Numbers of one of Australia's rarest birds, the regent honeyeater, are so low that during this breeding season, conservationists have been having trouble spotting any in the wild.
Birdlife Australia's NSW Woodland Bird Program manager Mick Roderick said it set alarm bells ringing.
"The official line is that there are 250 to 350 regent honeyeaters left in the wild, that's the ballpark," he said.
"However, not being able to find virtually any birds in the middle of a breeding season suggests that figure is optimistic. It's been a real worry for the people behind the recovery of the birds."
In a bid to save the critically endangered species from becoming extinct, a captive-breeding program is underway and recently there was a large-scale release of 50 birds near Kurri Kurri, in the Lower Hunter region of NSW.
Mr Roderick said it was a crucial part of efforts to bolster the wild population.
"Earlier this year the Australian National University in conjunction with other groups, did a population viability analysis, and that's basically mapping out the future of a species, " he said.
"They concluded if we did nothing the regent honeyeater would almost certainly become extinct within 10 years."
Successful release on Wonnarua country
The breeding program is led by Taronga Conservation Society Australia, Birdlife Australia and the NSW government's Saving our Species program.
The regent honeyeaters were released into an endangered forest type on Wonnarua Country, land owned by the Mindaribba Local Aboriginal Land Council (LALC).
It came after a similar large-scale release of 58 regent honeyeaters on the same land last year.
After last year's release, breeding activity was documented along with the assimilation of zoo-bred birds into wild flocks and this year's birds also appear to be thriving.
"It's a very productive area, the habitat is amazing, there's ironbarks flowering, and the birds are doing really well," Mr Roderick said.
"Now we've let this year's zoo birds go we have rediscovered some of the wild birds again … which is amazing.
"It's incredibly rewarding to see zoo-bred birds form mixed flocks with wild birds."
LALC chief executive Tara Dever said the successful release on Mindaribba LALC land reflected the deep relationship between the birds and the country at the foot of Mount Tomalpin.
"This country, like its people and this wonderful bird, has survived massive change and upheaval over the past 250 years," Ms Dever said.
"While conditions need to be just right to ensure the birds have enough food and shelter, the deep connection between First Nations people and this land has assisted with the success of the release."
Bird movements monitored
Tiny radio transmitters, designed to eventually fall off, were put on just over half the released birds to allow the program to track the birds over the next couple of months.
"Monitoring will involve a small radio-tracking crew, following transmitter signals and recording individual bird locations and behaviour to understand survival, breeding attempts and dispersal patterns," Mr Roderick said.
"Each day there's a team of about four or five people out there, tracking the birds, following their movements, trying to find birds that have wandered off, it's really exciting.
"Every day we discover new things about what these birds are doing."
Regent honeyeaters learn their song
One recent focus of the breeding program has been to help regent honeyeaters find their unique voice and song culture again.
Due to their declining numbers, wild regent honeyeaters have been gradually losing their distinctive mating song, as younger birds haven't got many older birds to learn from.
The Taronga Conservation Society has been trying to teach the captive-bred birds the species' unique call.
"Juvenile zoo-bred regent honeyeaters are now housed in aviaries with wild adult birds to be exposed to their wild regent honeyeaters song prior to release," the society's wildlife conservation officer Monique Van Sluys said.
"This crucial step allows the birds to learn and refine their distinctive song."
Mr Roderick said there was evidence the approach was working.
"We are trying to teach some of the really young birds that have hatched this year, the proper wild regent honeyeater song because a lot of the birds that are released to our ears don't sound a lot like wild regent honeyeaters.
"So potentially there might be an issue with the zoo birds attracting a wild female mate .... so this tutoring is happening and the results have been pretty astounding really," he said.
"Just from spending time with some of these birds, from this release I have already noticed a big difference between this year's cohort and last year's cohort … some of the males out there this year definitely sound better than last year's birds."
Since 2008, a total of 400 regent honeyeaters have been released into the wild in the Hunter Valley and predominantly in north-eastern Victoria, in the Chiltern-Mt Pilot National Park.
