Aerial 1080 drops have proven key to the survival of kiwi chicks in a North Island forest, shows the first long-running study of its kind.
Department of Conservation (DoC) researchers tracked hundreds of North Island brown kiwi and their offspring through four large-scale joint OSPRI/DOC 1080 operations in Tongariro Forest over 22 years.
They also monitored 142 radio-tagged kiwi through four aerial 1080 operations, finding that none were poisoned.
Results showed how just over 50 per cent of kiwi chicks in the 20,000ha forest survived to six months old in the first breeding season after aerial 1080 treatment - and 29 per cent the year after.
In the following three years, before the next five-yearly 1080 operation, kiwi chick survival halved to 15 per cent, well below the 22 per cent survival required to maintain this kiwi population.
DoC principal science advisor Dr Hugh Robertson said the results showed how aerial 1080 drops "unequivocally" benefited kiwi by knocking down numbers of possums, rats and stoats.
"Stoat attacks are the leading cause of death for kiwi chicks and without pest control as few as five per cent of chicks survive to adulthood," he said.
"Our research shows that aerial 1080 pest control, significantly improves the survival of kiwi chicks for two years before dropping off when rat and stoat populations begin to recover to pre-control levels."
"The 1080 operations knock down all resident stoats and likely all ferrets too and allow kiwi to survive to levels that can build their population."
The research supported DoC shifting in 2014 to a three-year cycle of aerial 1080 predator control in the forest to help the kiwi population grow.
"Population modelling shows that to get the kiwi population growing by at least two per cent, which is the target in our new Kiwi Recovery Plan, we needed to increase pest control operations to once every three years."
The study began in 1992 and monitored radio-tagged adult male brown kiwi as well as 207 kiwi chicks hatched in the forest between 1996 and 2014.
The kiwi chicks were monitored until six months old when they reach a size where they can fight off stoat attacks.
Researchers also looked at the effects on nesting success of fantail over 11 years.
The results followed a similar pattern to kiwi with fantail nest survival highest in the first two years after a 1080 operation, when rat populations were low and dropping significantly after that.
Breeding success of fantails was significantly better than in untreated areas in nearby forests.
The findings come as DoC is planning its biggest-ever predator control programme, in response to the most significant beech masting event in 45 years.
This year's $38 million operation will target rats, stoats and possums over about one million hectares or 12 per cent of conservation land.
Priority sites include Kahurangi, Abel Tasman, Arthur's Pass, Westland, Mt Aspiring and Fiordland national parks, the Catlins and Whirinaki.
More than 66,000ha will be covered with trapping – and the rest with aerial 1080 poison drops.