The technical side of bat surveying

Selection of heterodyne bat detectors

Unlike most other species bats require some technical kit to successfully determine which bats are using our nature reserves

Being nocturnal bats navigate and hunt, not using vision but sound - although contrary to the popular saying they do have good eyesight. Bats make high pitched calls and by listening to the echoes build up a detailed picture of their surroundings, this system is called echolocation and works the same way as sonar on a ship.

It’s too dark for us to see them (and flying bats are almost impossible to identify to distinguish to species this way) and their calls are pitched too high for the human ear to hear so we cannot simply look and listen for them as we do with birds. Instead we use electronic bat detectors to pick up the bats calls and convert them to audible sound and/or record them.

Anabat and heterodyne bat detectors

Field kit: A Magenta heterodyne detector (here set to hear bats calling around 50kHz) & Anabat SD2 recording detector (Photo from S. Miller)

In the field we use basic heterodyne detectors which mix the bat call with an internal signal from detector to create an audible sound. By changing this internal frequency we can scan through different frequencies until the difference gives a loud, clear sound. The disadvantage of this is that if they aren’t tuned into the frequency a bat is calling at you can’t hear it. Noctule bats call at the lowest frequencies, around 20kHz, whilst the soprano pipistrelle call is strongest around 55khz and the other species in our area are somewhere in between. Obviously you cannot tune to both these frequencies at once on a single heterodyne detector. There are other types of bat detector that let you hear calls in the field across the full range of frequencies bats used (frequency division and time expansion) but these tend to be more expensive and come with other foibles of their own.  We use several different makes of heterodyne detector for fieldwork, they all do the same job but people tend to develop their own preferences. For instance I love my old Mini 3 unit which I’ve been using since I first got into bats over 10 years ago –  I find the  large backlit dial, which I can easily use one handed in either hand, very useful.

In order to cover all call frequencies we also use recording bat detectors, the sonograms of the bat calls can them be analysed in relative comfort back at the office using specialist software. In previous years we’ve only used Anabat units which are great bits of kit, but some are finally beginning to show their age. These detectors save the calls as zero-crossing (ZC) files which are analysed using bespoke Analook software. This system has worked perfectly well for several years and will continue to work well as long as the detectors survive.  Recently we have been having some issues with some of these detectors not recording properly and whilst we do get them repaired they are very expensive to replace, so last year and this year we've been testing out a cheaper alternative, the PeerSonic RPA3.

This differs to the Anabat in that instead of creating zero-crossing files it records the full spectrum of sound, giving greater detail but also creating much larger files. The Anabat buttons are easier to accidentally change the settings on, but are also easier to re-set if you do. The PeerSonic can be set to play audible sounds too although it is easier to just use both as recording detectors and listen on separate units as this prevents accidental changing of settings. Both can be pre-set so just need to be turned on and are ready to use in the field.

Soprano pipistrelle WAV file from the PeerSonic (analysed with TF32)

Soprano pipistrelle sonogram WAV file from the PeerSonic (analysed in  TF32)

Back in the office the different file types means that volunteers have to learn to use different software to analyse the bat calls, which takes time to get to grips with. The basic call shape and parameters remain the same but the way they are looked at varies. One thing that is annoying about the software we’re currently testing is the inability to tag files with species names.  This means results need to be written out which can easily lead to transcription errors when dealing with large numbers of files. The files can be converted to zero-crossing files and analysed in Analook, but lose dome qualities this way. We are investigating other available software but most have the same issue, or different ones!

Soprano pipistrelle ZC file from Anabat (analysed in Analook)

Soprano pipistrelle sonogram ZC file from Anabat (analysed in Analook)