The once rare mohua is now the most common native bird in south Westland's Landsborough Valley.
Only 14 of the birds, also known as the yellowhead, were counted when monitoring there began in 1998. But after predator control in the valley, 444 birds were noted during the Department of Conservation's last count in November.
Native bird numbers overall have doubled in the valley in the past 21 years.
Department of Conservation principal science advisor Colin O'Donnell said the most recent data from the long-term study of native birds showed seven species were increasing in numbers, four remained stable and two had declined.
"For the first time in 21 years, mohua have become the most common bird counted, which is what they would have been in this valley prior to European settlement," Dr O'Donnell said.
"The results are exciting because year by year we're seeing a re-balance of the valley's birdlife and don't know when population limits will be reached."
Numbers of mohua or yellowhead, tūī, bellbird or korimako, brown creeper or pīpipi, rifleman or tītitipounamu, grey warbler or riroriro and kākāriki or yellow-crowned parakeet had steadily increased over the past 21 years.
Kākā, tomtits/ngirungiru, fantails/pīwakawaka, and kererū/wood pigeons had stable populations that had not declined as would have been expected without predator control.
Two species, the tauhou/silvereye and the migratory long-tailed cuckoo/koekoeā, had declined.
For silvereye it could be due to greater competition for nectar from the more aggressive tūī and bellbirds, Dr O'Donnell said.
Long-tailed cuckoo migrate to the Pacific Islands every winter and might be affected by conditions there. They return to lay eggs in mohua and brown creeper nests in late spring.
Predator control in the Landsborough Valley - a large beech-clad valley in South Westland which runs parallel with the Southern Alps for 50 km before joining the Haast River - began in 1994 after the impact of predators on birdlife was observed.
Since then the department carried out valley-wide trapping and six aerial 1080 operations timed to suppress rodent levels, with the most recent in 2014 and 2016 covering the entire valley.
Due to this year's heavy beech mast the valley is a priority for aerial 1080 predator control.