New Study Questions Sugar Glider Predation on Swift Parrots, Citing Habitat Loss as Main Threat

A new study has called into question the extent to which introduced sugar gliders prey on critically endangered swift parrots in Tasmania, pointing instead to habitat loss as the overarching risk to the species.

The Commonwealth-funded study, by landscape ecologist Matt Webb and veterinary ecologist Leanne Wicker, included acoustic monitoring, motion cameras and glider trapping over two breeding seasons on mainland Tasmania.

They could not find examples of sugar gliders killing swift parrots.

"Gliders were detected at, and inside, nests that successfully fledged young, confirming that presence or visitation alone is not a reliable indicator of impact."

The study found both successful and failed nests in the same areas, including where sugar gliders were present.

But rather than sugar glider predation, there was evidence of birds, including native currawongs and introduced European starlings, preying on swift parrots in nests, although "no species determined nest success or failure".

The study authors put this down to competition-related aggression, as well as predation, exacerbated by "ongoing habitat loss and fragmentation".

Researchers take 'contemporary' approach

For more than a decade, sugar gliders have been considered a main threat to the swift parrot on mainland Tasmania, reducing their survival rate to as low as 17 per cent, based on earlier studies.

But Dr Webb said this did not align with his experiences in the field.

They obtained a grant from the Commonwealth's Saving Native Species program to build on the evidence around the interactions between sugar gliders and swift parrots.

"Understanding glider diet using forensic necropsies, to understand why swift parrots died when we found nest failure.

"We were able to interpret that from the context of two decades of monitoring swift parrots in the wild."

Acoustic monitoring identified where swift parrots were nesting, and cameras were set up at 20 nests for periods of more than two months each.

Necropsies were carried out on swift parrots that had died as well as on trapped gliders, to determine whether they were eating the parrots.

Dr Wicker said the dead parrots did not display signs of being killed by sugar gliders, while the glider diet did not contain any evidence of parrots.

Government wanted to paint gliders as 'the main threat'

Based on previous studies, the Tasmanian government has sought to emphasise sugar gliders as the major risk to swift parrots, as opposed to habitat loss from native forest logging.

This included suggested edits to the swift parrot recovery plan, in which the state government wanted predation by sugar gliders listed as "the main threat to the survival of swift parrot".

The government threatened to withdraw its endorsement of the document unless a "narrative focus" on its forestry management system was addressed.

While earlier studies pointed to sugar gliders as killing female parrots and eggs, they also stated that this was less of a risk in wider swathes of mature forest.

Dr Webb said the narrative around swift parrot survival had since changed to make sugar gliders a "scapegoat".

"Sugar gliders are just one of the threats, but all of them sit underneath like an umbrella of habitat loss," he said.

Dr Webb said this made it difficult to obtain research funding for swift parrots, compared with funds for sugar glider research.

Focus on habitat protection

Mark Holdsworth first identified the potential risk posed by sugar gliders during research into the critically endangered orange-bellied parrot in Tasmania over two decades ago.

His work prompted further studies.

Dr Holdsworth said the new study showed the sugar glider did not appear to be as important a predator as previously thought, and that there were other pressures.

But he said addressing all predators would be impossible.

"A lot of those predators we can't do anything about; they're there in our landscape, they've been well established," Dr Holdsworth said.

Sugar gliders are believed to have been introduced to Tasmania in the late 19th century.

In a review earlier this year for the journal Australian Forestry, Simon Grove assessed previous studies to compare the "forest habitat narrative" with the "predation narrative", arguing there was more evidence for the latter.

He suggested sugar gliders could be eradicated through gene drives, while also acknowledging forest harvesting "may not make things any easier for the parrots".

The latest study has come out after his review.

Scrutiny on logging system

The swift parrot's status was uplifted from endangered to critically endangered in 2016 due to concerns about the impact of sugar gliders.

In its recovery plan, sugar gliders are mentioned first in the threats section, followed by loss or degradation of habitat through logging, land clearing and wildfire.

Swift parrot numbers could be as low as 500.

Tasmania's forest logging system includes increased habitat protection in swift parrot important breeding areas, but studies have shown the overall extent of logging is greater in these areas.

Foraging habitat is protected and potential nest trees are mapped prior to logging.

Trees above a certain size are intended to be excluded from logging but can still be cut down for safety reasons.

The area is then burnt and re-sown after logging, but studies have shown a progressive degradation of habitat over time.

Public forestry company Sustainable Timber Tasmania has previously stated that the swift parrot faces "multiple interacting threats", while describing sugar gliders as "the principal driver" of population decline.