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Bat ‘Passes’

When surveying for the NBMP you are asked to estimate the number of ‘passes’ each bat makes.

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.

Video clip 1 illustrates a single Daubenton's bat pass. A torch is used to confirm it as Daubenton's bat (see next page for guidance on torch use).

In video clip 2 two Daubenton's bats are seen, one flying behind the other. Count this as two passes.

Video clip 3 one Daubenton's bat is seen flying to the right and another to the left. Count this as two passes.

Video clip 4 illustrates 2-3 pipistrelle passes (bat unseen). The "wet,slappy" sound of these calls distinguishes them from the dry clicks of a Myotis species. Therefore these bat passes should not be recorded on your survey form.

If you hear rapid, dry clicks similar to those in clips 1-3 but you are not able to tell whether the bat is skimming over the water, then count the number of passes and record as "Unsure Daubenton's".

There is currently little information on how number of bat passes relates to numbers of individual bats.

For monitoring relative abundance over time, it does not matter if it is possible to determine whether a count of say five bat passes corresponds to five different animals, or to just one animal passing five times.

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.



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Play Video Clip 2 Click Here

Play Video Clip 3 Click Here

Play Video Clip 4 Click Here


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