Mohua numbers in South Island's Landsborough valley on way back to pre-European times

Mohua numbers in the Landsborough Valley in South Westland have increased 30 fold in 21 years.

A bird once on the brink of extinction is now the most-commonly-heard native bird in a remote South Island valley after decades of intensive predator control.

And those monitoring the phenomenal recovery of the mohua, or yellowhead, in Landsborough valley, whose numbers have risen 30 fold in 21 years, say it is how the valley would have sounded prior to European settlement.

Native bird numbers overall in the South Westland valley have doubled since monitoring began in 1998, according to the Department of Conservation.

Principal science adviser Dr Colin O'Donnell said the long-term study charts the response of 13 native bird species following sustained control of rats, stoats and possums.

"Our most recent bird count data from last spring shows seven native bird species are still increasing in numbers, four species remain stable, and two have 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.

"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/yellowhead, tuī, bellbird/korimako, brown creeper/pīpipi, rifleman/tītitipounamu, grey warbler/riroriro and kākāriki/yellow-crowned parakeet had all steadily increased in the monitoring period.

Mohua went from just 14 birds to 444 counted in the study area last November.

Kākā, tomtit/ngirungiru, fantail/pīwakawaka and kererū/wood pigeon had all remained stable and not declined as they would have been expected to without predator control.

Two species, tautou/silvereye and the migratory long-tailed cuckoo/koekoeā, had declined.

For silvereye this could be due to greater competition for nectar from the more aggressive tuī and bellbird, O'Donnell said.

Long-tailed cuckoo migrate to the Pacific islands each winter, and could be affected by conditions there. It returns to lay its eggs in mohua and brown creeper nests in late spring.

Predator control began in the Landsborough Valley in 1994, and has since involved valley-wide trapping, and six aerial-1080 operations timed to suppress increasing rodent levels. The latest, in 2014 and 2016, covered the entire valley.

Due to this year's heavy beech mast the Landsborough was seen as a priority for aerial-1080 predator control.