"Occasionally they do fly over from the Poor Knights or the Hen and Chickens, but as far as we know they haven't nested on mainland Northland since the early 1980s."
In the 1880s kakariki were so common their feathers were used to stuff mattresses, he said.
Unlike other birds so far re-introduced to the Ipipiri islands, kakariki could easily fly to the mainland, so if they bred well they could once again become a common sight in back yards around the Bay, travelling in flocks and making a "cheerful chatter."
On the mainland, however, they would be vulnerable to predators.
"So it's up to all of us if we want kakariki back in our everyday lives," Mr Baigent-Mercer said.
The mainland Bay of Islands had a chain of pest control projects that had taken many years to establish, from Purerua Peninsula in the north all the way around to Rawhiti, but there were still gaps, and everyone could contribute by trapping rats and possums in their own gardens, or by joining a pest control project targeting the four worst offenders - possums, rats, stoats and feral cats.
That would not only benefit kakariki but would also boost numbers of tui, grey warblers, kukupa and ruru (moreporks).
"We have only the crumbs of what there used to be ... With pest control we can increase the number of birds that are around and bring back the ones we've lost," Mr Baigent-Mercer added.
In the right conditions kakariki were prolific breeders, making their nests in holes in trees or banks.
They could lay several clutches of eggs between early spring and late summer, with three to nine eggs per nest, so numbers could increase quickly if predators were absent, as shown by saddlebacks on Urupukapuka and Moturua.
Project Island Song volunteers only re-introduced saddlebacks in 2015, and already they were "absolutely everywhere" on the islands.
