Revisionist history: rewriting the story of Indian birds

Research recently published in BMC Evolutionary Biology presents the first known example of bird groups that have speciated within India, challenging traditional notions that few animal groups originated in the subcontinent. Here the authors discuss the significance of their findings.

India is a biogeographic crossroads — a meeting place of different regions with distinct types of species. Although India is biodiverse, the high species richness of the subcontinent is often attributed to the fact that groups from the surrounding regions have been able to colonize and occupy various ecosystems. Furthermore, the conventional wisdom held that most animals in the Indian subcontinent were representatives of groups that were more diverse in other regions and that there were few, if any, distinctly Indian groups — ones that originated and speciated within the landmass.

Recent studies are beginning to challenge these ideas, particularly in some groups of amphibians and reptiles. In our recent study, we now show that there are in fact two groups of birds, Sholicola and Montecincla, that have also diversified within India. This is surprising because birds are thought to be really well-studied and these species have been know to science for a long time. Nevertheless, scientists had underestimated the true diversity of these groups and therefore the uniqueness of the avifauna of this region.

conventional wisdom held…there were few, if any, distinctly Indian groups

To address the history of taxonomic confusion surrounding these birds, we devised a strategy to examine their evolutionary relationships. Using a combination of modern analyses and gathering a suite of data – genetic, morphometric, song, and plumage – we examined the distinct differences between populations across their range and determined their phylogenetic position to decipher their closest relatives.

Our results conclusively showed that these two groups were older and more distinct than previously thought and had diversified into multiple species. These two groups of birds, now referred to as Sholicola and Montecincla, represent the first known examples of birds that have speciated within the Indian subcontinent as endemic radiations.

Our results have substantial implications for our knowledge of biodiversity and biogeography. First, it indicates that there is still much to be discovered about birds, even in well-inhabited regions. Second, the divergence patterns of these birds reveal the history of the Indian landscape and the role of climatic events in driving diversification. Finally, the dramatic increase in unique species with narrower ranges pushes the urgency of conservation in this vulnerable ecosystem.

Diversity hiding in plain sight

Both Sholicola and Montecincla are endemic to the Western Ghats, a coastal mountainous region in southwestern India. Their specific habitat is restricted to the highest elevation zone in what is called Shola forest — a unique form of evergreen forest interspersed with natural grassland that is found only on this mountain range. Species like Sholicola and Montecincla, which are restricted to these high montane areas, have patchy distributions within and across these ‘sky islands’ where montane habitat is similar to oceanic islands separated by deep valleys of unsuitable areas.

…there are still many mysteries to be uncovered in birds.

We conducted detailed field surveys across all the sky islands over a period of eight years and examined an unprecedented number (800) of individual birds for this project. Over this period, we measured body features, recorded song, noted plumage variation, and collected blood for genetic analysis.

Whereas previously each group was thought to be one or two species (taxonomists disagreed), we show with ample evidence each group had diversified across the mountain range and had more species than currently recognized. This meant we had to revise and describe new species – an exciting yet daunting challenge given the debate amongst ornithologists on how to recognize distinct species (species concepts). There were multiple opinions, even amongst our co-authors, but we ultimately converged on an approach to delimit species that fulfilled the requirements of multiple species concepts.

Sholicola ashambuensis
Sholicola ashambuensis
Shashank Dalvi.

To complicate matters further, our phylogenetic analyses showed that neither of the two groups we studied belonged with the genera in which they were traditionally placed. Although they were originally thought to be shortwings and laughingthrushes respectively, Sholicola and Montecincla do not belong with these groups. Their new positions in the bird tree of life, as distinct lineages and highly divergent from their closest relatives (and not near the genera they were previously classified in), make it imperative to place them into new genera.

Why so much excitement about new genera and species of birds? Some of this is due to expectation. For some other groups, like beetles, the rate of new species discovery is still steep and therefore adding new species to the list is fairly commonplace. For birds, on the other hand, many believe we have already discovered 99% of all the living species. New studies like this one highlight that there are still many mysteries to be uncovered in birds. This study also shows the value of phylogenetics and systematic analyses. We may have described the vast majority of species, but the taxonomy of birds is far from being clear, and this can mislead subsequent studies.

Past connections and current isolation

One of the big mysteries of Indian biogeography is how organisms got to the Western Ghats. The closest mountain range, the Himalayas, is several hundred miles to the north and intervening is a large expanse of dry, seasonal, lowland habitat. One theory, known as the Satpura Hypothesis, proposes that montane groups were able to migrate from the Himalayas using the hill ranges in central India as a corridor. Although wetter than other parts of peninsular India, the Satpura hills are much lower and many high elevation species do not currently occur on them. However, we know that suitable habitat for these species might have existed in the past when global temperatures were cooler and India was covered in moist forest. One way to test the Satpura Hypothesis was to examine the origin of the Western Ghats birds.

Remarkably, our analyses showed that the pattern and timing of divergence in these two groups of birds were nearly identical and occurred at similar times. This lends support to the idea that landscape level changes, as implied by the Satpura Hypothesis, led to the isolation of these populations on their respective sky islands. Both Sholicola and Montecincla diverged from their closest relatives in the Himalayas and Southeast Asia around 11 million years ago. This coincides with a period of heightened tectonic activity in the Himalayas and Tibetan Plateau that led to the aridification of the Indian subcontinent, thereby shifting from moist to dry forest. We hypothesize that due to this drying, these cool-adapted birds were isolated in the Western Ghats, the only suitable habitat at the time. Subsequent climate shifts, which limited dispersal across the sky islands, lead to further isolation events across the deep valleys of this mountain range.

The Shola forest and grassland mosaic of the Western Ghats. Camel’s Hump Mountains, Kerala.
The Shola forest and grassland mosaic of the Western Ghats. Camel’s Hump Mountains, Kerala.

A biodiversity hotspot becomes hotter

India, being a developing nation with over a billion people, has to reconcile tremendous pressures on natural ecosystems and its extraordinary biodiversity. Nowhere is this more apparent than in the Western Ghats where the interests of people, agriculture, and wildlife collide. This biodiversity hotspot is home not just to unique species but also a diversity of cultures, all of which are threatened by anthropogenic impacts such as economic development and climate change.

We were the first research team in decades to obtain permissions to handle birds across this landscape

The crucial need for additional scientific studies to study this distinct region is becoming increasing apparent as modern studies like ours uncover dramatically more biodiversity. The type of study we conducted is rare in India and was only possible with persistence and some luck in acquiring the necessary permissions to handle birds, which is challenging in a country that is apprehensive about interference with wildlife. Nevertheless, the field crew, led by Robin and Vishnudas, were supported by a long-term research permit specifically from the Forest Department in the state of Kerala that was able to grasp the importance of this work. We were thus the first research team in decades to obtain permissions to handle birds across this landscape. We hope the results of our study makes the need to facilitate scientific studies an important and urgent venture across all states in the face of increasing threats.

Finally, we hope our study will help to highlight this exceptional and diverse region in terms of the need for more conservation efforts. In additional to their scientific value, birds can generate enormous interest in non-scientists from avid bird-watchers to casual nature observers. We gave our new taxa names inspired by local words for these birds and their habitat. This has already aroused national pride –our publication was released on Republic Day and enthusiastically picked up by supporters and newspapers with the hashtag #MadeInIndia! We hope these new discoveries will not only elevate the protective status of these birds and their shrinking habitat but also encourage interest in studying and preserving the long-term survival of other Western Ghats fauna and flora.

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