Before the first humans and later waves of pests arrived on our shores, New Zealand's forests could have been described as a three-dimensional jungle.

Think of thick undergrowth teeming with reptiles, kiwi and weta.

Or a night-and-day cacophony of the rustlings, calls, booms, whistles and hoots of nearly 200 native bird species.

Even in 1769, those aboard Cook's visiting HMS Endeavour were greeted by a chorus said to be almost deafening.

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New Zealand would never be that way again.

Roughly a third of our native bird species have vanished since that day, as have many other endemic plant and invertebrates.

What remains is in a sorry state: 1000 animal, plant and fungi species are now considered threatened.

The rats, mice, possums and stoats that infest our wilderness are responsible for the slaughter of some 26 million native birds every year.

To protect what's left, scientists have begun a project to create, in unprecedented detail, a virtual picture of our forests to boost efforts to slow the slaughter.

The study will build on 30 years of ecological work on the dynamics of beech forests, which are known to fuel pest booms when their floors become littered with seed during increasingly devastating "masting" events.

Manaaki Whenua – Landcare Research scientist Dr Susan Walker said these forests were our coldest and, ecologically, also our simplest.

They were like "fridges" that still stored endemic species such as rifleman, mohua, kaka, kea, and whio.

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"New Zealand has learned to prevent catastrophic declines of these bird in cold beech forests by coinciding predator control with rodent and stoat plagues following beech masts."

Yet those vast forests that are comparatively warmer, and which once supported our most diverse bird communities, have fallen silent.

They have been a headache for conservation efforts as they hold more predators, more of the time, and scientists have a poorer understanding of the complex food resources that sustain them.

Like elsewhere in the country, our response to these pests has been to beat them back using 1080 poison drops, which have so far yielded short-term gains.

A major drop undertaken in Northland's Russell Forest last September knocked the resident rat population down to almost nothing, as did another operation at Cape Brett in the Bay of Islands.

But before conservation efforts can properly intervene and sustain our species, scientists have to look at the deeper picture.

In their new study, Walker and colleagues will build on large rodent tracking and management data-sets gathered from around the country over two decades.

The ship rat is a key predator that has proven immensely difficult to control in the long-term over large warm forest areas.

"These data provide a powerful key to explaining variation in food-resource drivers, predators and birds because ship rat dynamics are tightly coupled with productivity."

Ship rats not only prey on birds, but also suppress mice, while supporting populations of apex predators like stoats and cats.

"Ultimately we want to be able to forecast predator threats and native forest bird responses to a whole range of different environments, fluctuating food resources, and forest management regimes," she said.

"We aim to discover the crucial drivers of forest predator and bird productivity and whether we can develop scalable spatial predictors of them, as well as to identify key processes involved in predator and bird population responses."

The ship rat is one predator that has proven immensely difficult to control over large warm forest areas. Photo / D.Mudge
The ship rat is one predator that has proven immensely difficult to control over large warm forest areas. Photo / D.Mudge

They also seek to understand how climate change will affect our native birds.

"Climate change is already upon us: reports from all over New Zealand tell us that forest plants are flowering earlier, more heavily, and at unusual times," she said.

"This will certainly complicate our linking of drivers of past rodent dynamics to present phenology."

Out in the field, new data will be collected from long-term rodent tracking sites around the country.

"Targeted field experiments will help us to understand just how climate limits ship rats, and how refuges and immigration affect population growth," she said.

"Detailed field measurements of selected forest bird populations, including their dispersal in big forests following reintroduction, will allow us to understand and model their variable recovery potential.

"As anyone in this area knows, there is a lot of tough but rewarding field work ahead, as well as many technical challenges.

"So we view the programme as important step in a long term national effort for New Zealand - and we will just make as much progress as possible.

"The aspirational goal of our partners is to develop more cost-effective methods to reintroduce and sustain forest birds in warm forests at large scales."

Being able to better forecast predator threats and bird responses will help innovators target the right interventions to the right places and times to boost recovery of forest birds, as well as to reduce predators.

The study is supported with a $9 million grant from the Ministry of Business, Innovation and Employment's Endeavour Fund.