DoC scientist says result helped breeding birds but is disappointed some pests survived.

A major 1080 poison offensive succeeded in wiping out 95 per cent of rats and 85 per cent of stoats in targeted forests around the country, but the result was still below what a Department of Conservation scientist was hoping for.

As part of a $21 million project dubbed The Battle for Our Birds, DoC staff launched 27 aerial pest control operations between last August and February over hundreds of thousands of hectares of beech forest, mainly in the South Island.

It was an urgent response to a tide of rats, stoats and possums - fed by a one-in-15-year beech seeding event last spring - that was poised to cause an ecological disaster by ravaging native bird populations.

DoC scientist Dr Graeme Elliott, who will this week be presenting detailed results to the New Zealand Ecological Society's 2015 conference in Christchurch, said the amount of seed generated in the beech "masting" event was equivalent to about 50 million seeds - or 250kg - per hectare.

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This was a "huge pulse of energy" going into our beech forest, which covered about 4 million hectares, roughly two-thirds of New Zealand's total forest.

The 1080 poison drops across targeted forests obliterated nearly all local rat populations and 85 per cent of stoats, which are harder to kill with 1080 as they do not tend to directly eat bait, though they do eat poisoned rats and mice.

Dr Elliott said though some birds were also thought to have perished in the drops, it was a small number compared to those that otherwise would have been killed by pests.

In the Marlborough Sounds, all of a group of 24 South Island robins monitored during the drop survived.

"Before the drop, most robins didn't even manage to keep their eggs going until they hatched, because they were killed by predators, but after the drop, half of the nests got right through to fledging."

Similar successes were observed in populations of riflemen - all 30 monitored birds survived the drop, and nesting rates shot up nearly to 100 per cent afterwards - and in mohua (yellowhead), for which the post-1080 nesting rate climbed to 89 per cent.

In Kahurangi National Park, rock wren nesting rates were 80 per cent in drop areas compared to just over 20 per cent in untreated areas.

However, 22 of the monitored 49 rock wren disappeared when a bad storm hit, and Dr Elliott expected at least some of the birds would have been killed by 1080.

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"Overall, it was fantastic and we achieved what we wanted to achieve - we stopped a real biodiversity disaster happening at those places we treated," he told the Herald.

But he remained disappointed that some small numbers of rats and other pests were able to survive the drops, which hadn't been the case in previous operations.

Dr Elliott believed the drops could have been even more devastating had they begun earlier.