In 1981, only 5 black robins remained. Check how scientists used other species to save them from extinction

Only five Chatham Island black robins remained before the 1981-82 breeding season, according to the Predator Free NZ Trust. The species was considered the rarest bird species on the planet at the time. A small team of New Zealand wildlife officers caring for the birds then attempted an unprecedented breeding strategy to prevent their extinction.

The surviving birds consisted of two pairs, Old Blue and Old Yellow, and Green and White, along with one chick that had survived from the previous year.

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Black robins pushed to the brink

The Chatham Island black robin, scientifically known as Petroica traversi, was once found across several parts of the Chatham Islands, a remote archipelago about 800 kilometres east of New Zealand's South Island.

The species had nearly disappeared from much of the islands after rats and cats were introduced by Europeans. The remaining birds survived mainly on Little Mangere Island, a small, steep and windy outcrop.

According to wildlife biologist Don V. Merton's paper in the New Zealand Journal of Ecology, about 18 black robins remained on Little Mangere Island by 1973. Continued forest degradation reduced the population to seven birds before the last robins were moved to nearby Mangere Island in 1976. More than 120,000 native trees had already been planted to restore their habitat, the paper notes.

The relocation did not immediately reverse the population decline. Diseases, adverse weather and bad luck continued to reduce their numbers, leaving only five black robins towards the end of the 1978-79 breeding season.

Old Blue and the last-ditch breeding strategy

One of the surviving females was Old Blue, a 10-year-old robin in 1980. She had already lived considerably longer than the species' average lifespan of four to five years, according to the Predator Free NZ Trust. Her mate was Old Yellow, while Green and White formed the other pair.

Merton, a conservationist and leader of the Wildlife Service team working with the birds, realised that simply protecting the remaining robins and allowing them to breed would not be enough. Black robins normally produce only one clutch of eggs each year, making more frequent nesting essential for the population's recovery.

Eggs transferred to foster birds

Merton adopted a technique known as cross-fostering. Before the 1981-82 breeding season, the team began removing the first clutch of eggs laid by a pair and transferring them to another bird species. The aim was for the foster birds to incubate the eggs and raise the chicks, while the black robins were encouraged to lay again.

The Chatham Island warbler was initially used as a foster parent. The warbler could incubate the eggs and care for the chicks during their first few days, but could not raise them through the fledging stage, the Predator Free NZ Trust explains.

The Chatham Island tomtit was also used, with birds from neighbouring South East Island taking on the foster-parenting role and warblers serving as backups. Removing eggs from one of the world's rarest birds carried considerable risk because it was not known whether the parent robins would lay another clutch.

The strategy succeeded. By the end of the 1981-82 breeding season, when tomtits had taken over most of the foster-parenting duties, the black robin population had increased to 12 birds.

From five birds to around 300

Old Blue continued to breed for several more years before being moved to South East Island in 1983. She was last seen in December that year and was estimated to be 13 or 14 years old. All black robins alive today are descendants of the six chicks she and Old Yellow raised during her final breeding seasons.

The population continued to grow in the following years. The Predator Free NZ Trust notes that about 250 black robins descended from Old Blue were living on Mangere and South East islands by 2013.

The current population stands at about 300 birds across the two islands, according to BirdLife International. In 2022, the IUCN downgraded the black robin's status from endangered to vulnerable on its Red List, marking a significant change for a species that had once numbered only five birds.

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Black robin recovery offers conservation lesson

Despite the recovery, the species has very limited genetic diversity because its entire population descends from a small number of birds. According to BirdLife International, this leaves the black robin vulnerable to predators, natural calamities and other shocks.

Conservationists regard the recovery as an example of how a small group can prevent a species from becoming extinct despite having limited resources. Techniques developed during the black robin conservation effort have since been used to help other endangered birds across the Pacific.