The Dodo, that remarkable flightless bird, has become an icon of extinction. However it was far from the only unique island bird to become extinct in the era of European exploration in the Indian and Pacific Oceans.
Of the many species lost in those times, the Spotted Green Pigeon is one of the most mysterious. It is known to us today from just a single museum specimen. Over 200 years after it was first described we are still unsure of where this pigeon lived, its relations to other birds or even if it was actually a unique species.
However new research, published today in BMC Evolutionary Biology, uses DNA taken from this one remaining specimen to not only resolve these mysteries but also potentially to teach us new things about the evolution of the Dodo.
Gone but not forgotten
We only know that the Spotted Green Pigeon ever existed because of two specimens that found their way into the private collections of two members of London’s Royal Society.
The specimens were officially described in 1783 by the prominent ornithologist John Latham, who pronounced the Spotted Green Pigeon a new species. Sadly, the collectors of these specimens were not careful note-takers and failed to record exactly which island they found this bird on. As both collectors focused heavily on the Oceania area, it has been largely assumed the bird was found on an island in this region.
Of these two specimens, one was lost while the other found its way to the World Museum in Liverpool, where it remains to this day (hence the species’ alternative name of the Liverpool Pigeon).
Debate has periodically arisen around this specimen. Some have speculated on the exact island it lived on (Tahiti being a popular, if disputed, choice). Others have suggested it is not a unique species at all but merely a juvenile Nicobar Pigeon, a bird still found today on a number of Indian Ocean islands.
Such debates have of course been impossible to resolve with such meagre evidence to go on. Now however, the advance of ancient DNA research appears to have resolved at least some of these mysteries.
Bringing ancient DNA to life
A team of researchers, led by Tim Heupink of the ancient DNA facility at Griffith University, Australia, set out to extract DNA from two feathers of the Liverpool museum specimen. Unsurprisingly, having been dead for over 200 years, the DNA from the specimen was highly fragmented; standard PCR techniques were unable to produce usable DNA fragments.
The researchers overcame this by using a novel DNA extraction technique. They characterised three ‘mini-barcodes’ located on the mitochondrial 12S gene. Despite being made up of short DNA sequences, these mini-barcodes are very informative; crucially, most bird species have unique barcodes.
By comparing the sequence of the Spotted Green Pigeon’s mini-barcode with those from its most likely relatives (including both the Dodo and Nicobar Pigeon) the researchers were able to produce a phylogeny of the evolutionary relationships between these species.
The results confirm that the Spotted Green Pigeon is a unique species; its mitochondrial barcode is unlike those of any other species. Those who claimed they were juvenile Nicobar Pigeons were not so far off the mark however; the results show that the Nicobar Pigeon is the closest living relative of the Spotted Green Pigeon.
Island hopping pigeons
The results also suggest that the Spotted Green Pigeon and Nicobar Pigeon were close relatives of the Dodo and its closest relative the (also extinct) Rodrigues Solitaire. The researchers suggest that this might tell us something about the origins and evolution of the Dodo.
Most species in the extended Dodo clade, apart from the Rodrigues Solitaire and the Dodo itself, share certain characteristics; semi-terrestrial habitats, an affinity for islands and the ability to fly. The researchers suggest that the addition of the Spotted Green Pigeon, also seemingly sporting these traits, to this group supports the ‘island-hopping’ hypothesis for the origins of the Dodo.
This theory proposes that the Dodo’s ancestors, from India or South-East Asia, flew from island to island across the Indian Ocean before eventually arriving in the Mascarene Islands, off the coast of Madagascar. Here, on Rodrigues and Mauritius respectively, they lost the ability to fly and eventually evolved into the Rodrigues Solitaire and the Dodo. One of the stops on the way would have been the island where the Dodo’s ancestors evolved into the Spotted Green Pigeon.
A suitable epitaph for an unusual bird
These findings confirm that a lone specimen in a Liverpool museum is all that remains of an entire species. While this is a sad end for the Spotted Green Pigeon, we can at least be consoled that, remarkably, it can still teach us about the evolution of other species, over two hundred years after the last of its kind flew over an island somewhere out in the Indian Ocean.