A bat pass is defined as a sequence of greater than two echolocation calls made as a single bat flies past the microphone.
Obviously in the dark it is difficult to see whether one bat is flying past your detector microphone. However you can usually hear the sound start off faintly, become louder and then fade again.
In places where there is a lot of bat activity or where several bats are circling, it can be difficult to count the number of bat passes, but try to make an estimate.
Watch and listen to the video clips. You can hear how the bat sound gets louder as it approaches. A couple of times it fades away slightly before getting louder again. This is three passes.
In video clip 1
the passes are from a pipistrelle bat (unseen).
There is currently little information on how number of bat passes relates to numbers of individual bats.
Provided the average number of passes per bat does not show a trend over time, number of passes can be taken as an index of number of bats.
In the following sections specific techniques for identifying species encountered in the field survey will be described along with the survey method – so please keep going!
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