By Liam Templeton, Species on the Edge Project Officer for the Solway Coast, Amphibian and Reptile Conservation
As part of our Species on the Edge work on the Solway Coast, we are monitoring roosts of brown long-eared bats (Plecotus auritus) – otherwise known as “The Whispering Bat”, so-called because its echolocation calls are barely-audible when using a heterodyne bat detector. If you’re lucky, you might detect a faint crackling not dissimilar to radio static or a Geiger counter.
The soft-spoken nature of the brown long-eared bat is apparently the result of an evolutionary arms-race through which the invertebrates that are frequently predated on by bats (e.g. moths) developed the means to detect bat echolocation calls and take evasive action to avoid being eaten. In retaliation, the brown long-eared bat gained the ability to echolocate in near-silence and detect its prey by virtue of its acute hearing abilities alone.
Thankfully, the roost count is a largely visual survey, during which the surveyor simply counts the number of bats they observe exiting the roost. Bat detectors can be useful for verifying the species of bat that are being counted; however, this can also be inferred by the relative size of the animal, and the brown long-eared bat has an instantly recognizable “horned” silhouette by way of its ears!
It’s recommended that you know the exact exit point of the roost before attempting a survey. If it’s not immediately obvious (bats are capable of squeezing through some remarkably tight spots), you can visit the suspected location of the roost shortly before dawn and should be able to observe the bats swarming around the entrance as they return to the roost before day-break. Some roosts can have more than one exit point, in which case it might help to have more than one observer!
Relatively little is known about the location of brown long-eared bat roosts on the Solway coast, and I just about exhausted all of my contacts trying to track down a handful of sites to survey. One such site is on the grounds of the Threave Estate – Scotland’s first and only dedicated bat reserve and home to at least eight out of a possible ten species of bat currently known to be found in Scotland.
Threave Estate is owned and managed by the National Trust for Scotland. In order to conduct the survey, I’m met by one of their rangers, Mary Smith. We’re also accompanied by a handful of volunteers who also happen to be members of the Dumfries & Galloway Bat Group. As the sun sets, I quickly go over the methodology. We break off into pairs and position ourselves at opposite ends of the building. The building in question is only a short walk from the car park and features amenities such as picnic tables and public toilets, making it relatively accessible as far as survey sites go!
Before leaving the roost for the evening, brown long-eared bats are known to fly around to raise their core body temperature. Shortly after sunset, low-frequency social chatter is picked up on the bat detectors and it’s assumed that the bats are getting warmed up. Before long, the bats begin to emerge in single file – swooping down in a low arc before disappearing into the night.
Armed with a bat detector in one hand and clicker in the other, we count the bats as they emerge – some in quick succession, others at intervals of a few minutes. Some even appearing to return to the roost soon afterwards, but stopping just shy of reentering. It’s suspected that the reason they do this to leave scent markings at the entrance of the roost so that they don’t lose their way back at any point during the night.
As the frequency at which the bats exit the roost slows to a crawl, and the twilight has dimmed such that it’s barely even possible to see them, we declare that the survey is finished. An hour has passed and we have counted at least a dozen bats, and detected many more pipistrelle and noctule as they fly past. The jackdaws noisily return to their own roosts and that signals to us that it’s time to go home.
Gathered around our cars, we reflect on what we’ve seen that night – and while not everyone was lucky enough to see the bats, we all agree that it was a pleasant enough way to spend an evening! We make plans to do a follow-up survey in a couple of weeks’ time, and some are even spurred on to do surveys of their own in the meantime. We all leave feeling more accomplished and deserving of a warm bed.
