ultrasonic microphone and then does one of two things with them.
The older, classic approach is heterodyne: you tune the detector to a target frequency – say, 45 kHz if you think you’re listening to common pipistrelle, or 82 kHz for a Greater Horseshoe – and it mixes that frequency with the incoming signal to produce an audible difference tone. The upside is instant feedback; the downside is that you can only listen at one frequency at a time, so the wrong species slips straight past you.
The modern approach is full-spectrum real-time recording. An ultrasonic recorder captures the whole band unaltered, stores the waveform, and lets you visualise each call as a sonogram afterwards. Call shape, duration, peak frequency and harmonic structure are all preserved, so species identification is based on call architecture rather than on guessing the right frequency in advance. The same data file also lets you count individual passes – which is how you go from “there’s a bat here” to “there are approximately forty bats using this site”.
Ecoleg brought a single detector of the modern type – one piece of kit doing both jobs at once: live confirmation in the cave, and a data file we could use afterwards for species ID and numbers.
What they found
All of us guides had suspected Weird Cave went back further than the outer chamber, but none of us knew how far. What we’d only ever floated the edge of during coasteers turned out, with lights and without swell slapping at your back, to be much bigger: a high corridor off the tidal chamber led to a run of dry inner galleries – high-ceilinged, with nooks, fissures and side tunnels. Exactly the kind of habitat horseshoe bats favour: thermally stable, dark, dry, undisturbed.
Elliot and Ben quickly identified the tell-tale signs of roost use – grease staining on the walls at roost height, guano accumulations under specific roof features, and acoustic hits on the detector as we moved through. They took a guano sample for DNA analysis. Within an hour they’d seen enough, and we left before disturbing anything further.
Their read on it was stronger than we’d hoped. It was plausible, they said, that Weird Cave held not just a roost but a maternity roost – where pregnant females gather in summer to give birth and rear pups. Maternity roosts for Greater Horseshoe bats are of national conservation significance. Everything pointed that way, but nothing was yet confirmed. That was what Phase 2 would be for.
Phase 2: the emergence watch
A few weeks later we were back – same two SUPs, same ecologists. This time we weren’t going in. We were sitting outside as dusk fell and counting every bat out.
It was one of those evenings the Pembrokeshire coast does properly. Flat calm sea, not a breath of wind, the sun dropping behind the cliffs so the limestone went from gold to pink to grey above us. We anchored the SUPs a short paddle off the cave entrance, Elliot and Ben set up the detector on a ledge with a clean acoustic view of the opening, and we waited.
The first bat came out at civil twilight – a single quick silhouette against what light was left in the sky. Then two more within a minute. Then four. Then we stopped counting individually and started trying to keep pace – a steady trickle of small dark shapes pouring out of the entrance, pausing to orient, then fanning out along the cliff line to hunt. The detector clicked and chirped through the speaker beside us. The trickle thickened into something like one bat a second, for the better part of twenty minutes.
When Ecoleg worked up the data the next morning, their estimate came in at around 200 individuals emerged – a number that put Weird Cave firmly into maternity-roost territory for the county, and right at the upper end of what any of us had dared to hope for.
Sitting on a board off the cliffs in a flat sea, watching two hundred rare bats spiral out of a hole in the rock as the light went, is the kind of evening that doesn’t really come round twice.
What came of it: the Concordat update
The most important outcome wasn’t the survey itself but what it triggered. Pembrokeshire’s commercial coasteering operators work under the Pembrokeshire Coasteering Concordat – a voluntary agreement with the National Trust, Pembrokeshire Coast National Park and Pembrokeshire County Council that sets site-by-site guidance for where and how we work. Once Ecoleg had confirmed the find, the responsible thing was to feed it back into the Concordat so every operator on the coast – not just TA – was working to the same protection.
TA supplied the Concordat working group with annotated maps of the Lydstep stretch, photographs of the approach and cave entrance taken from the water, and a draft site-note based directly on Ecoleg’s recommendations. The group adopted the wording with minimal change. The Lydstep site guide now carries the line:
“To avoid disturbance, do not enter the cave and pass the entrance quietly.”
One short sentence, but behind it is a protection that now applies across the whole commercial coasteering industry in Pembrokeshire. One operator flagging a find, specialists confirming it, a voluntary framework propagating the protection – this is what self-regulation is meant to look like when it works.
Where this leaves us
We’re writing this up publicly because the general story matters, but we’ve deliberately not named the cave, not dropped pins, and aren’t encouraging anyone to go and have a look. Disturbing a roosting or hibernating bat can kill it – waking one costs weeks of fat reserves it doesn’t have – which is why Natural Resources Wales withholds roost locations from public datasets.
If you want to see Pembrokeshire bats properly, the Bat Conservation Trust’s guided walks, a local wildlife trust event, or signing up as a volunteer for the National Bat Monitoring Programme are the right routes. You’ll see far more, far better, in the company of people who know what they’re doing.
For us, the lesson of Weird Cave has been simple: guides spend more time in these caves than almost anyone else, and that puts us in a useful position if we stay in our lane. Specialists bring the licences, the kit and the identification skills. We bring the access, the safety cover, and the local pattern-recognition that says “this one looks different – come and check.” Partnered with Ecoleg, that’s the model that got this roost protected. We’d like to do more of it.
Four years on, we went back in with the same ecologists and surveyed three more caves along the coast – read the 2026 follow-up here.