Bat Pass Duration Measurement: An Indirect Measure of Distance of Detection

Christian Kerbiriou,Romain Lorrillière,Jean-François Julien,Justine Mougnot,Yves Bas,Isabelle Le Viol

doi:10.3390/d11030047

Abstract

Few reports have been published on detection distances of bat calls because the evaluation of detection distance is complicated. Several of the approaches used to measure detection distances are based on the researcher’s experience and judgment. More recently, multiple microphones have been used to model flight path. In this study, the validity of a low-cost and simple detectability metric was tested. We hypothesize that the duration of an echolocating-bat-pass within the area of an ultrasonic bat detector is correlated with the distance of detection. Two independent datasets from a large-scale acoustic bat survey—a total of 25,786 bat-passes from 20 taxa (18 species and two genera)—were measured. We found a strong relationship between these measures of bat-pass duration and published detection distances. The advantages of bat-pass duration measures are that, for each study, experimenters easily produce their own proxy for the distance of detection. This indirect measure of the distance of detection could be mobilized to monitor the loss in microphone sensitivity used to monitor long-term population trends. Finally, the possibility of producing an index for distance of detection provides a weight for each bat species’ activity when they are aggregated to produce a bat community metric, such as the widely used “total activity”.

Highlights

In the 1970s, the general opinion among bioacousticians was that bat species identification from echolocation signals was difficult [1]
Strong relationships were detected between measured bat-pass duration and published detection distance for both survey methods
A consistently strong correlation was found between the bat-pass durations measured from the car-transect survey and those from the point-count survey (r = 0.988, p < 0.001)

Summary

Introduction

In the 1970s, the general opinion among bioacousticians was that bat species identification from echolocation signals was difficult [1]. Knowledge and methods of acoustic identification of bat species have matured [2,3,4]. The cost of ultrasonic bat detectors and recorders has decreased, resulting in the development of passive acoustic sensors that are able to record throughout the night. To respond to this large number of records, several reliable quantitative methods for detecting sound events, extracting numerous acoustic features, and automatically identifying bat species have been developed [5,6,7,8]. Since the late 1990s, this non-intrusive method has been widely used by researchers to investigate habitat use by bats [9,10] or to evaluate the impact of various anthropogenic pressures, such as (1) agriculture [11,12]; (2) forestry [13,14]; (3) habitat fragmentation [15]; (4) non-lethal impacts of wind turbines, such as the disturbance of commuting and migration routes, local habitat loss [16,17,18], and ambient noise [19]; or (5) artificial light at night [20,21].

Methods

Results

Conclusion