Soprano Pipistrelle call variations

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Soprano pipistrelles (Pipistrellus pygmaeus) calls are very similar to those of the Common Pipistrelle except that they are higher in frequency.  Typically the terminal frequency is around 55kHz rather than 45kHz for the Common Pipistrelle.  It is not surprising therefore that only with the advent of DNA analysis was it possible to determine that these were two different species rather than clades of a single species.

As with common Pipistrelles, bats flying in open areas use quasi-cf pulses (often quite constant but sometimes with a gentle slope) about 9ms in duration.  On detection of something of interest, or in more cluttered situations they introduce a frequency sweep at the start of the pulse and this fm component can become extend to cover a very wide frequency band including a significant amount of 2nd harmonic.  During an interception manouevre the cf portion decreases and the extent of the fm portion increases, and the total pulse duration decreases until the interception 'buzz' of pure fm pulses, often in groups, increasing in rate and descending in frequency.

Often the fm-cf type pulses end with a slight frequency up-turn, giving a hook-like pulse.  

The main variations encountered are the use of very short, pure fm pulses, somewhat higher in frequency than the typical pulses and often with a very wide bandwidth.  These may be single pulses or groups of pulses inserted into an otherwise normal echolocation sequence, or occasionaly may occur as an isolated pass.  Alternatively the bat may insert one or more 'dropped' pulses where the frequency is significantly lower than the rest of the sequence, moving gradually from an echolocation call to a type-C social call.

Radar studies, that allowed accurate measurement of the emitted pulse frequency by adjusting for Doppler shifts due to the bat's speed toward or away from the microphone, showed that Pipistrelles will change their emitted frequency upwards by an amount equalt half the Doppler shift due to their flight speed when they observed a target of interest.  If the target was not desirable (possibly because it was moving away rather than closing) the sequence would revert to the previous cruising frequency, but if it was potential food the next pulse would move further up in frequency and stay there as the bat went into an interception manoeuvre.

It is also often possible to see an alternation of frequency, and/or amplitude of pulses in a sequence although the amplitude variations could be due to the bat scanning his head from side to side.

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