Bat detectors are the most common way to identify the species of flying bats. There are distinct types of call which can indicate the genus, and variations in pattern and frequency which indicate the species. For readers not familiar with the different types of bat detector, there is further information below and elsewhere.

Bats also make social calls, which are less useful for species identification. They sound different from the echolocation calls and do not have the same frequency patterns. Fuller details on the types of call and other clues to species identification follow below but Pipistrelles (or "Pips") give good examples of what can be discovered with a bat detector and make a good start to learning how to identify bats.

Bat detectors pick up various signals in the ultrasound range, not all of which are made by bats. To distinguish bat and bat species it is important to recognise non-bat species.

Captured bats can be exactly identified in the hand but in many countries a licence is required before bats can be captured.

Types of call There are four basic types of bat echolocation call.

FM calls The term "frequency modulation" (FM) refer to the "chirp" type of bat call. On a bat detector it sounds like a sharp click. Tuning a heterodyne detector does not change the sound much, but the level varies.

This is a typical call from a Myotis species. It sounds like hard dry clicks. It was recorded at 40 kHz which was not critical, but this was chosen because it was above the crickets and below Pipistrelles.

CF calls These calls were recorded using a heterodyne bat detector tuned to an appropriate frequency.

The constant frequency (CF) call is a series of peeping calls.

The bat emits the calls at a constant frequency, but the heterodyne bat detector exaggerates the doppler effect as it flies.

Hockey stick calls Some bats call with a composite call, starting with an FM component and ending in a constant frequency. This is often called a "hockey stick" call from its appearance on a spectrogram.

The Pipistrelle call is an example of a "hockey stick" composite type FM and CF call with a fast falling frequency FM part ending with a constant frequency, CF section.

A spectrogram is a graphic representation of frequencies against time. The colour represents the loudness of each frequency. This spectrogram shows a falling call which becomes a steady note. The yellow and green blotches are noise.

On a heterodyne detector this sounds like a ploppy click, but when it is slowed down by eight octaves, a sharply falling call can be heard, slowing to a single note.

A heterodyne bat detector will only handle a small range of bat frequencies, so it is necessary to keep retuning the heterodyne frequency to find the point of maximum loudness or, in the case of bats with a hockey stick call, the frequency which gives the lowest sound. This gives the lowest plop sound from the CF end of the calls.

Using a heterodyne bat detector A heterodyne bat detector simply downshifts the bat frequencies by the amount shown on the dial or the display on the more posh detectors. For instance with a hockey stick call ending at 45 kHz, this will produce a near zero end frequency when tuned to 45 kHz, If it is tuned too low, or too high, the difference frequency rises as illustrated in Tuning the heterodyne in the Pipistrelle section below. It will only reproduce a limited range of frequencies, typically only 10 kHz out of a bat spectrum of over 90 kHz.

A heterodyne bat detector does not give a very accurate measurement of the frequency of a bat call, One reason is that the call frequencies can easily vary by 1 kHz or more due to the doppler shift. To track these changes and to get a more precise frequency, a frequency division bat detector or a time expansion bat detector is used using a computer with sound analysis software. Time expansion detectors are beyond the scope of this article but are described in bat detector

Using a frequency division bat detector A frequency division bat detector analyses each bat call and re-synthesises it at typically a tenth of its frequency to make them audible as explained in the bat detector article. It can produce anomalies with sounds with a random structure and can only process periodic or tonal calls with a measurable frequency, but a recording can be used to measure the frequency of all parts of the bat call using a spectrogram display as illustrated below.

A FD detector does not require manual tuning as it works on a full range of bat frequencies, but some, like the Duet, also have a heterodyne detector built in. The entire call is preserved and can be recorded on an audio recorder and studied later on a computer. A spectrogram and other analysis software can also show the repetition rates and patterns of the calls which is also useful for species identification.

This is what a Pipistrelle sounds like on a frequency division bat detector:

The sound is different from a heterodyne recording and is a squeaky "teek" a bit more like a heterodyne tuned off frequency. With an FD detector the user cannot easily tell the difference between 45s and 55s by ear.

And this is what that recording looks like on a spectrogram up to the feeding buzz.

We can measure the end frequency (selected) which comes out as 44.6 kHz after multiplying by 10. The call rate speeds up as a food target is approached ending in a very rapid "feeding buzz". Note that the call becomes FM only during the feeding buzz.

Social calls This is a wide subject and there is still a lot to be discovered about bat social communication and how they use social calls in roosts and when flying. Generally a bat social call is not tonal, in other words it does not consist of a musical type note. Some bat detectors do not produce an accurate recording of a bat social call. Typically bat social calls use a lower frequency range than echolocation calls, and can thus be heard further away. Sometimes a bat will make a social call while echolocating which can cause confusion.

We can see and hear how the lower frequency social calls are heard at a greater distance than the higher echolocation calls as the bat approaches and departs. Zooming in on a spectrogram, social calls are atonal and repeated rapidly about five times in each call. The social calls are interleaved between the echolocation calls. They show a ragged frequency distribution around 20 kHz. Note the FD detector divides frequencies by 10.

The echolocation calls are single "hockey stick" calls at a higher repetition rate. At this scale the hockey stick shape is not very clear, but the end frequency can be measured as 45.2 kHz. A doppler shift is recorded as the bat approaches. The frequency was measured as it passes.

Flying speed Calculating flying speed involves some technicalities. A spectrogram can easily reveal a doppler shift in a passing bat. A heterodyne detector will show some shifts in the calls above, but doppler shifts are most readily heard in CF calls such as the Horseshoe's. A rough estimate the speed of a bat can be made from the doppler shift as it flies past. The rule of thumb is that at around 50 kHz a shift is 1 kHz indicates about . A passing bat will produce a total shift of about double this.

The Pipistrelles below showed an estimated shift of around 1.5 kHz indicating a speed just over 5 m/s or a bit under .

UK species

This section is also a practical introduction to the recognition of bat species, using Pipistrelles as an example. More detailed and technical information is given below.

Seventeen species of bat are regarded as resident in the UK. The species most often seen and heard are the Common Pipistrelle and the Soprano Pipistrelle, and are a good reference point for comparison with other bat species. In fact it is worth taking time to get familiar with the various calls of the two common species.

Common Pipistrelle Pipistrellus pipistrellus

Soprano Pipistrelle Pipistrellus pygmaeus

These two species are considered together here. This section also acts as a tutorial for analysing bat calls. The only technical knowledge needed is that kHz is the number dialled into a bat detector.

These two pips are distinct species but the frequencies of their calls are very variable and are not an exact indication of the species. They are frequently referred to as "45 Pips" and "55 Pips" from the calls as heard on a heterodyne detector.

Note the "ploppy" sound of the call and the "feeding buzzes" as it homed in on insects. The Soprano Pipistrelle's call sounds very similar, but at a higher frequency setting on the heterodyne bat detector.

Tuning the heterodyne As heterodyne bat detector only shifts a limited range of bat call frequencies, it needs to be constantly retuned so as not to miss some species and to identify those heard. One solution sometimes used in bat surveys is to use a second heterodyne detector tuned to a different frequency to detect other species such as Horseshoe bats if these are likely to be present. With Pipistrelles, if it is tuned too low or too high, the difference frequency rises as illustrated in the following example.

The first and last sections of this edited recording at about the same frequency as the last part of the hockey stick call, This produces a deeper and wetter ploppy sound. The second section is with the detector tuned too low - this brings the end frequency up and gives a squeaky click sound. The third section is with the detector tuned too high and also gives a squeaky sound but a bit harder. By tuning up and down, the deepest sound as in the fourth section is again produced, and this indicates the approximate frequency of the end of the bat's call. This is important for species identification.

45 and 55 Pips To distinguish between P. pipistrellus and P. pygmaeus, if a call is around 45 kHz it is a Common Pipistrelle and around 55 kHz it is a Soprano. The rare P. nathusii calls at around 39 kHz and so is easier to distinguish. The problem is that there seems to be an almost continuous spectrum of Pip frequencies from 43 kHz to 59 kHz. More studies need to be done on the call frequency ranges of each species.

Another small problem with differing frequencies is the doppler shift and a Pip passing by at 3.4 m/s (8 MPH) will show a doppler shift of about 1 kHz.

For bat workers with a suitable licence, an examination in the hand or close up, shows distinct characteristics between the 45 and 55 Pips:

Muzzle colour and shape P.p has a more bulbous muzzle and darker fur. Set of eyes P.p has eyes more deeply set in dark fur. P.pg has thinner fur or bare skin next to the eyes. Wing venation This is difficult to distinguish and can only be seen on a captured bat. If this check is held to be definitive, it does not always correlate with a definite call frequency criterion.

Resolving calls The following recording was made on a Duet combined heterodyne and FD detector. The heterodyne frequency was 53 kHz and the corresponding track sounds like this:

What can be heard is a lot of background noise from crickets and a rather muddled pass by what is probably a pipistrelle.

A spectrogram of the FD track reveals what happened:

Another bat was completely missed by the heterodyne detector which was set to the wrong frequency for that bat. It was a Lesser Horseshoe emerging from its roost.

The muddle following the LHS resolves into two Pipistrelles flying together with frequencies of 47.7 kHz and 54.5 kHz, in other words, a Common and a Soprano Pipistrelle.

Three bats have thus been positively identified using an FD recording