Colin O’Donnell ambles towards the edge of silver beech forest near the Landsborough River, drawn by the high-pitched, repetitive call of a mohua. It’s a call the Department of Conservation ecologist has been following for more than 30 years.

Ford Flat, overlooked by the Solution Range of mountains, is a common place to wait for the river to recede. In sections of the forest above there’s an ominous ripple of red – signs of a coming mast seeding. Swirling sandflies are ever-present and insistent.

“While it’s there I might just cheat,” O’Donnell says of the chattering mohua, producing from his pocket a portable speaker loaded with bird calls. “It might not work but we’ll give it a go.”

It does work. Within a minute two bright-yellow-headed birds dance animatedly in the trees above the speaker, hoping to glimpse their new friend. Flitting branch-to-branch and tree-to-tree, they make a hard target for an amateur wildlife photographer. But their nests, in tree holes, are easy pickings for predators like stoats, rats and possums.

That concern was the genesis for intensive pest control in the Landsborough Valley. Stoat trapping started there in 1994, and the first aerial 1080 poison drop, to kill rats and possums, occurred in 1998. That year was also when O’Donnell and his survey team started what has become one of DOC’s longest-running bird monitoring operations.

Beech forest hugs the Landsborough River valley. Photo: David Williams

Over two days last November the quartet of counters complete 174 five-minute counts. Some are done down at the bush edge, while others are way up the back, where the hills steepen. The results are officially out today. DOC says it’s the first time in at least 21 years that mohua are the most common bird counted. The “count” was 444 mohua.

“That is a record,” O’Donnell says with satisfaction, after the second day of counting. “They’ve been going up every year that we come and do it – but that’s another leap up.”

What does it tell us?

“The predator control’s working. Anywhere else where you don’t do control they’re gone. Basically, they’ve gone from about 99 percent of their former range, now – with a few mainland sanctuaries like this one still, and then a bunch of offshore islands that we’ve been putting them on.”

It could have been so different.

“I’d read these reports about them being common as dirt down here in the ‘70s. We just didn’t find any.” – Colin O’Donnell

It was the prospect of large-scale logging that first brought O’Donnell to South Westland in 1985.

The Forest Service was considering logging between 600,000 and a million hectares of forest. O’Donnell was tasked with predicting the effects of logging on forest birds.

“One of the birds that we expected to find in abundance was the mohua, or yellowhead. I’d read these reports about them being common as dirt down here in the ‘70s.

“We just didn’t find any. Eventually we found a population here, in the Landsborough Valley. It just really stood out as the only place left in the whole of South Westland, even then, with a population of mohua.” (Another bird found in abundance was the large native parrot, kākā.)

Without people really noticing, mohua, one of the South Island’s most common and conspicuous birds, had declined so dramatically that it had become locally extinct in some areas. “A lot of our forest birds have been like that,” O’Donnell says, adding: “We all know about kākāpō and takahē.

“But a lot of these other birds have just been slowly but surely going down the gurgler for a long time. So that was a bit of a shock.”

(Predators weren’t just killing vulnerable birds, the veteran DOC scientist explains. They were also eating their “ice cream” plants, like mistletoes, fuchsias and rata flowers. Yellowheads eat invertebrates in the main, especially caterpillars and spiders, but occasionally consume small fruit.)

Experts considered mohua as a threatened species for the first time. Then they had to work out what the problem was. O’Donnell and Peter Dilks researched yellowheads at Arthur’s Pass and another DOC veteran, Graeme Elliott, worked in Fiordland.

The birds seemed productive, with up to five fledglings from a single nest in Fiordland. Nothing seemed to be eating them.

But in the fourth year of the study there was a big predator plague – a mast year, when predator numbers explode after high amounts of tree seed drop to the ground.

“In conservation circles everyone knows about mast years now and what effect it has,” O’Donnell says. “But in ’85 to ’90, people had forgotten about that sort of phenomenon.”

Mohua numbers plummeted in the Landsborough. In 1991, after a big seed fall, counts dropped to 24. The following year counters heard just 14 individuals. Something had to be done.

Intensive trapping initially trialed in the Eglinton Valley, near Te Anau, and the Dart River Valley, at the head of Lake Wakatipu, was rolled out at the Landsborough.

The fightback had begun.

Colin O’Donnell conducting a five-minute bird count in the Landsborough. Photo: David Williams

In the Landsborough beech forest last November, O’Donnell’s head swings from side-to-side, his pen hovering over a sheet of paper.

“There’s two that way,” he says, after listening intently. “A bellbird just over here. I’ll just tally those,” he says, marking pre-printed boxes. “A mohua’s still calling just down the way there. There’s chaffinches rattling away down that way, too.”

A five-minute bird count is just that – counting birds in a predetermined spot for five minutes. It’s a technique developed by the now defunct Department of Scientific and Industrial Research in the 1970s. The weather needs to be fine and still – or as O’Donnell puts it, “good listening conditions”.

There are fancier ways of doing it, O’Donnell admits, but they might be more labour intensive. “I believe in simplicity, really.” Rather than a census, the counts are an “index of relative abundance”, he says. Double-counting isn’t knowingly done but is feasible.

Counters have to know all the bird calls, obviously, and have good concentration. “But the other part of it is how you mentally map how many birds you might have. The trick is standardising how you do that.”

The forest isn’t exactly a deafening chorus that afternoon but there’s constant bird chatter. Distant whistling, punctuated by pauses, echoes across the valley. There’s a nearby warble from the lower branches. The occasional high-pitched call of alarm pierces the air.

O’Donnell looks down at his sheet: mohua, rifleman, tomtit, grey warbler, brown creeper, fantail, bellbirds, chaffinches, and tūī.

“One kākā just called right at the end – screeched in the background.”

Counting birds requires good concentration. Photo: David Williams

When the trapping programme started in the Landsborough counters were getting, on average, about six birds on a five-minute count. Now, 21 years later, the average is up to about 15 or 16.

This afternoon it’s 17 birds. “That’s good for a five-minute bird count anywhere in the country.”

Someone else might get a different tally from the same count, O’Donnell says, because people perceive things differently. Some people count lower, others count higher – or “slightly more optimistic”. Consistency is the key.

“One of the good things about this project [is] four people have done 90 percent of the counts over the 21 years.” (They’re O’Donnell, Paul van Klink, Glen Newton, and Ron van Mierli.)

On both sides of the Landsborough River, and along some ridgelines, 280 double-set traps are dotted every 200m – covering 56 kms. But they can’t do the job alone. There have been six 1080 poison drops between 1998 and 2016.

Rat plagues that aren’t “managed” by a pulse of aerial poison can be devastating. There was such an “irruption” in the Landsborough in 2010 – as the global financial crisis kicked in and the National Government of the day cut millions from DOC’s baseline budget. The numbers of mohua and other native species dropped markedly.

“That one caught us,” O’Donnell says. It took seven years for native bird numbers to recover.

(Severe rat plagues in 2000 and 2001 wiped out mohua on Mt Stokes, in the Marlborough Sounds, and devastated numbers in the Eglinton. In its May 2002 issue, NZ Geographic said seven of the nine largest remaining South Island populations suffered serious decline. O’Donnell saw one silver lining: “That’s when people really got serious about things, when they saw just how bad these increased rat plagues could be.” DOC boosted pest control and started transferring mohua to predator-free offshore islands.)

Taking in the results of last year’s Landsborough count, since 1998 seven native species – mohua, tūī, bellbird/korimako, brown creeper/pīpipi, rifleman/tītitipounamu, grey warbler/riroriro and kākāriki/yellow-crowned parakeet – have increased. Kākā, tomtit/ngirungiru, fantail/pīwakawaka and kererū/wood are stable, and tautou/silvereye and long-tailed cuckoo/koekoeā have declined.

Fluctuations aren’t always because of predators. There might be competition between birds for nectar. Birds can suffer because of icy-cold winters or too-dry summers. “There are lots of factors that affect the survival of wildlife and they fluctuate all the time,” O’Donnell says. The lesson there is don’t read too much into short-term data.

Mōhua flit well over short distances but are surprisingly poor fliers. Photo: David Williams

Aerial 1080 has become increasingly controversial in recent years, more from public unease over poison use than a weight of published scientific work. In 2011, former Parliamentary Commissioner for the Environment Jan Wright backed the chemical compound as effective and safe – so much so that, in her opinion, more should be used.

But protests have intensified, including threats and harassment against Department of Conservation staff. Even Prime Minister Jacinda Ardern said she’d received a death threat.

Last year, Auckland Council’s plan to drop the poison over the Hunua Ranges was temporarily halted because of a court challenge.

Despite assurances the poison – the chemical compound sodium fluoroacetate – is doing more good than harm, that’s only when it’s dropped in the right place. Last year, eight cows died after a 1080 drop on a Waikato farm.

Given the crossover between hunters and anti-1080 sentiment, it’s no surprise there’s strong opposition on the West Coast. In Haast, the nearest settlement to the Landsborough Valley, one local pub was, for a time, selling ‘Fuck 1080’ T-shirts.

Back in the Landsborough, the sun has gone down behind one mountain and, from the other direction, the moon has risen, sending shafts of light towards our shelter nestled in beech forest. A wooden spoon scrapes beef burrito mix from a large frypan and water’s boiled for a cuppa.

After 21 years of scientific field work and reading the literature, O’Donnell is unequivocal. In the Landsborough, 1080 use has resulted in wildlife increasing, he says. Some species have increased by 15-to-25 times since control started.

“It’s a beautiful glimpse of what New Zealand used to be like, where birds were really common things,” he says.

(A journal entry on January 17, 1770, by botanist Joseph Banks, who travelled to New Zealand with Captain James Cook on the Endeavour, said of the dawn chorus in Queen Charlotte Sound in the Marlborough Sounds: “This morn I was awakd by the singing of the birds ashore from whence we are distant not a quarter of a mile, the numbers of them were certainly very great who seemd to strain their throats with emulation perhaps; their voices were certainly the most melodious wild musick I have ever heard…”)

O’Donnell, who has a PhD in zoology from the University of Otago, says he hears arguments about silent forests all the time. “And I know that the silent forests are a result of rats and stoats and possums eating all the wildlife. I’ve watched it myself hundreds of times, with the videos we use for monitoring. Those rats and stoats and things are the problem.”

The poison drops are doing the job, he says, and he thinks it’s “a bit irresponsible” that DOC is forced to spend more and more taxpayer money on monitoring dead non-target animals when, “every time”, it shows that wildlife bounce back. (Last month, however, DOC announced two of 13 monitored kea died of suspected 1080 poisoning in a research trial by Zero Invasive Predator in the Perth River valley. West Coast boss Mark Davies said the elevated risk to kea was accepted because of the trial’s importance.)

O’Donnell says there’s a vast amount of literature on 1080 – plenty of which was referred to in Wright’s 2011 report. “I believe the paper that I read that said there’s more 1080 in the cup of tea you drink every day than you’ll ever get in a river after an operation.”

Your correspondent looks down at his cup of tea. “Milk?” O’Donnell asks, cheerfully.

Colin O’Donnell first visited the Landsborough Valley in 1985. Photo: David Williams

Mohua have thrived on predator-free offshore islands in places like Fiordland, around Stewart Island and the Marlborough Sounds – so much so that they’re being re-introduced to the mainland. The bird’s official conservation status is at-risk/recovering. There are thought to be fewer than 5000 individuals left, in strongholds such as the Landsborough, Catlins, some beech forests in Fiordland, and Canterbury’s Hawdon and Hurunui Valleys.

DOC clearly sees the Landsborough as a success. Mohua have recovered remarkably and, unlike in other parts of South Westland, kākā have remained stable. O’Donnell wonders when there’ll be so many birds in the Landsborough that making accurate counts will become challenging.

Isn’t it better, then, for DOC to spend its time – and our money – in places where mohua populations are struggling?

O’Donnell sticks to the science. He says more studies are needed to demonstrate to the public the benefits of what it’s doing. Long-term studies can teach us a lot about what’s possible, and how far we can go, to restore bird populations, he says.

Questions remain about the effect of climate change and whether that will lead to more frequent rat plagues and what breakthroughs can be expected from new pest control technology.

In 1990, as mohua numbers were set to plummet in the Landsborough, there were 50 hectares of stoat control across the whole South Island. This year, DOC’s website has confirmed pest control operations covering almost 1.2 million hectares. Ninety-three percent of that, including 46,750 hectares of the Landsborough, is in the South Island.

“I find that really exciting,” O’Donnell says. “Because when people say we can’t control these things, or the aspirations of predator-free New Zealand seem ridiculous, I just look back on my career and say, well, in 25 years we’ve gone from 50 hectares to more than a million hectares. Who knows what’ll happen in the future?”

DISCLAIMER: DoC paid for the writer’s helicopter ride to and from the Landsborough Valley. It also provided a sleeping bag and tent.

David Williams is Newsroom's environment editor, South Island correspondent and investigative writer.

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