Bat species identification
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.
The term "frequency modulation" (FM) refer to the "chirp" type of bat call.
A spectrogram and other analysis software can also show the repetition rates and patterns of the calls which is also useful for species identification.
The call rate speeds up as a food target is approached ending in a very rapid "feeding buzz".
Generally a bat social call is not tonal, in other words it does not consist of a musical type note.
At this scale the hockey stick shape is not very clear, but the end frequency can be measured as 45.2 kHz.
A rough estimate the speed of a bat can be made from the doppler shift as it flies past.
The Pipistrelles below showed an estimated shift of around 1.5 kHz indicating a speed just over 5 m/s or a bit under 14 mph (23 km/h).
The Soprano Pipistrelle's call sounds very similar, but at a higher frequency setting on the heterodyne bat detector.
With Pipistrelles, if it is tuned too low or too high, the difference frequency rises as illustrated in the following example.
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.
For bat workers with a suitable licence, an examination in the hand or close up, shows distinct characteristics between the 45 and 55 Pips: 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.
From other recordings taken at the time, the insistent "chuck" sound was associated with the Soprano Pipistrelle at around 20 kHz, which habitually made this social call while flying.
The following minimally edited recording was made in a small nature reserve in North Devon, UK, in September.
Footsteps and contents of pockets like keys and small change can be very noisy at ultrasound frequencies.
These are very approximate figures and bats which call at lower frequencies can hear and be heard at much greater distances.
Conversely a bat like the Lesser Horseshoe which calls mainly at about 110 kHz is more difficult to detect over 10 metres.
Directionality of the ultrasonic sensor in the bat detector can also have a large effect on detection distance.
With a small target like an insect, the level of the reflection coming back falls with the inverse fourth power of the distance.
A large object such as a tree, a building or the ground can be detected by the bat at much greater distances.
An FM call gives a very precise time measurement and enables a bat to detect distances to the accuracy of tens of centimetres.
Some species such as the Pipistrelles start their call with an FM component but the rate of change of frequency slows to an almost CF end part.
Bats fly mostly at night but some indication of the species by sight at dusk or dawn can be given by size, flight patterns and proximity to known roosts.
The power of an IR floodlight drops rapidly with distance, and a spotlight will miss bats not in the beam.
Affordable imagers will not detect distant bats and this method is unlikely to be better than IR illumination except with the most expensive high definition equipment.
They are very expensive to deploy and the licensing arrangements for mobile zoological ground radars is unknown.
Anecdotal reports suggest that the most sophisticated radar systems with detection software can identify the presence of bats at a range of around 1 kilometre.
