Parininihi Forest kōkako project manager Konrad O'Carroll, of Ngati Tama, searches for the elusive bird in dense forest.

Bush-bashing in search of critically endangered birds is not a pursuit for the light-hearted or unfit.

Last month, Ngāti Tama's Tiake Te Mauri O Parininihi Trust project manager Konrad O'Caroll and trust chairman Davis McClutchie tramped and scrambled up and over steep ridge tracks, and pushed through dense native vegetation searching for the kōkako.

They, along with a group of other people were surveying the forest for the 20 birds which were released into the Parininihi Forest in North Taranaki in May and July this year.

GRANT MATTHEW/STUFF Ecologist Dave Bryden said the Parininihi Forest kōkako project was "exciting".

The survey found the majority of the kōkako released have paired up with partners in anticipation for the breeding season, O'Carroll said.

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"They have established their territory and are getting ready to nest."

GRANT MATTHEW/STUFF Volunteer kōkako searchers, fromleft, Charlotte Crummack, Davis McClutchie, Chris Hannett, Dave Bryden, and Amanda Rogers.

The birds are most vocal early in the morning when the haunting call, followed by a series of "tuk, tuk" calls, can be heard the loudest.

The birds can be easily identified through binoculars by the colour combination leg bands which were applied before they were released into the 1300ha Parininihi Forest on the western boundary of the Mt Messenger Range, 60 kilometres north of New Plymouth.

The Tiake Te Mauri O Parininihi Trust, with the help of Tiritiri Matangi Island staff, released the 20 birds in two 10 bird lots in May and July - the first North Island kōkako to inhabit the area since 1999 when the last remaining kōkako, Tamanui, was removed.

GRANT MATTHEW/STUFF Project manager Konrad O'Carroll, foreground, and Tiaki Te Mauri o Parininihi Trust chairman Davis McClutchie survey for kōkako in Parininihi Forest.

Ten of the birds released recently are progeny of Tamanui after a breeding programme was undertaken at Mt Bruce Wildlife Centre.

The remaining 10 birds came from a separate kōkako breeding programme in Pureora Forest in the Waikato.

A further 20 birds from Pureora will be released in 2018 to widen the gene pool of the remnant population.

O'Carroll said two bird surveys in September had established kōkako population numbers had remained steady since the releases.

Parininihi Forest is one of 24 kōkako projects operating in the country.

The projects had boosted population numbers from a critically endangered 400 birds to the current 1500-1600 birds.

The two Parininihi Forest surveys, undertaken with the help of four wildlife contractors, were timed to occur before the up coming breeding season, O'Carroll said.

The searching involved bush-bashing through dense undergrowth into steep gullies within a two to three kilometre radius of the original release area.

Once in a position the searchers play a tape of a kōkako call and listen for any response.

While the birds can be heard calling back, some areas were in steep terrain which made it difficult to locate the bird, he said.

Alternatively searchers follow bait lines along steep ridges and play back the recorded call every 200 metres.

O'Carroll, who has been visiting the area for the last decade, said the survey results of the kōkako surviving in the forest among a variety of predator including rats, stoats, and possums, vindicated the extensive predator poisoning programme undertaken in the Parininihi Forest by the Department of Conservation, and Taranaki Regional Council.

"At the beginning I was against having 1080 dropped to control pests but when you see the change in the vegetation growth and the increase in other native bird species, such as kiwi, tui and kereru, it shows the drops are working," he said.

There is plenty of evidence of kiwi probe holes in the ground where kiwi are feeding in an area where a recent 1080 helicopter drop occurred, he said.

"1080 is the most effective method we have to bring down predator numbers."

A definitive "musky" smell of a kiwi is evident in one area along a ridgeline.

"The smell, and the large probe holes show there is a healthy kiwi established there," he said.

Other methods such as bait stations, trapping stoats, rats and cats have also proved effective, he said.

Rimu and miro seedlings had flourished in the forest undergrowth since the predator numbers had been cut back, and provided a good food source for birdlife, he said.

"It's a very unique area for both fauna and flora," he said.

"There are plants and animals which are at their northern most, and southern most limit of their territories."

O'Carroll said it was pleasing to see that some of the released kōkako had paired up and established territories.

There were nine females and 11 males released from the original 20 birds, he said.

Of the 13 kōkako visibly found by searchers, six had paired up.

A lone bird had been seen which had not yet paired.

The remaining seven birds had yet to establish their territories, or had been slower to pair up, he said.

Once the breeding season was completed surveys will monitor nests for eggs, and later fledgings.

"The aim is to monitor the nests to gauge the success rate of the breeding period, and then we can leg band the chicks," O'Carroll said.

Nesting areas are "ring fenced" off with bait and traps to protect from predators attacking the chicks.

One of the searchers, ecologist Dave Bryden, has been studying kōkako in the wild for 10 years.

Bryden said it had been difficult to establish population numbers because the kōkako had been a "little shy"

"There were 8 pairs trans-located among the released birds and most of those have found new partners.

Kōkako stay together for around 10-15 years, and live to be 25 years old, he said.

He expected the summer nests will provide two to three eggs each if the predator control was sufficient.

Falcons and harrier hawks also prey on the kōkako nests.

"The successful breeding rate is eight per cent where there is no predator control, and more than 80 per cent where this is control.

"It's an exciting project and it's fantastic kōkako are back in Parininihi after almost 20 years but only predator control will enable them to survive."