Quantitative acoustic differentiation of cryptic species illustrated with King and Clapper rails

Abstract

Reliable species identification is vital for survey and monitoring programs. Recently, the development of digital technology for recording and analyzing vocalizations has assisted in acoustic surveying for cryptic, rare, or elusive species. However, the quantitative tools that exist for species differentiation are still being refined. Using vocalizations recorded in the course of ecological studies of a King Rail (Rallus elegans) and a Clapper Rail (Rallus crepitans) population, we assessed the accuracy and effectiveness of three parametric (logistic regression, discriminant function analysis, quadratic discriminant function analysis) and six nonparametric (support vector machine, CART, Random Forest, k‐nearest neighbor, weighted k‐nearest neighbor, and neural networks) statistical classification methods for differentiating these species by their kek mating call. We identified 480 kek notes of each species and quantitatively characterized them with five standardized acoustic parameters. Overall, nonparametric classification methods outperformed parametric classification methods for species differentiation (nonparametric tools were between 57% and 81% accurate, parametric tools were between 57% and 60% accurate). Of the nine classification methods, Random Forest was the most accurate and precise, resulting in 81.1% correct classification of kek notes to species. This suggests that the mating calls of these sister species are likely difficult for human observers to tell apart. However, it also implies that appropriate statistical tools may allow reasonable species‐level classification accuracy of recorded calls and provide an alternative to species classification where other capture‐ or genotype‐based survey techniques are not possible.

VOCAL SIMILARITY AND DIFFERENCES IN FOUR SPECIES OF THE GENUSANTHUS: ACOUSTIC FEATURE ANALYSIS OF SOME COMMON CALLS

“…This paper showed that two closely related species, with an overall similar vocalization, nevertheless possess fine, but significant differences of acoustic features that may be important to transmit species-specific information and to discriminate between conspecific and heterospecific vocalizations, like found by Elfström (1992) for A. petrosus and A. pratensis . This finding is confirmed by other authors for some species of the genus Anthus (Fijen 2014; Garner et al 2015) and for bird species belonging to other taxa (Stiffler et al 2018; Tietze et al 2008). On the other hand, many bioacoustics researches found interspecific communication mediated by various bird call types (Johnson et al 2003; Goodale and Kotagama 2005; Magrath et al 2007; Templeton and Greene 2007; Magrath et al 2009; Fallow et al 2011; Fallow et al 2013).…”

Descriptions of the vocalisations of the Chestnut-backed Button-quail Turnix castanotus

“…The analysis provided here for the Chestnut-backed Buttonquail and the methods described provide a foundation for this application and future work on other species of button-quail. The use of acoustic detection has proven effective for other highly cryptic but vocal species (Stiffler et al 2018;Znidersic et al 2020). The Chestnut-backed Button-quail, like others of the genus, appears highly vocal at certain periods of the year, which lends itself favourably to this application.…”

Cryptic Species in Ecotoxicology

“…; De Queiroz, 2007), providing researchers with plenty of other potential means of species identification besides morphology. Consequently, cryptic species have been differentiated by various approaches ranging from molecular genetics (Fišer et al, 2018), to proteomics (Wilke et al, 2020), to pheromones (Lassance et al, 2019), and acoustic signals (Stiffler et al, 2018), to name a few. Naturally, some methods are taxon specific whereas others have broad applicability across the tree of life, making them ideal for standardization efforts.…”