Quailthrush are a group of songbirds widespread across Australia and the island of New Guinea. They are primarily ground-dwelling birds that forage for seeds and insects in habitats from rainforests to deserts.
Quailthrush got their English name due to their thrush-like proportions but quail-like rapid, low, noisy flight when disturbed.
In 2007 we began to study these remarkable birds. We hoped they might show us whether Australia’s arid zone fauna and flora had evolved from ancestors in New Guinean and Australian rainforests. To find out, we planned to use the birds’ DNA sequences to build their evolutionary tree.
Our research since that time has taken us on a journey with many unexpected twists and turns.
Quailthrush in Australia and New Guinea
When we began to study these special birds, researchers only recognised five different species. Over time our DNA studies revealed three more, bringing the total to eight. The extra three had originally been lumped with their similar-looking relatives.
Painted Quailthrush live in the rainforests of New Guinea. Spotted Quailthrush occur in eastern Australia’s eucalypt forests. Chestnut-breasted and Western Quailthrush can be found in stony red-rock mulga country on opposite sides of the continent. They live more or less east and west of Lake Eyre Basin, respectively.
The Cinnamon Quailthrush of Lake Eyre Basin are very closely related to Nullarbor Quailthrush. The latter are the only birds that live only on the Nullarbor Plain and nowhere else.
Chestnut Quailthrush and Copperback Quailthrush round out the eight species. But more on them in a moment.
By 2012, the DNA evidence supported the idea that quailthrush had evolved in the rainforests of Australia and New Guinea before expanding into to the arid zones of Australia. We also found that the ages of the arid zone species matched the formation of the Australian deserts. The arid zone species started diverging from each other around six million years ago.
This is interesting because the record of evolution of the environment, plants and animals in Australia is very sparse during the period between six and 10 million years ago. Also, it is consistent with estimates of first divergences in other arid zone vertebrate groups inhabiting the same stony deserts as quailthrush.
Splitting a quailthrush species in two
Our DNA studies revealed the Chestnut Quailthrush, with its fairly well understood geographical variation in plumage, was actually two different species that look very similar.
They are now known as the Chestnut Quailthrush, which occurs in south-eastern Australia’s Flinders Ranges and Murray Mallee, and the Copperback Quailthrush, which occurs from just west of the Flinders Ranges all the way to Shark Bay in Western Australia. The genetic difference between the two species is large. DNA reveals there were no genetic connections between the two populations for close on one million years.
But we could see that within Copperbacks, geographical patterns of their genetic and reassessed plumage variation didn't match. So, in just published research, we used DNA from specimens in museums and research collections to take a closer look at their genetic history. Improvements in technology for analysing DNA from toepads of the feet of older museum specimens made this possible.
We found quite a surprise.
A genetic robbery in Australia's quailthrush
In the distant past, Copperback Quailthrush populations between the Flinders Ranges and the Indian Ocean split into two separate populations. These populations, roughly west and east of the Nullarbor Plain, have since come back into contact. They connect genetically around a very broad region including the Great Victoria Desert and western Eyre Peninsula.
Eventually, birds from the western population began to move east, invading the range of the eastern population. We cannot be sure when this started, but it was probably within the last few hundred thousand years. As they did so, the mitochondrial DNA (mtDNA) of the eastern population began to move in the opposite direction, back into the original, geographical range of the invading western populations. Eastern mtDNA is completely replacing the western mtDNA: we are witnessing a genetic robbery in Australia's quailthrush.
This unusual situation can be explained by population genetics theory. We think the cause is a neutral process arising from the wave front of the western birds’ populations fragmenting into smaller subpopulations as they move east.
A competing idea, which further research might address, is that there is a survival advantage for birds that have the eastern mtDNA.
What next for quailthrush research?
Our quailthrush research journey has a few more stops ahead. We know next to nothing about the newly discovered Spotted Quailthrush population around Atherton in north Queensland. Are they as geographically isolated as we think? How different are they in plumage and genetic makeup from more southern populations? Will studies of their genetic and plumage differences mean that these birds are a different species from the birds further south? Will this parallel recognition of Chestnut and Copperback Quailthrushes? Only careful research can answer these questions.
In New Guinea, other researchers have noticed large genetic differences among Painted Quailthrush populations despite not a lot of plumage variation. Are we are looking at one species? Or are these separate species under natural selection to remain looking very much alike, perhaps because of their need to be camouflaged on the rainforest floor?
We have a way to go yet on our quailthrush research journey!
Leo Joseph is director of the Australian National Wildlife Collection at CSIRO. Read his article about Kangaroo Island's Glossy Black-Cockatoos.