Tracking European bat species with passive acoustic directional monitoring

We have developed a method of animal localisation that detects the angle from a sensor towards the direction of an animal call. The method is as simple to use as deploying a conventional static sound recorder, but provides tracking information as well as sound recordings. The principal of operation is to detect the phase difference between microphones positioned closely together. The phase is detected by converting the signals to their analytic form with a Hilbert transform. The angle is then calculated from the phase difference, frequency and microphone separation. Angular measurements provide an indication of flight paths above the sensor, and can give details of activity and behaviour that are not possible with a single channel static recorder. We recorded flight paths for 5 bat species at a site in Denmark (Pipistrellus nathusii, Pipistrellus pygmaeus, Eptesicus serotinus, Myotis daubentonii and Nyctalus noctula). The median error in angular measurement for the species was between 3 and 7 degrees. Calls at high angles from normal, corresponding with a poor sensor sensitivity, had larger errors compared to calls recorded in the centre of the field of view. Locations in space could be estimated by combining angular measurements from two or more sensors.