The black robin/kakaruia/karure population on the Chatham's Rangatira/Hokorereoro/South East Island is reaching capacity, highlighting the need for more predator-free habitat.
There are only two populations of black robins in the world – on two reserve islands in the Chatham archipelago, 800 km offshore from mainland New Zealand.
This iconic species' recovery from the brink of extinction is an internationally renowned success story. There were only five black robins left in 1980, with just one breeding pair – Old Blue and Old Yellow – remaining. Following intensive conservation efforts, the population has increased to 445 birds, with approximately 45 birds on Mangere Island and another 400 birds on Rangatira.
Dave Houston, Department of Conservation Technical Advisor, says the upward trend for black robin on Rangatira is heartening, but the species risks becoming a victim of its own success.
"There's a limit to how much food and habitat is available for these birds on Rangatira," says Dave. "Native bush on the island is slowly regenerating, but the black robin population is growing faster than the bush."
A second population of black robin, on Mangere/Maung' Rē Island, is not faring as well as its neighbours.
"A decade ago, there was a population of around 50 birds, but this has dropped to 45 as female survival over winter has been low, and there are genetic problems stemming from the extremely small gene pool. Habitat and food availability on Mangere are also problems because, even though planting has been ongoing since the 1970s, there are still only very small, forested areas suitable for black robins."
DOC's focus over the past five years has been on site restoration and monitoring. Teams are sent to monitor the birds on each island during pre-breeding and post-breeding seasons, capturing and banding juveniles to determine survival rates after winter.
DOC is exploring options suitable for black robin populations on both Rangiura/Pitt Island and main Chatham Island, but the presence of predators such as mice means there is no easy solution.
As it stands, says Dave, a rat incursion or natural disaster on Rangatira or Mangere could devastate those populations.
"Our eggs are quite literally all in two baskets," says Dave. "We really need to establish some more populations to safeguard the species, but we need sustainable, predator-free habitat first.
"Rangatira and Mangere are free of predators, which is why they work so well. One really bad week could wipe out hundreds of birds found nowhere else in the world, and erase decades of hard work. After how far we've come, we just can't let that happen."
Background information
South East Island/Rangatira/Hokorereoro and Mangere Island/Maung' Rē are both predator free islands, home to a range of rare plant and animal species.
All of today's black robins are descended from the last breeding female, Old Blue. She was one of the longest-lived robins known, reaching 14 years old.
The fostering programme used to save the black robin was such a fantastic success that it has been used as a case study on how to save endangered birds around the world.
