Answering Curlew Questions
Plans to save the threatened Bush Stone-curlew are turning to genetics for answers to some enduring questions, says the Museum’s Robert Mason.
I looked on with an uncomfortable mix of excitement and disappointment as, after a year of painstaking work, the Bush Stonecurlew’s DNA unveiled its story.
Work by our team – ornithologist Walter Boles and geneticists Rebecca Johnson, Mark Eldridge and myself – revealed that the species, one of Australia’s most charismatic and vulnerable birds, had suffered a sudden drop in its population at some point from which it has not yet recovered. Known as population bottlenecks, such crashes destroy genetic diversity, leaving the species vulnerable to inbreeding, genetic diseases and malformations.
The bottleneck was the first of several surprises in our study, which had set out to resolve a 100-year-old mystery about the status of these birds: were they one species or several subspecies?
Gangly and spooky
The Bush Stone-curlew, Burhinus grallarius, is a striking, brownspeckled bird with gangly legs and large, spooky eyes supremely adapted for nocturnal vision. It’s a ground-dweller that likes woodlands with a messy understory of fallen wood and logs for foraging, roosting and camouflage.
It also has a remarkable courtship dance – wings outstretched, tail upright and neck stretched forward, stamping its feet for an hour or more at a time, all the while accompanied by loud and constant calling.
People in rural Australia know its haunting ‘koo-loooo’ evening wailing call – actually a means of group communication during its nightly feeding forays searching for seeds and small animals. Once common throughout most of mainland Australia, the species has somehow held on in the south despite widespread clearance of its woodland habitat and predation by feral dogs, cats and – most damaging of all – foxes. Its last stronghold is in the fox-free far north of Australia.
Bird watchers began to notice the Bush Stone-curlew’s disappearance in southern Australia in the 1940s, and by the 1990s there were only small, scattered populations left across the southern mainland. Many long-time landholders became alarmed that this familiar character was slowly blinking out, and a number of community groups swung into action. A re-introduction plan was hatched to place them in fenced, predator-free areas. The NSW Government formed a threatened species plan in 2006 to comprehensively address the different threats to this species.
But these plans were complicated by the mystery of whether the birds were a single species or several subspecies across their range; introducing a subspecies into another’s area can, through interbreeding, cause the loss of unique genetic characteristics.
First described in 1802, the Bush Stone-curlew was split into a number of subspecies 110 years later by GM Mathews, one of the first experts on Australian birds. He recognised four subspecies: two in northern Australia, one in south-western Australia and another in south-eastern Australia.
In 1980, scientists re-examined the morphology of many specimens and concluded that there were no consistent groups that could be subspecies, though one author later conceded, in 2004, that there might be two subspecies in mainland Australia, northern and southern.
Conservationists realised that new DNA technologies could help solve the mystery. In 2006, Catherine Price from the (then) Department of Environment and Conservation of NSW contacted the Australian Museum, knowing that our expertise in animal DNA analysis might be able to help.
Catherine put out a call to a network of volunteers to send in any feathers found in Bush Stonecurlew habitat to provide a source of DNA. Meanwhile, using a technique originally developed for forensic human DNA identification, we slowly began working through the bird’s genome.
The meticulous process took many months. First, we had to isolate ‘microsatellites’ – tiny segments of repeated DNA from within the one billion-plus letters of the bird’s chromosomes.
The first time this was done was the longest and hardest, taking more than two weeks and using DNA from a single bird found dead near Ingham, Queensland. But once completed, we could manufacture ‘primers’, small sections that fluoresce under laser light, enabling the process to be repeated quickly for many other individuals.
This method gave us an estimate of the genetic variation in each animal – how similar or different they were to each other – and enabled a whole suite of other insights.
Specialised computer programs crunched the numbers to reveal the population bottleneck – the indicator of a rapid population crash. But the second result was just as disconcerting: a mysterious phenomenon known as the Wahlund effect – a genetic signal that a population has been fragmented into several smaller isolated populations. Again, that’s bad news because the isolated populations are unable to share genetic diversity, putting them at risk of inbreeding (much as pedigree pets express the characteristic weaknesses of their breed).
The final surprise has brought us somewhat closer to answering the century-old question about the number of subspecies. Examining mitochondrial DNA (passed on only by a mother to her offspring) we found that Bush Stone curlews in northern Australia are almost identical to those in the south.
So if there are no subspecies, then perhaps we can restock southern areas with birds from the north without affecting genetic integrity. We can’t be sure though until we repeat this test using the powerful ‘microsatellite’ DNA approach (which sometimes tells a different story from the mitochondrial DNA). But it’s still an important step in solving the mystery.
Modern genetic techniques are increasingly important for understanding the conservation and monitoring needs of threatened species like the Bush Stone-curlew. But as this species continues its struggle for survival in southern Australia, it will also need continued strong action to control its predators and conserve its habitat.
Technical Officer, DNA Laboratory
First published in Explore 33(1).
Michael Hugill , Online Producer