The tomtits would incubate and raise the transferred black robin chicks.
Meanwhile, because their original eggs had been removed, the black robins could lay another clutch.
In theory, one female could therefore contribute to more than one round of reproduction in a single breeding season.
BirdLife International describes the technique as a pioneering intervention that effectively allowed conservationists to increase reproductive output.
The technique was not the only intervention.
Conservationists also carefully monitored nests and eggs.
Some female black robins had an unusual tendency to lay eggs on the rim of their nests, where the eggs could fall and be lost. Researchers intervened by moving vulnerable eggs toward safer positions within the nest.
These interventions might appear extremely hands-on by modern standards.
But in 1980, the alternative was potentially losing the entire species.
There was no large population capable of recovering naturally.
There was no second major wild population waiting somewhere else.
There was no genetic reserve.
There were only five birds.
Every egg mattered.
Every chick mattered.
Every breeding opportunity mattered.
From Five Birds to a Growing Population
The intensive programme worked.
Historical records show that the population had increased to approximately 116 birds by early 1990, just a decade after the species had been reduced to five individuals. Contemporary conservation accounts attributed the dramatic increase to intensive management, including nest manipulation, cross-fostering and the establishment of a larger population on South East Island, now known as Rangatira/Hokorereoro.
By 1989, the black robin population had reached around 80 birds.
That milestone was important because conservationists could begin asking a new question.
Could the birds now reproduce without such intensive human intervention?
The answer was encouraging.
The intensive nest-manipulation programme was eventually scaled back. According to the Department of Conservation’s historical account, the population continued to increase, reaching more than 200 birds by 1998.
The species had moved from an emergency rescue operation toward something resembling a self-sustaining population.
But the black robin was not suddenly safe.
The problems that had nearly destroyed it had not disappeared.
They had simply been brought under control.
Why Predator-Free Islands Matter
The black robin’s modern survival is closely tied to two remote islands in the Chatham archipelago: Mangere/Maung’ Rē and Rangatira/Hokorereoro, also known as South East Island.
These islands are protected nature reserves and are free from introduced predators and pests, providing refuges for rare endemic species. Access is tightly controlled because even a seemingly small biosecurity failure could have serious consequences.
This island strategy reflects a broader feature of New Zealand conservation.
The country has many native species that evolved without the mammalian predators that later arrived with humans.
Rats, cats, mice and other introduced mammals can prey on eggs, chicks and adult birds, compete for food or alter entire ecosystems.
For species already reduced to tiny populations, the consequences can be devastating.
That is why predator-free islands have become some of New Zealand’s most important conservation strongholds.
The Department of Conservation manages numerous offshore islands, many of which have been cleared of introduced pests and are being restored as refuges for threatened wildlife.
For the black robin, these protected islands provide something that the species desperately needs: a place where conservationists can control one of its most immediate threats.
But even predator-free habitat has limits.
A Conservation Success Story That Is Not Finished
It is tempting to describe the black robin as a species that has been “saved.”
That is true in one sense.
A bird that had only five individuals in 1980 now exists in the hundreds.
But conservationists are careful not to treat that recovery as the end of the story.