Acoustic detection and acoustic habitat characterisation of the critically endangered white‐bellied heron (Ardea insignis) in Bhutan

Abstract

Growing developmental activities, such as hydropower construction, farm roads, and other human activities, are affecting the critically endangered white‐bellied heron (WBH). Out of a known global population of 60, 28 individuals inhabit the river basin area and freshwater lakes and ponds of Bhutan. Several constraints impede continuous monitoring of endangered species, such as the isolated and cryptic nature of the species and the remoteness of its habitat; to date, there are no long‐term reference data or techniques implemented for continuous monitoring of this species. In this study, we designed acoustic detection and habitat characterisation methods using long‐duration recordings from three habitat areas in Bhutan. Acoustic indices were extracted and used to implement a species‐specific call detector and to generate habitat soundscape representations. Using WBH calls annotated in month‐long recordings from a known site, a novel indices‐based detector was implemented and tested. A total of 960 hr of continuous audio recordings from three habitats in Bhutan were analysed. We found that a species call detector implemented using a combination of acoustic indices (that includes measures of spectral and temporal entropy and different angles of spectral ridges) has a correct detection rate of 81%. Additionally, visual inspection of the species’ acoustic habitat using long‐duration false‐colour spectrograms enabled qualitative assessment of acoustic habitat structure and other dominant acoustic events. This study proposes a combined approach of species acoustic detection and habitat soundscape analysis for holistic acoustic monitoring of endangered species. As a direct outcome of this work, we documented acoustic reference data on the critically endangered WBH from multiple habitat areas and have analysed its temporal vocalisation patterns across sites.

Six steps towards operationalising freshwater ecoacoustic monitoring

“…Approaching automatic detection from a completely different angle, two further studies describe detection of water‐dependent terrestrial birds and anurans using acoustic indices, bridging the gap between single‐species bioacoustics and ecoacoustic approaches. To detect the highly endangered white‐bellied heron ( Ardea insignis ) in Bhutan, Dema et al () employed a deep learning algorithm that selected a combination of acoustic indices. A similar, but even easier approach was chosen to classify calls by three frog species in Northern Australia (Indraswari et al, )—three easily calculated indices were combined to distinguish calls of two hyliids and a myobatrachid anuran.…”

“…This can be overcome by two techniques described in this special issue. Linke, Decker, et al () demonstrate that small subsamples are adequate to characterise soundscapes (see below), whereas two other studies introduce modern data visualisation techniques to freshwater monitoring (Dema et al, ; Indraswari et al, ).…”

“…This information can be used to build automatic classification algorithms, as can the characterisations by Grabowski, Young, and Cott (2020), who analysed spawning calls by soniferous burbot in northern Canada. Automatic detection algorithms are now widespread in terrestrial and marine systems, F I G U R E 3 Ecoacoustics can detect and monitor: (a) water-dependent birds and amphibians (Dema et al, 2020;Dutilleux & Curé, 2020;Indraswari et al, 2020;Linke & Deretic, 2020); (b) activity of aquatic insects and other invertebrates (Desjonquères, Rybak, et al, 2020;Gottesman et al, 2020;Linke, Decker, et al, 2020); (c) communication between fish (Grabowski et al, 2020;Hanache et al, 2020;Higgs & Humphrey, 2020;Linke, Decker, et al, 2020;Roca et al, 2020;Rountree & Juanes, 2020); (d) biophysical processes such as sediment transport and gas exchange (Gottesman et al, 2020;Linke, Decker, et al, 2020); (e) anthropogenic disturbance, for example boat and engine noises (Desjonquères, Rybak, et al, 2020;Higgs & Humphrey, 2020) [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…Ecoacoustics can detect and monitor: (a) water‐dependent birds and amphibians (Dema et al, ; Dutilleux & Curé, ; Indraswari et al, ; Linke & Deretic, ); (b) activity of aquatic insects and other invertebrates (Desjonquères, Rybak, et al, ; Gottesman et al, ; Linke, Decker, et al, ); (c) communication between fish (Grabowski et al, ; Hanache et al, ; Higgs & Humphrey, ; Linke, Decker, et al, ; Roca et al, ; Rountree & Juanes, ); (d) biophysical processes such as sediment transport and gas exchange (Gottesman et al, ; Linke, Decker, et al, ); (e) anthropogenic disturbance, for example boat and engine noises (Desjonquères, Rybak, et al, ; Higgs & Humphrey, ) [Colour figure can be viewed at wileyonlinelibrary.com]…”

“…classified spawning noises emitted by two different species of Catastomidae, in this issueDutilleux and Curé (2020) present an automated detection algorithm for the underwater vocalisations of the spadefoot toad (Pelobates fuscus) based on the temporal patterning of acoustic energy in a target frequency band. They find it feasible to monitor presence of this species with largely automated acoustic methods.As a consequence of their continuous monitoring, they present a substantially richer picture of temporal dynamics and variation in the P. fuscus breeding season.Approaching automatic detection from a completely different angle, two further studies describe detection of water-dependent terrestrial birds and anurans using acoustic indices, bridging the gap between single-species bioacoustics and ecoacoustic approaches.To detect the highly endangered white-bellied heron (Ardea insignis) in Bhutan,Dema et al (2020) employed a deep learning algorithm that selected a combination of acoustic indices. A similar, but even easier approach was chosen to classify calls by three frog species in Northern Australia(Indraswari et al, 2020)-three easily calculated indices were combined to distinguish calls of two hyliids and a myobatrachid anuran.…”