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In October, the Department of Conservation (DOC) carried out an aerial possum control operation over 25,000ha of Waitutu Forest using cereal pellet baits laced with biodegradable 1080.
Field teams made several monitoring trips into the area after that operation and found that possum, rat and stoat numbers had plummeted.
In addition, female kaka that were tracked by radio prior to the operation were all thriving - with nests full of healthy chicks.
"This is great news for the forest and its birdlife," said DOC's Murihiku area ranger Colin Bishop said.
"Species that were heading for extinction within this forest now have a good chance of survival."
So many nesting female kaka had been killed by predators that they were thought to be heading for extinction when surveyed six years ago.
"They were in desperate trouble -- it looked like curtains for our perky forest parrot. We helped them along four years ago with trapping in a small area of the forest, but now this 1080 operation means we've covered a huge area and given more birds more of a chance," said Mr Bishop.
"Already this year we're seeing the best fledging success for decades - more chicks are surviving until they're old enough to look after themselves. "That means more females, more chicks down the line, and a real chance to get our kaka back from the brink."
While possums were the primary target of the poison operation, high kills of rodents and stoats also achieved another positive spin-off.
Possum numbers have been reduced by 99.5 percent within the treatment area and the number of rodents and stoats have been reduced to a level where they are not detectable with standard monitoring methods.
Southland Conservation Board chairman Robin McNeill said using aerial 1080 was the last real chance to save Waitutu.
"Believe me - possums, rats and stoats have made Fiordland a green desert. Good science must prevail to save a precious oasis for us all to enjoy," he said.
Local birdlife was monitored both before and after the operation including, tomtits, grey warblers, rifleman, mohua, kaka and ruru.
No birds were found to have been killed by the poison.
There was no detectable reduction in bird numbers as a result of the operation, and it was clear that bird numbers were now on the increase.
Water samples were also tested from 12 sites in the area and no traces of 1080 were detected.