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Tweeting in different dialects
Like humans, birds use language in the form of calls and songs to communicate with each other. A bird ‘call’ is short, simple, and is used all year round and serves a range of functions from keeping members of a flock together to alerting them of predators. Bird songs, however, are longer, complex and are used mainly during the breeding season. Males typically ‘sing’ either to defend their territory or to impress the females with their diverse sounds. Each species has their own song, but did you know that these songs can differ even within the same species?
Scientists believe that such differences in songs may be the first step in the evolution of two different species. Sometimes, because of an obstruction, like a deep-sea or a large valley, the birds of a species can be prevented from socializing between two groups, thus increasing their differences. With time, instead of one, they might evolve into two different species. In nature, it is hard to find enough evidence to prove whether this is true. However, through a carefully designed study, Dr. V.V. Robin and his colleagues tried to find out just how differently close relatives of a species sing and took us a step closer to understanding the first steps to evolution.
The White-bellied Sholakili are birds found only on sky islands in the Western Ghats. Sky islands are patches of forest lands situated high up on the mountains and have different degrees of connectivity between them.
An illustration of sky islands with different degrees of connectivity
Three groups of White-bellied Sholakili were selected from patches of Shola forests around Ooty on one island, and Grasshills and Kodaikanal on another island respectively. The Ooty (Nilgiris) population has been separated from the forests on the other island for over 5 million years by the Palghat Gap, an ancient mountain pass cutting through the Western Ghats. On the other hand, genetic studies suggest that Grasshills and Kodaikanal may have been isolated due to deforestation only
for the last 100 years.
So, what effect did being separated and lack of socializing (listening to and replying to each other) have, on the songs of White-bellied Sholakili of Nilgiris, Grasshills and Kodaikanal?
To find out if there are any differences, researchers recorded songs from each group of birds in the wild. In all, they recorded almost 2000 songs! These songs were analyzed by listening to them carefully and visualizing them through graphical representations called spectrograms.
What they found was that the songs of birds from Nilgiris were distinctly different from those of Kodaikanal and Grasshills! Songs of birds from Kodaikanal and Grasshills were much longer and more complex than songs of birds from Nilgiris!
Songs of sholakilis to the north and south of Palghat gap. Do put on your earphones! :) Videos by Prasenjeet Yadav
These populations, which have been separate for about 5 million years because of the Palghat gap, also show distinct genetic variations! In fact, it is not surprising that birds from these two sky islands (although considered as the same species when this study was conducted), even look different! It is possible that such long isolation and subsequent genetic differences could have resulted in differences in songs.
Consequently, in 2017, a team again led by Dr. Robin conducted extensive genetic studies of all Sholakili populations found in the Western Ghats. Since then, Sholakilis to the north of the Palghat gap (Ooty) has been identified as a separate species and renamed as ‘Nilgiri Sholakili’ and those to the south (Kodaikanal and Grasshills) are renamed as ‘White-bellied Sholakili’.
Interestingly, songs of birds from Kodaikanal and Grasshills populations, too, showed some distinct differences in their frequencies! These populations have been separated from each other for only around 100 years. The birds of these populations look the same and are genetically very similar.
Authors believe that the differences in songs can be because of the ‘cultural differences’ between the two groups. Apart from an innate tendency to sing (genetic impact), birds also learn to sing from parents and others in their community (cultural impact), just like how humans learn to speak a language! Such striking differences in songs of not only genetically different birds which were separated by 5 million years of isolation but also between genetically similar birds which have been separated only a hundred years ago has led researchers to identify this as a case of ‘evolution in action’.
Consequently, will the cultural differences in songs of populations of Kodaikanal and Grasshills be so strong that the birds stop identifying and interbreeding even if given an opportunity? Will this then make the birds genetically different and result in the evolution of different species of the ‘White-bellied Sholakili’!?
What a wonderful example of how geographical barriers can lead to a species splitting into separate species, and what a great way of finding the first clues for evolution—through differences in songs!! Who would have thought that we would identify ‘evolution in action’ in a bird species, right in our own treasured western ghats! Thanks to Dr. Robin and colleagues for bringing out this fascinating example of evolution in action! For more information about such bird-related exciting stories and to know more about the super exciting projects in his lab, visit Dr. Robin’s lab page here.
Robin, V. V., Katti, M., Purushotham, C., Sancheti, A., & Sinha, A. (2011). Singing in the sky: song variation in an endemic bird on the sky islands of southern India. Animal Behaviour, 82(3), 513-520.
Robin, V. V., Vishnudas, C. K., Gupta, P., Rheindt, F. E., Hooper, D. M., Ramakrishnan, U., & Reddy, S. (2017). Two new genera of songbirds represent endemic radiations from the Shola Sky Islands of the Western Ghats, India. BMC Evolutionary Biology, 17(1), 1-14.
Original paper: https://bmcecolevol.biomedcentral.com/articles/10.1186/s12862-017-0882-6
Written by Nandana Chaudhuri, edited by Devica Ranade