New research offers hope for saving orange-fronted parakeets (kakariki), whio (blue duck) and mohua (yellowhead) from extinction. Operation Ark, an emergency response plan to stoat and rat plagues, was unveiled by Conservation Minister Chris Carter this week.

WHEN Mike Aviss walked his stoat traps up Mt Stokes in Marlborough to protect 90 mohua for the 1999 summer breeding season, he was in for a shock. Only 27 of the chirpy yellow-and-brown songbirds were there to greet him. Something had happened during the winter.

Trapping that summer at Mt Stokes provided the clues: 80 stoats, more than expected, and 22 ship rats, the first recorded appearance of more than one rat in the area.

“It highlighted for the first time that rats were a serious threat to mohua,” recalls Mr Aviss, Department of Conservation biodiversity manager for the Sounds Area. “That’s even in high altitude beech forest which rats normally find inhospitable.”

“We had thought if we trapped stoats the birds would recover. We put the high rat numbers that year down to unusually heavy fruiting in beech forest in autumn, that happens every now and again, and we would watch out for it in the future.”

No heavy beech fruiting occurred on Mt Stokes the next year, and Mr Aviss and his staff breathed a sigh of relief - too soon, however. Of the 27 remaining mohua, none survived. Scientists are still trying to determine the cause of the second rat incursion.

It was a bitter pill for Sounds Area staff. The Mt Stokes mohua had been discovered in 1985 in cold, wet, steep terrain. The six pairs were the only mohua north of Hurunui and Hawdon in North Canterbury at the time. Stoat trapping allowed the birds to recover till 1998-1999, when 39 chicks fledged of 17 nesting pairs.

Mohua had also disappeared from the Hurunui River north branch, and were dramatically reduced in numbers in the Eglinton Valley (Fiordland) during the two breeding seasons. In both areas, heavy beech fruiting had occurred two years running, a one-in-20-year event. As at Mt Stokes, the rat plague in the Hurunui was the first recorded such event.

Fewer than 5000 mohua now remain at a handful of isolated sites in the South Island, down from 10,000 birds three years ago.

Attention has been drawn to mohua this year by another critically endangered bird, orange-fronted kakariki. Only 100-200 remain, down on 700 a few years ago. Present at three sites in North Canterbury beech forest, the bird could be snuffed out like the Mt Stokes mohua if more stoat and rat plagues occurred.

Such knuckle-biting figures spurred Conservation Minister Chris Carter to visit the Hawdon Valley on Friday 19 September, where he unveiled a plan to prevent further bird extinctions on the mainland.

“Operation Ark is about protecting our most at-risk species from stoat and rat plagues at the most crtical sites,” Mr Carter says. “I’m talking about the orange-fronted kakariki, mohua and whio, as well as takahe and long-tailed bat (peka peka).”

“What is proposed is early detection of predator plagues, such as early detection of beech masting events and warm winters, and intensive trapping at key sites for early detection of rats. We will ensure that we have the resources for intensive trapping and precision poisoning, and have them ready to go if and when the time comes.”

“Operation Ark will envisage a level of pest control on a scale never before seen in New Zealand.”

Why has it taken so long to get to this point? Cost, Mr Carter says, and, more importantly, the time needed for scientists to understand what’s been happening to hole-nesting birds such as mohua and orange-fronted kakariki, as well as riverbank-nesting whio.

Charting the cause of mohua decline reads like a whodunnit – there’s more to the story than rat and stoat predation, and more to killing rats and stoats than dropping poison over large areas.

Mohua – aka the bush canary for its bright colour and cheerful song - were once a common forest bird throughout South Island forests, says DOC scientist Graeme Elliot.

DOC had known for decades that mohua were in gradual decline. Then, from the 1970s, huge gaps suddenly started appearing in mohua distribution. They disappeared from Nelson Lakes National Park by the mid-1970s, Hope River in North Canterbury (mid-1980s), Moeraki in Fiordland (1990), Burwood Bush near Te Anau (mid-1990s), Waikaia forest in Southland (2000).

It had long been assumed that stoats taking adult female mohua and eggs on the nest were the main cause of decline, and that trapping stoats would manage the problem, Dr Elliot says.

The news that rats, not stoats, were the key predator of mohua and kakariki added a piece to the jigsaw, he says. Another was that stoats prefer eating rats to native birds. By trapping stoats, DOC may have unwittingly made things worse for mohua by allowing rats to multiply more easily. Current thinking is that stoats and rats must be controlled simultaneously.

Joint stoat control and rat monitoring in the Hurunui and Hawdon valleys, key strongholds for kakariki and mohua, has been trialled since last year. Stoat traps are set up at 100m spacings, says DOC Canterbury Conservancy advisory scientist Andy Grant. Rat traps at 50m spacings on lines 150m apart allow early detection of rats.

“As soon as rats appear in our traps, and we don’t have any at the moment, we plan to control them with toxins delivered in parakeet-proof bait stations, at 25m spacings on lines 150m apart. That’s 100km of bait station lines in the Hawdon valley and 75km in the Hurunui, a huge effort. Rat plagues like we’ve seen only crop up once every few decades but when they do they have a devastating impact on our birds.”

Operation Ark would go a few steps further than Dr Grant’s work.

At Hurunui and Hawdon, it could include 75km of traps, on top of carefully targeted rat poisoning, intensive trap and bait station monitoring, as well as aerial 1080 drops in a buffer zone around the area to prevent rat and stoat reinvasion of protected areas.

Mr Carter says he would work with the department to build an emergency response capacity into Vote Conservation as a key part of the Government’s Biodiversity Strategy.

There is an epilogue to the mohua tale. DOC has a stop gap for the birds– building breeding colonies on offshore islands. Currently, there are many hundreds of mohua on the islands of Breaksea, close to 100 on Chalkey and less than 50 on Anchor in Fiordland; 20-30 on Ulva and 30-40 on Codfish off Stewart Is, and 10 on Centre Is in Lake Te Anau.

Four Mt Stokes birds escaped slaughter in 2000-2001 by being translocated to Nukuwaiata Island in the Marlborough Sounds. Of that group, a male and female remain, as well as three offspring hatched on the island.

© Scoop Media