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.

Watch and listen to the video clips. You can hear how the bat sound gets louder as it approaches and then fades away. This is one pass.

In video clip 1 the pass is from a pipistrelle bat (unseen).
In video clip 2 it is from a Daubenton's bat that can be seen passing.

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.

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!

Play Video Clip 2 Click Here