There are killers in central Victoria’s box-ironbark forests, and they are going after the eggs of birds already in decline.
Researchers have been delving into the private lives of woodland birds, trying to understand more not only about why some avian numbers are dropping, but what might be breaking into their nests.
A new Deakin University study, published in recent weeks, focused on areas around Heathcote, Maldon and Rushworth as well as three other more agricultural habitats.
Researchers deployed artificial nests to monitor eight of the more common farm and woodland birds, including scarlet robins, eastern yellow robins, golden whistlers and willie wagtails.
The carnage was high.
Most of the nests were raided within days.
That was something that shocked Desley Whisson, a Deakin senior lecturer in wildlife and conservation who was part of the research team.
“We are talking about, normally, a 12 day incubation period for some of these birds, maybe a little bit longer,” she said.
“So if they are being depredated within three days that is not giving any chance whatsoever for fledglings.”
The culprits were many and varied.
“We actually did find sugar gliders found our artificial nests quite quickly. Brush-tailed possums were another group,” Dr Whisson said.
“Even ring-tailed possums were observed at some of our nests.”
Yet, it was ravens and white-winged choughs that were the most common nest raiders, along with currawongs.
Researchers had wondered whether there would be differences between habitats near small towns and those in agricultural tracts, reasoning that different species have their own preferences when it comes to the company of humans.
Sadly, it appears that it does not matter where birds choose to make their nests.
“Nest predators were pretty much super-abundant wherever you went,” Dr Whisson said.
So, are predators entirely to blame for falling bird numbers?
Dr Whisson put a few caveats on the research. The nests used were artificial and it was not clear whether there would have been a different result if real ones were used.
There are currently no statistics for real nests, but it is a study that Dr Whisson said needs to take place.
There was also a chance that researchers monitoring nests may have inadvertently drawn predators’ attention.
As far as whether predators are entirely to blame for dropping bird population rates, the answer is no.
There are a number of factors that may help explain falling bird numbers. Some of Dr Whisson’s other research near Victoria’s coast has explored the way habitats can be disturbed, say, by koalas stripping too much vegetation off trees.
We are talking about, normally, a 12 day incubation period for some of these birds, maybe a little bit longer. So if they are being depredated within three days that is not giving any chance whatsoever for fledglings.Dr Desley Whisson, Deakin University.
She says another reason some populations could be shrinking is because of habitat loss.
That’s a point that Anne Bridleymakes too. She has been bird-watching and conducting wildlife surveys in central Victoria for 30 years and says birds are sensitive to habitat loss – especially those known to migrate north to Queensland, where a lot of land is still being cleared.
Then, of course, there are local areas where tracts of bushland have been replaced by housing, like in Jackass Flat and Bendigo East, or where there are more properties clustered around habitats, like at One Tree Hill.
“Some birds are very sensitive to disturbance,” Ms Bridley said.
Some species, like the hooded robin, have declined over most of the district as the climate has become drier.
Others are moving in, These days you are more likely than ever to see red-capped robins in the Wellsford State Forest. Those birds have widened their range from the Mallee.
However, even with all these problems that birds have faced, there is one event that Ms Bridley rates more significantly than any others.
“The Millennium Drought really reduced bird numbers,” she said.
“They have really never recovered to the numbers they were in the 1990s.”
Glimmers of hope
On the bright side, things can change for the better. Just consider the willie wagtail.
That species struggled during the Millennium Drought, with numbers plummeting around Bendigo. Then the dry broke and the birds returned.
Other birds are coming back too.
"Last spring was the first time since some time during the drought that I had all the cuckoos I'd expect to see in Bendigo," Ms Bridley said.
Those birds lay their eggs in other species’ nests. Last spring, cuckoos made their local comeback in part because other birds moved here to escape drought conditions in New South Wales.
"So when you are thinking about what is happening with birds, it's very complicated and can be affected by factors well outside your particular area," Ms Bridley said.