Atop the Western Ghats, lilting melodies of the White-bellied Shortwing can often be heard echoing through the mountain air.
This threatened songbird species is endemic to the Shola forests of the sky islands – green clusters on mountaintops separated by valleys – in the southern Western Ghats and are found in small isolated populations. So cut off are these populations from each other that their birdsong sometimes varies from one mountaintop to another.
The Western Ghats, which run along the western coast of India, extend from Tamil Nadu to Gujarat. In the southern part of the mountains, ecologists said there are two reasons for such isolations.
Some populations of White-bellied Shortwings have been cut off from one another by natural barriers such as valleys and rivers for millions of years, resulting in genetic and cultural differences in the species.
This not only includes variances in their physical appearance – the shortwing population found in Ooty would look a little different from that found in Brahmagiri, for instance – but also in their breeding songs, which are a cultural trait.
Other populations have been isolated more recently – for just over a 100 years – because of deforestation to make way for human activity on the mountainside. Ecologist VV Robin said that between the hilltops of Munnar and Kodaikanal, for example, tea and coffee plantations have come up in the last 100-200 years, disrupting the contiguous flow of Shola forests. As a result, different populations of the species have been cut off for the last century or two, because of which there is no breeding between the various populations.
“Genetically they are still similar,” said Robin. “But this time is enough to cause cultural differentiation, in the sense that birds are beginning to sing different kinds of songs across this landscape.”
In a study published in Journal of Ecology and Evolution in September, Robin and fellow ecologist Chetana Purushottam examined these differences in birdsongs in populations that have been isolated historically or because of contemporary reasons.
Robin explained that birds pick up breeding songs in a manner that is similar to how humans learn languages. Children pick up words and accents through constant association with others and birdsongs are similarly learnt through cultural transmission.
“You can take a genetically different bird and put it in another population, and it will start singing the song of the other population,” he said.
Robin said that ecologists who study isolated populations typically look for only genetic differences in the species, which take centuries to show up. However, cultural differences, such as in the breeding songs, can be detected much faster as these are learnt through association, he said.
Different kinds of cultural differences
The extent of differences in the bird song depends on the kind and duration of isolation, said Robin.
Between shortwing populations that have been isolated for millions of years, there is a significant difference in the grammar of the songs – how they are arranged or how the notes are strung in a sequence when sung, much like how humans order words in a sentence.Here is what the birdsong of the Anamalais shortwing sounds like:
This is quite different from the song of the Nilgiris shortwing, in the link below. These two populations have been isolated for around five million years by the Palghat Gap – a break in the Western Ghats between the states of Kerala and Tamil Nadu – said Robin.
For shortwing populations isolated relatively recently, owing to man-made factors such as construction or deforestation for instance, another kind of variation is seen even though the grammar of the song largely remains similar.
The voice box or syrinx of the shortwing, explained Robin, has two sides that can be controlled independently. The bird, therefore, can produce two different sounds at the same time. So their birdsong typically consists of intervals where both sides of the syrinx are used.
After travelling to several mountaintops and sampling every known population of shortwing in the South Indian mountain ranges, Purushottam and Robin found that often the use of the dual syrinx differed across populations, including those isolated for just over a century.
“What we found was that the frequency of making two sounds at one time seemed to be dependent on isolation,” said Robin. He compared this cultural trait with the guttural "R" in French produced at the back of the vocal tract, which is also used in different frequencies in other European languages.
It took the researchers around one-and-a-half years to identify this difference.
"With a voice recorder in hand, we would walk for hours each morning exploring the forests, following trails and listening for the distinctive song of the shortwings," said Purushottam. "The forests were very thick and these birds are very shy. I only managed to see a handful of the shortwings I was listening to. The few times I did catch a glimpse of them singing, it was truly beautiful."
Purushottam and Robin believe that their findings open up a number of possibilities for further research.
Primarily, it shows how man-made changes such as deforestation and construction activity can isolate various populations of a species, thereby bringing about differences in their cultural traits, such as birdsong. This in turn would shed light on how species are affected by changes in their habitat.
Robin said that by identifying that the usage of the syrinx is a cultural trait, they can now look for other isolated populations of the shortwing.