Thursday, 20 October 2016

Radio tracking at Ashwellthorpe Woods

This year intrepid volunteers from the Wymondham Nature Group (WyNG) teamed up with bat experts from the Norfolk Barbastelle Study Group (NBSG) in an attempt to gain an insight into the secretive nocturnal chiropteran activity in our local Norfolk Wildlife Trust nature reserve, Lower Wood Ashwellthorpe. In particular, we wanted to discover more about the ecology of the barbastelle bat, one of the rarest mammals in the UK.

Barbastella barbastellus, (“star beard”) is the evocative Latin name for this distinctive bat with a pug-like face, broad ears, silky dark fur and a delicate beard of frosted white hairs. Since 2013, we have recorded barbastelles annually in Lower Wood as part of the Norfolk Bat Survey organized by the British Trust for Ornithology (BTO). The method relies on identifying specific echolocation calls. For the barbastelle the calls occur at a frequency of around 32 kHz and are described in acoustic circles as sounding like short, hard smacks. Were the bats just foraging in the wood? Could they be roosting?  Was it possible that they were part of a maternity colony? To put these questions into some sort of perspective, it is worth noting that in 2001 Bristol University reported that there were just 5 colonies known in the UK. Since then Jane Harris and others from NBSG have been working hard to identify and study new colonies in Norfolk, particularly in the woodlands in the North of the county. However, for the claylands of South Norfolk there was little or nothing known. WyNG’s ambitious aims were to attempt to identify a new colony in Lower Wood and to contribute to existing information about habitat selection and foraging behaviour. We were to do this by trapping, radio-tagging and then tracking two female barbastelles. Here’s how we got on….

Our Big Bat Diary began on the 23rd of April when Jane and Lotty Packman from NBSG, spent a Saturday afternoon inducting ten complete novices into the mysteries of bat radio-tracking. Armed with receivers and antennae held aloft, we fought against the clock to locate strategically placed stationary transmitters.  Attenuation, polarization, back gearing, peak signal, triangulation….. We struggled in daylight with non-moving objects, how on earth were we going to cope in the dark with swiftly flying bats?

At the beginning of May, undeterred by our tracking shortcomings, Jane put out detectors in the places where barbastelle calls had been recorded previously and where the trapping nets would be positioned. However, it had been a pretty cold spring so far and the bats seemed to be in no hurry to head to the maternity roosts. A week later though, and a flurry of barbastelle calls were picked up just after sunset - time for the mist nets to go up! In the evening we volunteers gathered in the crepuscular light eager with excitement, anticipation and insect repellant. Between 9pm and 11pm it was all systems go as we darted between the three sets of nets checking for captives, transfixed as 18 bats were bagged, weighed, measured and released by the licensed bat workers: common pipistrelles, soprano pipistrelles, natterer’s, brown long-eared and then finally, to our great delight, a barbastelle… but it was a male and therefore not suitable for our tagging project. Nevertheless what a privilege to get our first glimpse of that magnificent fur and endearing face!  
As May drew to a close, we gave the trapping another go. In spite of the voracious appetite of the local mosquitoes, there was a distinct atavistic pleasure to be gained from being in the dark wood, late at night, intruding on an enigmatic world of hooting owls, barking foxes and silent mysterious bats. Once again there were pipistrelles galore, brown long-eareds too but still NO BARBASTELLES! It was time to hang up the nets and wait until the weather improved but there are strict wildlife laws to obey. The main bat maternity season runs from June and July and netting is forbidden during these sensitive months. We retreated from our woodland nightspot and hoped that our burgeoning chiropteran skills would not desert us over the summer.
We regrouped at the beginning of August when thoughts of success and dreams of discovery had begun to wane. Imagine then the whoops of frenzied delight and rekindled interest when, on the first attempt, Jane managed to trap and tag two female barbastelles at around 9.20 pm on the 4th.  Bat 1, who had a pup and Bat 2, who hadn’t, would be our companions for the next two weeks. It was finally time to put that tracking training to use in earnest and, perhaps more importantly, time to find out if we could remember any of it!

On the 5th of August armed with our trusty antennae we located the roost tree, an old oak in a group of oaks in the south west corner of the wood in the “non-intervention area”. Both our bats were in the same tree. Easy so far then! But our complacency wasn’t to last long. We had hoped to count the bats as they emerged around dusk but they popped out high up in the tree canopy and we could only estimate between 4 and 6 by the number of calls heard on a detector.  Back to the tracking and both bats foraged in the nearby ride for about 20 minutes before leaving the wood:  number 1 went east while number 2 went west. Enough for one day!

Saturday night, 6th August and both bats were back in the roost tree. We plotted to hit the road once they emerged and follow them with our trusty antennae: some tracking Bat 1, others on the trail of number 2. We would keep in contact via mobile phone……oh dear, another technique to get to grips with. Over the coming days, predictive texting, fat finger and emoticons would become the bane of my life. Welcome to the 21st century! Careering round in a Batmobile, antennae stuck out of windows, aural functions desperately straining to hear the peep-peep of the radio-tag may seem a strange way to spend an evening to some but it was nevertheless an exhilarating experience: “Blooming brilliant” announced volunteer Georgette after riding shotgun for the first time. We soon fell into a routine: Bat 1 always went east, Bat 2 west. But where exactly were they going? There was always a huge sense of excitement and achievement when we managed to follow our prey just a little further and desperate disappointment when we lost them again. One night Bat 1 headed towards the White Horse in Ashwellthorpe, then the Bird-in-Hand, Wreningham before settling on the Kings Head, Ashwellthorpe. We left her foraging happily in a hedgerow along the old railway line.  Clearly a bat with discerning tastes, she enjoyed the pleasures of Ashwellthorpe Hall and its environs too. On one occasion she “hung up” there for three quarters of an hour. I was rather alarmed when the tag became stationary for quite so long, particularly as there was a noisy tawny owl flying around on the look out for a tasty snack, but Jane thought the Hall may be an important place for socialising. Bat 1 certainly approved of it anyway and seemed quite set in her easterly flying ways. We had her sorted! Or so we thought.

Bat 2, on the other hand, lead us a merry dance and just seemed to vanish into thin air. One night, with all antennae blazing, and gain turned to max, we staked out her woodland exit, found her in a garden (thanks to the friendly, if rather mystified, owners who let me rampage over their lawn late at night brandishing an aerial whilst attempting to tune and text simultaneously in the dark) and finally tracked her to Wattlefield Hall (predictive text had a literal field day with that!) where she promptly disappeared.  Even when experts Lotty and Jane arrived with gigantic antennae, the signal could not be followed from the road and so, being unable to gain access (legally at least) to the grounds to find out exactly what she was up to, we had to be content with her 3.3 km flight path to this rather upmarket feeding ground.

Back to old favourite Bat 1 who by now, of course, had completely changed her habits and started going west. We had to start from scratch. Just to keep us on our toes, both 1 and 2 upped sticks to a new roost on more than one occasion causing minor panic attacks, (which in at least one case involved terrifying images of ferocious bat-eating owls), amongst us weary trackers when this happened the first time. A collective text-based virtual cheer went up, when our furry, flying friends were re-discovered in another oak.
As the end of the second week approached, the tag batteries diminished and we had to lay down our antennae. Sleep-deprived, mosquito-bitten, tick-ridden, but now equipped with some pretty impressive texting skills, I’d like to think our efforts will contribute at least some information to help conserve and protect this fascinating creature.

                                                                                                                                     Anne Edwards

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