Forest & Bird Otago projects manager Francesca Cunninghame said tītī (also known as muttonbird or sooty shearwater) at Sandfly Bay’s Sandymount Reserve were protected by 200ha of traps, including 30ha of intensified trapping immediately around the rare mainland colony.
On March 15, during routine monitoring, Ms Cunninghame discovered two recently killed chicks lying on the ground with telltale wounds to their heads.
After she found the first dead chicks, Ms Cunninghame checked nearby burrows the monitoring team had been observing and every one of those chicks had been killed as well.
"It’s always a horrible, stark reminder of what we are up against, and however hard we try we can still fail," she said.
"It’s a reality of conservation work — it’s not easy — but also giving up and doing nothing is not an option."
The day after the first dead chicks were found, additional traps were set and a more thorough survey was completed across the colony.
The team was able to confirm ferrets had killed 21 chicks throughout the colony, but there were also 17 "healthy" survivors.
The following Sunday morning, two days after the initial discovery, a ferret was caught in the new intensified trap network.
On Monday, a second ferret was caught.
A third ferret was caught in the traps on Wednesday.
Previously, Forest & Bird had only ever documented one ferret in the area at a time.
The good news, Ms Cunninghame said, was that no further chicks had been discovered dead at the site and there had been no further signs of the predators.
"But you can’t relax," she said. "These birds are not going to fly away until early to mid May.
"So, it’s still several weeks that they are vulnerable.
"And we do know that ferrets can cover huge distances."
They also kill more than they can eat.
Five years ago, in 2019, a single ferret killed every single chick that the conservation organisation was aware of at the colony.
To Forest & Bird’s knowledge, not a single chick fledged from the site that year.
However, that incident had led to the increased predator control now in place and "some really successful" breeding seasons when more than 100 chicks fledged from the more than 200 active burrows at the colony.
The Sandfly Bay colony was a very challenging one to monitor.
The monitoring team used a "burrow scope", which was a camera fixed to the end of a 2m length of tubing.
Looking for chicks involved "a lot of rolling around on the ground sticking your arm as far as you can" down a burrow, trying to stick the tube as far as possible into the deep burrows.
The burrows were deep in a sandy substrate, and many were too deep for the monitoring team to say with any certainty whether they were occupied or not.
For that reason, monitoring was limited to a subset of burrows which the burrow scope could reach into.
It was not until the end of the breeding season when the chicks came to the entrance of the burrows at night before fledging when the team could get a feel for the number of birds fledging from the colony in a year.
The total of 21 dead chicks and 17 surviving represented the total number of burrows the team could monitor.
More chicks could have been killed during the incursion and there still could be many chicks alive.
"We really won’t know until the end of the season."
The native birds numbered in the millions, but were declining, and largely limited to breeding on islands.
Only a few remnant colonies endured on the mainland.
The burrowing seabird colony ecosystems in coastal Otago were considered endangered, Ms Cunninghame said.
