But keeping our birds in large numbers can be achieved for a realistic price, says the Landcare Research scientist who has presented the startling figures to top ecologists.
A comparison of monitoring data, collected over periods in the 1970s and early 2000s, showed significant falls in well-known species such as brown kiwi, kokako, kaka, rifleman, tomtit and blue duck, or whio.
View original story from Jamie Morton, NZ Herald here
While numbers of tui, kereru and bellbird had risen in some deforested areas in the north, they had become scarcer in forests across the country.
Data on species close to the brink, such as takahe, kakapo, stitchbird and NZ fairy tern, wasn't included in the analysis because they were too rare.
Away from forests, there had also been drops in many modelled species of waders, terns and gulls; the banded dotterel population was shrinking to its last breeding stronghold in the Mackenzie Basin, the pied stilt was vanishing from the Waikato and King Country, and the black-billed gull's threat classification was now "nationally critical".
In the case of these birds, changes in land use were now foreclosing options for future recovery.
The data came from two sprawling atlases of national bird distribution compiled by the Ornithological Society - described by Dr Walker as New Zealand's "premier" bird biodiversity dataset.
Other large-scale surveys tended to be specific to locations or species.
Dr Walker said the data showed two central patterns driving decline.
Until the 1970s, losses had come with deforestation, while more recently the largest falls were being seen in those forests left - especially over vast swathes of bush on both islands - which were being emptied of endemic birds by pests like stoats, rats and possums. "Those forest refuges are really no longer refuges."
Dr Walker said the typical approach to conservation had been to try to rescue certain species once they dwindled to alarming numbers.
"So there is this whole stack of other species that are still lined up and are going to inexorably get there."
However, she believed avoiding this scenario was within reach, in both cost and capability.
She calculated the cost of triennial 1080 poison drops in the South Island, which would cover general protection for the full range of bellbirds and most other resident bird species, amounted to $14.7 million a year. "We can certainly slow decline, and from the numbers I've generated, it doesn't look like either cost or logistics are in our way at the moment."
It comes as the Department of Conservation ramps up its war against pests, with $21 million spent on 27 1080 poison drops last spring and summer over swathes of beech forest, mainly in the South Island.
It wiped out 95 per cent of rats and 85 per cent of stoats across most targeted forests, but now the next pest-fuelling beech flowering (mast) season is threatening.
Forest and Bird has urged the Government to allocate more funding for another major 1080 offensive.
Conservation Minister Maggie Barry said no firm decisions would be made until it was clear another mast season would happen.