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I’m All Ears…Bats on the Common

I’m All Ears…Bats on the Common

As the saying goes – we have two ears and one mouth, so should listen twice as much as we speak – and there has rarely been a better time to do a lot more listening and to hear brilliant bird song that was previously drowned out by the humdrum of our hectic lives. The power of listening is an incredibly important tool in conservation. Most people think it’s all about visual observation but what happens if you can’t see what you’re trying to survey?

Welcome to the wonderful world of bioacoustics!

What are bioacoustics? It may sound fancy but is simply the study of animal sounds. This applies to birds and mammals, as well as sound produced by insects. Whether you’re in the Amazon jungle or the concrete jungle, bioacoustics devices can be used for individual animal detection, species detection, species location, monitor populations1 and even to assess the health of an ecosystem.

The principle is simple; you set up the device in the area you want to survey, walk away (don’t forget to turn it on) and return in a day, week or month. Hopefully it’s recorded lots of data of what you were after. However, there’s always been a bit of an issue with bioacoustics and that’s been the cost. Professional devices can start at over £500 and easily take you into the thousands, just for one device. This isn’t often viable for larger conservation organisations let alone a smaller one like FoBC.

In recent years researchers have looked to partner with engineers to find low cost alternatives.  Open source software, (free for people to use, modify and enhance i.e. companies don’t keep it private and charge for it) has meant the development of less expensive bioacoustics devices.

Enter AudioMoth. In their own words AudioMoths are: 

A low-cost, full-spectrum acoustic logger, based on the Gecko processor range from Silicon Labs. Just like its namesake the moth, AudioMoth can listen at audible frequencies, well into ultrasonic frequencies. It is capable of recording uncompressed audio to microSD card at rates from 8,000 to 384,000 samples per second.

The best part is you get change from £50 for the device alone and their benefit across the conservation spectrum has already been well documented1,2.  Happy conservation days!

Bioacoustics are perfect for surveying bats. The only time you may be lucky enough to get a glimpse of one is at dusk as they come out to feed at night. The sound waves they send out (echolocation) to find food cannot be heard by the human ear as we can only hear from 20 Hz to 15-20 kHz (depending on age) and bat calls can range from 9 kHz to 200 kHz.

Traditionally the only non-invasive way to survey for bats (and without a Natural England survey license) is to walk a set route (transect) around an area with a bat detector that picks up their calls. However, you only pick up the bats that happen to cross your path at that specific time that you are there. Static detectors will pick up everything as they’re on site day and night.

Onto the fun stuff. I still wanted to try before you buy and last autumn we were lucky enough to be lent three AudioMoths by Philip Briggs from Bat Conservation Trust. As all 14 species of bat found in the UK utilise woodland3 (some only when near water), I felt this would be a good starting point. We placed one in the woodland near the old cricket pitch off Vine Road, one in the woodland between Common Road and Mill Hill Road. We placed the final detector by the Beverley Brook, as certain bats like to feed on insects around water.

The AudioMoths were left in situ for three weeks. Unfortunately, the weather wasn’t great, with lots of rain. Bats avoid flying in heavy rain as once they get wet, they have to use up much more energy just to try and stay warm4 and you end up with less data. The rain also managed to trigger the AudioMoths, so each detector had over 4000 recordings on it, with just 100 of these actual bat calls, so it wasn’t plain sailing. Thankfully (for us) Philip kindly analysed all of the data using specialised software (something we need to get trained up on) to ID the bat calls.

We found a total of seven different bat species across the three sites, including one that hadn’t been found on the Common before, the Brown long-eared bat (Plecotus auratus).

Barnes Common latest bat species list

The Common is clearly a key feeding and roosting site for a number of bat species. The AudioMoth along the Beverley Brook picked up four species alone, showing how important the brook is for feeding bats.

Apart from the Nathusius’ pipistrelle, where evidence is still lacking, all of the species listed above will utilise crevices and/or cavities in trees for roosting at different stages of their annual cycle 3. Increasing the availability of natural tree roosting sites for bats is something we’re working on and a blog for another day. But if you want to see the sort of tree features bats can be found in I highly recommend a visit to the Bat Tree Habitat Key Facebook page, which is an evidence-led research project into assessing what tree features different species of bats use.

If you’re interested in helping your local bat population there’s one simple thing you can do, even whilst in quarantine as you don’t need to leave the house: simply make sure to turn any outside lights off at night. Lots of people keep lights on in their garden or have outside lights on permanently through the night, rather than on a sensor. Bats have developed to hunt at night and artificial lighting can have a huge negative impact on bats. It’s easy to do and will be a huge help.

Watch this space as we have two AudioMoths on order!  When they will arrive is anyone’s guess, but hopefully we will be able to get them up and running before the end of summer and maybe even add a few more species to the list.



  1. Hill AP, Snaddon JL, Prince P, et al. AudioMoth : Evaluation of a smart open acoustic device for monitoring biodiversity and the environment. Methods Ecol Evol. 2018;9:1199-1211.
  2. Hill AP, Prince P, Snaddon JL, Doncaster CP, Rogers A. HardwareX AudioMoth : A low-cost acoustic device for monitoring biodiversity and the environment. HardwareX. 2019;6.
  3. Key, B.T.H. 2018. Professionals., Bat Roosts in Trees: A Guide to Identification and Assessment for Tree-Care and Ecology. Pelagic Publishing Ltd; 2018.
  4. Voigt CC, Schneeberger K, Voigt-heucke SL, Lewanzik D. Rain increases the energy cost of bat flight. Biol Lett. 2011;7:793-795.