Some of New Zealand's most threatened birds are being backed into colder corners of our forests, through a "thermal squeeze" from pest predators and climate change.

Conservation advocates say findings of a just-published study show another reason to ramp up the war on pests, in what is expected to be a horror year for our native biodiversity.

The paper, drawing on bird distribution data captured by the Ornithological Society of New Zealand between 1969 and 1979, and again between 1999 and 2004, explored how a warming climate will heap yet more pressure on threatened species hanging on in our remaining forests.

Researchers had suspected climate change would have been worsening the toll wreaked by predators, regardless of whether it influenced pest-fuelling mast seeding events, like the record one now under way in beech forests across the country.

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"We know that two key predators, ship rats and possums, do better in warmer sites," said study lead author, Dr Susan Walker, of Manaaki Whenua – Landcare Research.

"New Zealand's cool forests are mainly beech forests, which support plagues of predators in years following mast seeding, but not otherwise. In contrast, warmer forests at lower elevations support high numbers of predators every year."

Some of New Zealand's most threatened birds are being backed into colder corners of our forests, through a
Some of New Zealand's most threatened birds are being backed into colder corners of our forests, through a "thermal squeeze" from pest predators and climate change. Photo / Barry Pethybridge.

Walker and her colleagues searched for evidence that our most vulnerable forest species were less likely to occupy warmer forests than cooler ones.

They also looked at how species had vanished faster from warmer forests between the two periods, notably because they'd tended to be large, nested in holes, and dispersed poorly.

The data revealed a clear pattern of so-called "thermal squeeze", with vulnerable birds becoming confined into smaller, colder areas high up against treelines, and into cooler forest pockets.

Those birds most at risk included some of our rarest - kiwi, whio, weka, and kokako – while among the next most threatened were rifleman, mohua, kaka, kea, and kakariki.

"It's likely that bird populations have hung in longest in colder places because predators are fewer there most of the time," Walker said.

"Climate warming is likely to intensify the difficulty of keeping remaining forest bird populations in the back country, by making even more of our forests a great place for predators - especially ship rats."

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Mohua vanished from Marlborough's Mt Stokes during a
Mohua vanished from Marlborough's Mt Stokes during a "mega mast" seeding event 20 years ago. More populations are again at risk this year. Photo / DoC, James Rearden

Walker said the results showed why scaled-up responses to beech masts - like the Department of Conservation's 1080-focused, $38m operation this year - were so vital, as the resulting pest plagues could completely wipe out any bird populations left in cool forests.

"But our results also show the enormous loss of forest bird populations that has occurred across forest more generally, and the scale of the challenge New Zealand faces in halting endemic bird declines - much less restoring forest birds into warmer, more productive forests."

Manaaki Whenua – Landcare Research wildlife ecologist John Innes said the volunteer birdwatchers who'd gathered the data decades ago would be pleased to know it was now being used to conserve them.

The study also added a "sharp new conservation twist" to the threats from climate change, he said.

"Predators like ship rats and stoats are already a problem, but these results suggest that impacts may be greater in the future."

Forest & Bird spokesman Geoff Keey agreed warming temperatures were bad news for our birds.

"This is another reason why we need to take action to reduce global warming and also continue to ramp up pest control," he said.

"It's vital that the Zero Carbon Bill has strong, binding targets to cut emissions and recognises the role of nature in our response to climate change.

"As well as cutting emissions, we need to help make nature more resilient in face of unavoidable climate change."