With their efforts combined, before 1080 poison drops, rats and stoats used to clear 98 per cent of Kea nests in Kahurangi National Park.

A boom in the rat population threatening nesting native birds in Kahurangi National Park has been cut short after a pest-control program.

The Department of Conservation (DOC) performed scheduled pest-control over the weekend ahead of the summer breeding season. Originally the planned aerial 1080 drops were to cover 1710 square kilometres, but the range was reduced to just 397 square kilometres.

DOC Golden Bay operations manager Andrew Lamson said the area was calculated from beech seed measurements over last summer, which suggested the numbers of seeds would be medium to high. A high number of seeds leads to beech mast, and usually causes a boom in rodent populations.

SUPPLIED Stoats eat rats, so a booming rat population leads to a big stoat problem. Stoats which eat 1080-poisoned rats will also be poisoned.

However, despite the relatively high seed production, rats were less prolific than expected.

"Rat monitoring at various sites in the planned pest control area in May and August showed that at some sites rat numbers had not risen to predicted levels or were declining ... this led to the decision to reduce the area."

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SUPPLIED A single pair of rats can produce up to 2000 offspring in a single year, and can produce offspring after just three months.

Though the 1080 drop was targeting specifically rats, stoat numbers would also be affected. Stoats eat rats as well as native animals, and should suffer from secondary poisoning.

Kahurangi National Park is home to several at-risk species, including great spotted kiwi, kea, and giant snails.

"The benefits to native species of increased protection from predators extends over two breeding seasons for most species," Lamson said.

BRADEN FASTIER In the three years since aerial poison drops began, two kea are known to have died as a result.

Though DOC is monitoring great spotted kiwi, whio, and giant snails in the Kahurangi National Park, Lamson said it was "too early for firm results".

However, results are firm for kea, which have been monitored in the area for many years.

Lamson said from 2009 to 2014, when there was no aerial pest control in the Kahurangi park, only 2 per cent of monitored kea nests were successful. Aerial pest control measures were brought in before the nesting season in 2014, which lead to a higher success rate for nesting kea.

"In the 2015 and 2016 breeding seasons in Kahurangi National Park, on average 50 per cent of monitored kea nests successfully produced young kea."

In the three years since aerial 1080 pest control began in the Kahurangi area, only two kea are known to have died from 1080 poisoning.

"The greatest threat to kea survival is stoats preying on nests, juveniles and adults," Lamson said.