Playback experiments corroborate acoustic analyses.
An integrative approach to taxonomy is becoming more common. On this blog, I have covered several studies that used multiple lines of evidence to support a taxonomic decision (see for instance this lark species complex or the Gentoo Penguin). Combining multiple datatypes protects some taxonomists from their eagerness to split taxa into multiple species based on a few minor differences. For example, a morphological study on the bean goose complex found that it was possible to discriminate between the taxa fabalis and rossicus using just two measurements: the number of “teeth” in the upper mandible and the maximum height of the lower mandible. Some ornithologists argued that these differences were sufficient to elevate both taxa to species status. But how biologically relevant are these traits? Do they represent a plastic phenotype that is shaped by certain environmental conditions? Or are they heritable adaptations to a particular ecological lifestyle? Such questions need to be addressed before one can properly assess their taxonomic level. Similar reasoning can be applied to the songs and sounds produced by different taxa. Just because two birds sound different does not necessarily mean that they are different species (just think of all the dialects sung by certain songbirds). The biological relevance of bioacoustic differences needs to be investigated in more detail, such as conducting playback experiments. A recent study in the journal Avian Research did just that to evaluate the taxonomy of the Elegant Pitta (Pitta elegans) species complex.
Currently, the Elegant Pitta is classified into five subspecies (concinna, maria, vigorsii, elegans, and virginalis). Arya Yue and her colleagues took a closer look at these subspecies to see whether some might represent distinct species. An analysis of territorial calls revealed striking differences between the subspecies. Here is the succinct summary from the results section:
The widespread taxon concinna, resident from Lombok to Alor with the exception of Sumba, exhibited a distinct two-element motif. In contrast, most other taxa, including nominate elegans centered around Timor, maria and virginalis utter a three-element motif in which the first two elements are given in quick succession. The only taxon displaying variability in this trait was the far-eastern vigorsii, centered around the Tanimbar and Kai Islands, in which both two-element and three-element motifs were detected. However, neither of them resembled those given by the other taxa.
The bioacoustic differences were also apparent in the principal component analysis (PCA) where elegans, virginalis and maria formed one cluster distinct from the two other subspecies (concinna and vigorsii).
However, as I explained at the beginning of this blog post, just because two taxa sound different does not necessarily mean that they are different species. So, how biologically relevant are these differences in territorial calls? One of the co-authors (James Eaton) performed playback experiments over multiple years to see how the different taxa respond to one another. These experiments revealed that the three subspecies from the PCA-cluster (elegans, virginalis and maria) responded aggressively to each others calls, whereas they ignored the calls of the other subspecies (concinna and vigorsii). It would take me too long to describe all the species combinations, so you can check the table below for the results. In essence, the playback trials corroborated the bioacoustic analyses.
Based on these bioacoustic findings – and additional analyses of plumage patterns and morphology – the authors suggest to split the Elegant Pitta into three species:
- Temminck’s Elegant Pitta (P. elegans) with subspecies elegans, virginalis and maria
- Wallace’s Elegant Pitta (P. concinna)
- Banda Elegant Pitta (P. vigorsii)
However, this classification remains to be confirmed with genetic analyses. If the genetic patterns follow the bioacoustic results, it would indicate that the territorial calls contribute to reproductive isolation between the different taxa. Then they could undoubtedly be considered a biologically relevant and taxonomically informative markers.
Yue, A. Y., Ng, E. Y., Eaton, J. A., & Rheindt, F. E. (2020). Species limits in the Elegant Pitta (Pitta elegans) complex from Wallacea based on bioacoustic and morphometric analysis. Avian Research, 11(1), 1-12.
Featured image: Elegant Pitta (Pitta elegans) © Abdul Azis Gizan | Wikimedia Commons