A gap too wide to fly across
Two years ago, a bird species found in the high elevations of the Western Ghats was split into two different species. What was behind this rediscovery and the split? Sandhya Sekar finds out.
doi:10.1038/nindia.2012.35 Published online 23 March 2012
The White-bellied shortwing is a bird about the size of a sparrow found in the high altitudes of the Western Ghats. And nowhere else in the world.
There were only 78 records of this bird from 1881 to 2000.
In 2001 and 2002, biologists V. V. Robin and R. Sukumar surveyed the forest areas across the Western Ghats — covering Goa, Karnataka, Kerala and Tamil Nadu. Choosing sites based on available records, they chronicled over 200 sightings of Brachypteryx major, the white-bellied shortwing (WBS). They figured out its preferred habitat by noting down areas from where it was found.
The species was restricted to the high elevation areas (above 1000m) in the Western Ghats, a continuous chain of mountains parallel to the west coast of India. At this height, the shift in vegetation is quite dramatic, with evergreen forests transiting into a mosaic of rolling grasslands on hilltops, with forests (known as sholas) in the ridges. The shortwing occupies these shola forest patches.
For species inhabiting these patches, moving to another suitable patch is difficult. The high elevation areas are far flung, more than 80 km apart in some areas. For the sparrow-sized shortwings, these are huge distances. So they end up trapped in these islands of suitable habitat called 'sky islands'. When such patches are located close to human habitation, there is a high risk of extinction. Though the WBS is not hunted, the shola forests have been destroyed systematically over the last 200 years, and their habitat reduced to smaller and smaller patches.
Movement between islands
The WBS occupies sky islands all over the Western Ghats. Over such distances, observing movement physically is impractical. Short-term observations are a snapshot in time and not a holistic picture of the scale of movement.
Robin and his colleagues at the National Institute of Advanced Studies and the National Centre for Biological Sciences, Bangalore, decided to study movement indirectly — by using the genetic code (DNA). To obtain the DNA, they took a drop of blood from 33 shortwings from across its range.
The genetic code was used to check the relatedness between the populations from each of the sky islands. If the birds were moving between the sky islands, birds from all the islands would be related to each other. But, shortwings from distant patches were unrelated: which means, they had not been able to move between the patches. The further apart the birds were sampled from, the less related they were.
The 1600 km long Western Ghats is almost continuous, except at the Palghat Gap. This gap has proved too much for the shortwing. The researchers found two distinct groups, one to the north of the pap and the other to the south.
An old gap too deep
The difference in the DNA between the two groups also allows the scientists to estimate when they split up. Other studies have shown that there is an approximate constant rate at which the genetic code has been evolving — they call it a 'molecular clock'. Using the molecular clock in this case, Robin and his colleagues have found that the split between the northern and southern groups is as old as 5 million years. Also, the extent to which they have split was enough to call them two species.
The shortwings found to the north of the Palghat Gap have a reddish brown belly, and were named the rufous-bellied shortwing, Brachypteryx major. Those found to the south of the Gap have a white belly, and were called the white-bellied shortwing, Brachypteryx albiventris.
Singing a different tune
Shortwing males sing during the breeding season to attract females. Robin and his colleagues checked for differences between the song patterns of these two new species. They recorded about 572 songs from 23 shortwings in and around Kodaikanal (south of Palghat Gap) and near Ooty (north of Palghat Gap). Indeed, both species were singing very different tunes. Since the song is used for something as crucial as attracting mates, this is excellent support for the previous finding that shortwing is in fact two different species.
The songs are also different between two populations of rufous-bellied shortwing, one in Kodaikanal and one in the nearby Grass Hills. Even though there is no genetic difference between these populations, the songs vary significantly. Songs are passed on in birds like in human beings — as part of 'culture'. Emerging differences in song means there is not much movement between these patches. Deforested lands have separated Kodaikanal and Grass Hills for about hundred years now, and the shortwing is not able to fly between these sky island patches.
Both the shortwings occupy unique habitats restricted to small pockets in the Western Ghats. There is a lot of deforestation in and around their habitats, making the situation more grave. Shortwings represent a large number of understudied groups found in India. Detailed studies will help frame a better management policy to conserve such groups.
- Robin, V. V. et al. Status and habitat preference of White-bellied Shortwing Brachypteryx major in the Western Ghats (Kerala and Tamilnadu), India. Bird Conserv. Int. 12, 335-351 (2002)
- Robin, V. V. et al. Ancient geographical gaps and paleo-climate shape the phylogeography of an endemic bird in the sky islands of southern India. PLoS ONE 5, doi: 10.1371/journal.pone.0013321 (2010)
- Robin, V. V. et al. Singing in the sky: song variation in an endemic bird on the sky islands of southern India. Anim. Behav. 82, 513-520 (2011) | Article |