How a single female saved an entire species from the brink of extinction
In short: In 1980, the black robin was down to just five individuals. One female, known as Old Blue, became the sole ancestor of the entire modern population. This is the remarkable story of a genetic bottleneck, a daring conservation experiment, and a bird that refused to vanish.
The black robin (Petroica traversi) is an endangered inhabitant of New Zealand's Chatham Islands. Evolution in an environment free of mammalian predators left the species uniquely vulnerable to introduced cats and rats, leading to its disappearance from the main Chatham Island before 1871. By 1905, many believed the species had vanished entirely.
The species' survival is credited to the efforts of Don Merton and the Wildlife Service team. They utilized a technique called cross-fostering to boost reproduction: by removing the first clutch of eggs and placing them in the nests of Chatham Island tomtits, they encouraged the female robins to lay a second brood. This intervention helped expand a population that once dwindled to just five birds on Little Mangere Island.
Conservationists also faced a biological dilemma known as 'rim laying.' A maladaptive trait had emerged where over 50% of females laid eggs on the edges of nests, where they could not survive without human assistance. To prevent the species from becoming permanently dependent on humans, intervention was eventually halted. While 9% of birds still exhibited this behavior as of 2011, the population has grown to approximately 390 individuals by 2026.
Despite the extreme genetic bottleneck—where every living black robin descends from Old Blue—the species has not suffered from obvious inbreeding depression. This suggests the species may have survived similar population crashes throughout its evolutionary history, purging harmful alleles along the way. Today, the species continues to face threats from introduced starlings, which cause nearly 21% of nest failures on Rangatira Island.
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- Summary by Intellect · source: Black robin
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