Genomic and acoustic analyses of the species complex provide the first clues.
Somewhere in Myanmar, there is a white-faced population of the Limestone Wren-babbler (Napothera crispifrons). Ornithologists are not sure how to treat this population from a taxonomic point of view. Is it just a white morph or does it represent a distinct (sub)species? In addition to this mystery, the taxonomy of the Limestone Wren-babbler has been recently revised. Traditionally, this inconspicuous passerine was considered as a single species, comprised of three allopatric subspecies (crispifrons, annamensis and calcicola). However, analyses of plumage differences based on museum specimens suggested to split it into two species: the rufous-bellied N. calcicola and the grey-bellied N. crispifrons (containing two subspecies). An integrative approach is needed here, combining different data sources to support a taxonomic decision (see for example this blog post). And indeed, a recent study in the journal Molecular Ecology collected data on vocalizations and genomics to solve this puzzle.
Chyi Yin Gwee, Qiao Le Lee and their colleagues generated genomic sequences for 15 individuals and uncovered three deeply divergent lineages. They could confidently discriminate between crispifrons from Myanmar and western Thailand, annamensis from Vietnam and calcicola from northeastern Thailand. More detailed analyses indicated that there has been no gene flow between these three lineages, suggesting that they have been reproductively isolated for some time. The genomic results contradict the plumage-based classification which combined the subspecies annamensis and crispifrons. It turns out that annamensis is more closely related to calcicola than to crispifrons. This finding nicely illustrates the dangers of solely relying on morphological data.
The acoustic data supported the genomic patterns. Analyses of 10 vocal parameters showed that annamensis produces sounds similar to calcicola. Because these taxa look quite different – annamensis is grey-bellied, while calcicola is rufous-bellied – vocal differences might be less important in species recognition. Based on these results, the researchers concluded that “the Limestone Wren-babbler complex consists of three mitochondrially and genomically diverged lineages, each supported by a combination of plumage and vocal characters that would allow them to be diagnosed as different species under many species concepts.”
The White Mystery
And what about the white-faced population in Myanmar? The genomic data cluster it within the brown-plumaged populations of crispifrons. Comparing the genomes of white and brown individuals pointed to several outlier regions that contain a few candidate genes involved in pigmentation (including RAB3IP and SLC16A3). Research on other bird species has shown that a few genomic loci can drive drastic plumage differences (see for instance crows and warblers). At the moment, it is difficult to judge how stable the white-faced population is. The researchers might have captured the beginning of a diversification process between white and brown Limestone Wren-babblers, or the white-faced population might disappear in a few generations due to stochastic processes. Only time will tell.
Gwee, C. Y., Lee, Q. L., Mahood, S. P., Le Manh, H., Tizard, R., Eiamampai, K., Round, P. D. & Rheindt, F. E. (2021). The interplay of colour and bioacoustic traits in the differentiation of a Southeast Asian songbird complex. Molecular Ecology, 30(1), 297-309.
Featured image: Limestone Wren-babbler (Napothera crispifrons) © Francesco Veronesi | Wikimedia Commons