Genetic study argues to raise the status of great bustard subspecies to the species level.
It could be a typical question in a pub quiz: ‘What is the heaviest bird capable of flight?’ The answer is the great bustard (Otis tarda), which can reach weights of nearly 20 kilograms. Currently, ornithologists recognize two subspecies of this flying heavyweight: the Eastern (dybowskii) and the Western (tarda) great bustard. But this taxonomic arrangement might change…
East and West
The Eastern subspecies is restricted to Siberia, Mongolia and China where about 1,500 individuals live. The Western subspecies is more numerous – between 42,000 and 55,000 individuals – and ranges from Portugal to Xinjang (western China). Aimee Elizabeth Kessler and her colleagues collected samples across this distribution and sequenced two mitochondrial genes.
The genetic analyses showed a clear separation between both subspecies that probably diverged about 1.4 million years ago. Gene flow was estimated at less than one individual per generation. Interestingly, there was one ‘misplaced’ sequence from Xinjang, which suggests that there might be a hybrid zone at this location. Definitely something to explore further.
The genetic differentiation and low levels of gene flow are consistent with the proposal of considering dybowskii and tarda as distinct species. But genetic data alone are not sufficient to support this decision: ecological, behavioral and morphological studies are needed to make a convincing case. In 1874, Taczanowksi already described dybowskii as a separate species based on morphometric and plumage differences.
Indeed, there are some differences between the subspecies in the extent of white plumage on the wings and tail. And the plumes around the bill are differently placed in both subspecies. Perhaps this plumage variation allows females to discriminate between (sub)species during the elaborate displays of the males (see video below). That would push these bustards one step closer to the glorious status of species.
Kessler, A.E., Santos, M.A., Flatz, R., Batbayar, N., Natsagdorj, T., Batsuuri, D., Bidashko, F.G., Galbadrakh, N., Goroshko, O., Khrokov, V.V., Unenbat, T., Vagner, I.I, Wang, M. & Smith, C.I. (2018) Mitochondrial Divergence between Western and Eastern Great Bustards: Implications for Conservation and Species Status. Journal of Heredity.