If black robin eggs or chicks could be transferred to suitable foster birds, the black robin parents could potentially produce additional offspring while the foster parents took on some of the work of incubation or raising the young.
This became an important part of the recovery programme.
The approach demonstrated how conservation sometimes requires techniques that would be unusual in ordinary wildlife management.
When a species has thousands or millions of individuals, natural losses may be absorbed by the population.
When there are only five birds left, every egg matters.
Every chick matters.
Every breeding opportunity matters.
The black robin's recovery therefore became a story not only about the resilience of nature but also about careful, long-term human intervention.
From Five Birds to Hundreds
The results were extraordinary.
The population gradually increased.
The birds were eventually established on protected islands where introduced predators could be controlled or excluded.
Today, the black robin survives on two predator-free reserve islands in the Chatham archipelago: Mangere and Rangatira, also known as South East Island. The Department of Conservation currently puts the population at around 350 adults.
That number may seem small compared with common bird species.
But compared with five, it represents a remarkable transformation.
The species has increased by roughly two orders of magnitude from the crisis point of 1980.
And perhaps the most remarkable part is that the entire modern population descends from that tiny founding group.
The black robin therefore represents both a conservation triumph and a reminder of how narrow the margin between survival and extinction can become.
A Recovery That Is Not Yet Finished
It would be tempting to describe the black robin as a species that has been "saved" and leave the story there.
But conservationists emphasize that the situation remains fragile.
The Department of Conservation continues to classify the black robin as Threatened–Nationally Critical. Current threats include predation and disease.
The population's limited distribution is another concern.
There are currently only two populations in the world, both on reserve islands in the Chatham archipelago.
In 2025, New Zealand's Department of Conservation warned that one of the existing populations was approaching the capacity of its available habitat. At that time, DOC reported a total population of about 445 birds, including approximately 400 on Rangatira and 45 on Mangere.
Population estimates can change depending on the year, survey methods and management conditions, which helps explain why recent sources may give somewhat different numbers.
The broader point remains the same: the species has recovered dramatically, but it still occupies a very limited range.
That creates a difficult conservation problem.
If the population is concentrated in only a couple of locations, a single major event could have devastating consequences.
A disease outbreak, the accidental arrival of an invasive predator or a serious environmental change could affect a large percentage of the entire global population.
The Genetic Problem Created by Success
The black robin's recovery also presents an unusual scientific challenge.
Having a larger population is obviously better than having five birds.
But population size is not the only consideration.
The modern black robin population originated from an exceptionally small number of ancestors. That means genetic diversity is extremely limited.
New Zealand's Department of Conservation notes that modern black robins share similar DNA and warns that a single disease could potentially pose a serious threat.
New Zealand Birds Online similarly identifies severe loss of genetic diversity and long-term inbreeding as important concerns for the species.
This creates a fascinating paradox.
The species has recovered numerically, but its genetic history still carries the imprint of its near-extinction.
A population can go from five individuals to hundreds without immediately regaining the genetic diversity it lost.
That is one reason conservation does not end when a population begins increasing.
Scientists and conservation managers must continue thinking about disease, habitat, genetics, predators and the possibility of establishing additional populations.