Controversial poison 1080 might have killed two native kea in a trial on the West Coast.
The Department of Conservation (DoC) says it was "fully aware" of the risk to native birds in the area, but the losses "will be offset by significant increases to the kea population from improved nesting success and survival of young birds without predators".
"There is a healthy kea population in the vicinity of the Perth River valley, which is due in part to a long history of predator control in this area, including multiple 1080 predator control operations," said DoC West Coast operations director Mark Davies.
"Losing two out of 13 monitored kea is in the order of what was expected as an elevated risk to kea from the increased number of prefeed applications and the more intensive use of 1080 in the ZIP trial."
The ZIP trial aims to remove possums and rats, and significantly reduce the presence of stoats, across a 12,000ha area in southern Westland.
It differs from DoC's normal process by distributing two rounds of non-toxic bait, then a double load of toxins, followed by a normal round. The non-toxic rounds make kea feel sick, with the aim of training them to avoid the toxic dump.
A survey in 2018 found 13 kea, which were tagged. The two dead birds have been sent to Massey University to determine their actual cause of death, which is suspected to be 1080.
One of the birds had high levels of lead in its system, which could have made it more susceptible to 1080 poisoning, ZIP researchers said.
"We accepted the increased risk because this is an important research trial that could lead to a breakthrough in New Zealand becoming predator free at a landscape scale," said Davies.
"The knowledge gained by this work will ultimately lead to a better and more secure future for kea and other threatened native species."
1080 has a controversial history in New Zealand, but authorities say it's the best tool available for keeping predator numbers in check.