The foster parents would incubate the eggs and raise the chicks.
Meanwhile, the black robin parents could be encouraged to lay another clutch.
In other words, conservationists were effectively trying to increase the number of breeding opportunities available to the last surviving black robins.
The idea had never been straightforward.
Moving an egg is not like moving an object from one box to another. The timing of incubation, temperature, feeding, hatching and parental behavior all have to work.
The wrong foster species could reject the egg or fail to raise the chick.
The conservation team initially experimented with Chatham Island warblers. The approach showed limitations, and the programme eventually turned to Chatham Island tomtits, which proved much more suitable as foster parents. DOI
The work was extraordinarily delicate.
Historical accounts published by Forest & Bird describe conservationists studying potential foster species, monitoring nests and determining how the birds responded to unfamiliar eggs and chicks. Eggs and young sometimes had to be transported between islands, requiring careful coordination with weather and local boat operators. Papers Past
It was not a single dramatic intervention.
It was years of careful work.
Egg after egg.
Nest after nest.
Season after season.
And gradually, the numbers began to change.
The Programme That Changed Everything
Between 1980 and the late 1980s, conservationists used cross-fostering and other intensive management techniques to boost black robin reproduction.
The Department of Conservation explains that eggs and young were cross-fostered to other bird species in an effort to increase productivity. Eventually, the technique using Chatham Island tomtits proved successful. Department of Conservation
Scientific literature confirms the importance of the programme.
A peer-reviewed study published in Animal Conservation describes how the black robin population had fallen to five individuals by 1980, with only one pair successfully producing viable offspring. Intensive cross-fostering helped prevent extinction, while birds were subsequently reintroduced to Rangatira Island to establish a second breeding population. ZSL Publications
This was crucial.
A species living on a single small island remained extraordinarily vulnerable.
One disease outbreak, storm, predator introduction or other environmental disaster could potentially destroy the entire population.
Creating another population therefore provided an important layer of protection.
In 1983, black robins were established on Rangatira Island.
The new population became a major turning point in the species' recovery. Papers Past
The black robin was no longer confined to one fragile population.
There was now another place where the species could survive and reproduce.
From Five Birds to Hundreds
The numbers began to tell the story.
According to the International Union for Conservation of Nature's Red List assessment, the black robin population increased dramatically following intensive management. The species went from five birds in 1980 to more than 90 adults by 1990, nearly 200 adults by 1998, and roughly 300 mature birds by 2020. DOI
New Zealand's Department of Conservation now places the population at around 350 birds in 2025, while Forest & Bird reported around 350 adult birds following the 2026 breeding season. Department of Conservation
The difference between five and 350 may look like nothing more than a statistic.
It is not.
For conservationists, it represents decades of work.
It represents thousands of hours of monitoring.
It represents protected habitat, strict biosecurity, carefully managed breeding and the establishment of new populations.
Most importantly, it represents the fact that a bird once expected to disappear is still flying through the forests of the Chatham Islands.
The recovery has been so notable that the black robin is now frequently cited as a model for endangered-species conservation. The Department of Conservation describes the fostering programme as a successful case model that has been used to inform efforts to save endangered birds elsewhere. Department of Conservation
The Species Is Not Out of Danger
The story would be incomplete if it ended with the number 350.
The black robin has survived.
But it has not been completely rescued from danger.
New Zealand's Department of Conservation classifies the species as Threatened–Nationally Critical, the country's highest threat category. Department of Conservation
The IUCN Red List has historically assessed the black robin as Vulnerable, while noting its extremely restricted range and continuing vulnerability to introduced predators and other threats. DOI