Fewer than 250 regent honeyeaters are likely left in the wild. New research provides a vital clue in the efforts to save the critically endangered Australian bird.
Captured on one of Bianca McBryde's tree-mounted cameras, the brush-tailed possum crawls into the frame, lowers its head into the nest and bites into the egg. The snack was a shop-bought quail's egg and the nest was artificial – a crafty construction made of half a tennis ball, some brown paint and fibres from the husks of coconuts. But the interaction was a vital clue in the increasingly desperate efforts to stop Australia's critically endangered regent honeyeater from going extinct.
McBryde wanted to know how often these native possums were likely taking the eggs of this striking Australian songbird. The answer was: not very often, but often enough to matter. Over two weeks, possums raided only four of the 40 fake nests placed in trees at North Head in Sydney.
As little as three decades ago, when regent honeyeaters were far more common, such infrequent raids would have made little difference to the species. “It's likely possums have always done this occasionally and it wouldn't have had an impact,” says McBryde, a researcher at the University of Sydney. “The occasional egg here and there was probably just a part of the normal balance of things.”
But when McBryde fed her new data into a computer model, the effect of these occasional raids on the honeyeaters' prospects for survival was profound. If this happened in the small populations of honeyeaters that are clinging on today, McBryde's research found the loss of just a few eggs to the opportunistic possums could increase their risk of extinction within 20 years by about 35%.
A uniquely Australian bird
Regent honeyeaters are one of Australia's most threatened birds, with fewer than 250 likely left in the wild restricted to three sites on the periphery of the Blue Mountains, to the west of Sydney. Once seen in large and mobile flocks that would help pollinate eucalypt trees across vast distances, most of their woodland habitats have been lost to developments and farming.
“They're a uniquely Australian bird,” says Mick Roderick, the regent honeyeater recovery adviser at BirdLife Australia, which alongside the Taronga Conservation Society helped pay for McBryde's research. “It can mimic the calls of other birds and it's the only honeyeater on Earth that can do this. Their strategy was to arrive at blossom events and chase other birds away. Their flocking strategy doesn't work any more. A flock might just be three birds.”
That talent for mimicry is thought to be a symptom of the bird's demise because, with so few other male birds around them to learn from, regents are picking up the calls of other birds instead. In a captive breeding program at Taronga Zoo, the full song is played to young males through speakers so that when they are released they have a better chance of attracting a mate. Research earlier this year outlined the success of this “song tutoring” approach. While the bird's full song disappeared from the wild entirely, it was rescued by teaching the young captive males so they could reintroduce the song back into the wild.
Possums are not the only native animal likely raiding the nests of regent honeyeaters. Some gliding possums have also been seen raiding, as have native birds such as ravens, pied currawongs and butcherbirds.
McBryde's modelling of the impact of egg-taking by possums and gliders suggests that if solutions could be found to protect the nests, this could buy time to build up the wild population. “So their probability of extinction [in the next 20 years] is about 90%. But what happens if we take out that predation? If we can stop it or reduce it?” she said. “We could drop that probability to 65%. That gives us a bit more of a chance.”
Roderick says the recovery team is investigating a range of non-lethal interventions. Putting collars around the trunks of trees where the birds have built their nests could discourage possums from climbing but he said it would do nothing to deter gliders and could be difficult to deploy because the birds do not nest in the same trees from one season to the next.
The use of scent to confuse predators about the location of the nests is another line of inquiry but McBryde's research found the possums did not appear to be attracted to the smell of the nests. Another step being investigated is the use of ultrasonic sound to deter possums and gliders.
“It's a wicked problem,” says Dr Ross Crates, an ecologist and regent honeyeater expert at the Australian National University. “These are urgent stopgap measures to prevent extinction.” There are also projects to replant the trees favoured by the regent honeyeater but Crates says for the endangered bird “their situation is so dire that we don't have time to wait for the trees to regrow”. “They're in big trouble. We wouldn't want them to get any closer to extinction.”
