The mystery of the white-faced Limestone Wren-babbler: a leucistic morph or a separate (sub)species?

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.

Three Lineages

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.”

Genomic analyses of the Limestone Wren-babbler species complex indicated three distinct lineages (see K=3 graph in figure a). Acoustic data separated crispifrons fro the other two taxa (figures b and c), which are morphologically distinct. From: Gwee et al. (2021) Molecular Ecology.

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.

References

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

Rapid evolution of sexual dimorphism in island populations of the Sulawesi Babbler

Stronger intraspecific competition on islands might explain this finding.

A few weeks ago, I wrote about the molecular underpinnings of sexual dimorphism in birds (see this blog post). In most species where males and females look different, the dimorphism is very obvious. Think of the extravagant tail feathers of the male peacock (genus Pavo) compared to the dull brown female. In some species, however, the differences between males and females are more subtle and easily missed by the casual observer. For example, in descriptions of the bird family Pellorneidae (the jungle babblers), you will often read that the sexes are similar. But a closer look at these inconspicuous birds might reveal cryptic sexual dimorphism. Indeed, a recent study in the journal Biotropica uncovered subtle morphological differences between males and females of the Sulawesi Babbler (Pellorneum celebense).

Clustering

Fionn Ó Marcaigh and his colleagues examined the morphology of birds captured on the southeast peninsula of Sulawesi and on the smaller islands of Kabaena, Muna, and Buton. They applied a cluster algorithm to determine if the birds could be sorted into different subgroups based on subtle morphological differences. This approach of unsupervised clustering is well suited for detecting sexual dimorphism in body size for monochromatic bird species. The analysis separated the sampled birds into two groups that corresponded to males and females (confirmed with molecular sexing). Male babblers were consistently larger than females. Convincing evidence of sexual dimorphism in the Sulawesi Babbler.

The clustering algorithm identified two main clusters that correspond to males (green) and females (red). From: Ó Marcaigh et al. (2020). Biotropica.

Competition

The researchers also noticed that “island birds appeared more strongly dimorphic than their mainland counterparts in each of our five morphological traits.” These islands became isolated within the last 20,000 years, suggesting very rapid evolution of sexual dimorphism in the Sulawesi Babbler. The selection pressures responsible for these fast changes are probably related to intraspecific competition (i.e. between individuals of the same species). On islands, species can often occur in higher densities because there is less competition with other species. However, these higher densities might result in more fierce competition between members of the same species, potentially culminating in sexual dimorphism when males and females adapt to different ecological niches. An interesting phenomenon on the islands that Alfred Russell Wallace already described as “anomalous”. Who knows what other discoveries await us on Sulawesi?

Sexual dimorphism was more pronounced on the islands compared to the mainland, as illustrated by the diverging lines in these graphs (green = males, red = females). From: Ó Marcaigh et al. (2020). Biotropica.

References

Ó Marcaigh, F., Kelly, D. J., Analuddin, K., Karya, A., Lawless, N., & Marples, N. M. (2021). Cryptic sexual dimorphism reveals differing selection pressures on continental islands. Biotropica, 53(1), 121-129.

Featured image: Sulawesi Babbler (Pellorneum celebense) © Trinity College Dublin

Should I stay or should I go? Patterns of gene flow across land bridges in Southeast Asia

Land bridges can promote gene flow. But do the birds cross them?

The genetic patterns in present-day bird populations can often be explained by the glacial dynamics of the ice ages (during the Pleistocene, between 2.5 million and 11,000 years ago). On the Northern Hemisphere, huge ice sheets covered large parts of North America and Eurasia. These glaciers formed formidable barriers between plant and animal populations, which could not exchange genes any longer and started to diverge genetically. Hence, divergence times often align with glacial maxima on the northern half of our planet.

On the Southern Hemisphere, however, conditions were quite different, especially on Southeast Asian islands. The growth of expansive ice sheets in the north requires a lot of water, leading to a significant decrease in sea level. In the western part of the Indonesian archipelago, this drop in sea level resulted in the formation of land bridges between islands. These connections might allow previously isolated island populations to exchange genetic material. A recent study in the journal Molecular Ecology took a closer look at five bird species (two babblers and three bulbuls) on Singapore, Sumatra and Borneo to determine whether land bridges promoted gene flow between these island populations.

The Indonesian archipelago now comprises several islands, but during glacial maxima these islands were connected by land bridges (indicated in grey). Did this result in gene flow between bird populations on the different islands? From: Cros et al. (2020) Molecular Ecology

 

Five Species

To infer whether gene flow occurred during the ice ages, Emilie Cros and her colleagues generated DNA sequences for the following five species:

• Chestnut-winged Babbler (Cyanoderma erythropterum)
• White-chested Babbler (Trichastoma rostratum)
• Olive-winged Bulbul (Pycnonotus plumosus)
• Asian Red-eyed Bulbul (Pycnonotus brunneus)
• Cream-vented Bulbul (Pycnonotus simplex)

Genetic analyses indicated that the island populations of these species diverged well before the Last Glacial Maximum (ca. 20,000 years ago). However, two species did show signatures of recent gene flow, namely the Asian Red-eyed Bulbul and the Olive-winged Bulbul. What can explain these different gene flow patterns?

Each circle is an individual bird and the color represents its genetic make-up. There is clear genetic divergence between the islands populations (most obvious in C. erythropterum here). In some species, however, there are signatures of recent gene flow. This is nicely illustrated by P. plumosus where different islands share genetic variation. Adapted from: Cros et al. (2020) Molecular Ecology.

 

Ecology Matters

A closer look at the ecology of these species reveals a striking pattern. The babbler species reside in the understory of the forest, while the bulbuls can be found in the canopy. A study in South America showed that canopy species have lower genetic divergence values compared to understory birds. The researchers attributed this difference to higher dispersal propensity of the canopy species. The same reasoning applies to the birds in Southeast Asia: the understory babblers might not have dispersed far during the ice ages and rarely crossed the land bridges between the islands.

In addition to the position in the forest (canopy vs. understory), other habitat features explain the gene flow patterns. Forest specialists, such as the Cream-vented Bulbul, do not venture outside the forest and thus need forested areas to disperse. The land bridges probably consisted of open habitats, including swamps, woodlands and savannas. Not the ideal habitats for forest specialists. More generalist species, such as the Olive-winged Bulbul, could survive in these non-forested areas and travel between islands. This ecological explanation is reflected in genetic patterns: the generalist (Olive-winged Bulbul) showed higher levels of gene flow compared to the habitat specialist (Cream-vented Bulbul). Ecology matters!

 

References

Cros, E., Chattopadhyay, B., Garg, K. M., Ng, N. S., Tomassi, S., Benedick, S., Edwards, D. P. & Rheindt, F. E. (2020). Quaternary land bridges have not been universal conduits of gene flow. Molecular Ecology, 29(14): 2692-2706.

Featured image: Red-eyed Bulbul (Pycnonotus brunneus) © Lip Kee | Wikimedia Commons

 

This paper has been added to the Pycnonotidae page.