Three years after 31 endangered mohua, or yellowheads, were introduced to an island in the Marlborough Sounds, Department of Conservation rangers are delighted the birds are now starting to breed there.
A check last month found five pairs on Blumine Island, two with fledgling chicks and another pair incubating eggs, indicating they are gaining a foothold on the sanctuary.
The previous year only nine individuals were spotted but no chicks.
"It was a delight to see the fledglings at close range for a good length of time, and they were being well fed by their parents," said ranger Dan Palmer.
"We now believe they had been breeding as the male of the pair incubating eggs was unbanded. All the mohua moved to Blumine Island had bands put on their legs to identify them so this unbanded bird must have hatched on the island."
Mohua are classified as nationally vulnerable, and today are only found on the mainland in beech forest south of Canterbury.
Thirty-one were moved to Blumine from Otago's Blue Mountains in October, 2013.
It appeared mohua had taken a couple of years to adjust to life on Blumine, and their numbers declined at first, Mr Palmer said.
Numbers were still uncomfortably low and they could be wiped out by a predator incursion or even a storm.
"But knowing the mohua can breed to build up their numbers gives us encouragement they can have a secure future."
There is also a breeding mohua population on one of the Chetwode Islands, in the northern Marlborough Sounds.