This essay is adapted from Traversal.

“In the great chain of cause and effect,” Alexander von Humboldt wrote as he was teaching science to read the poetry of nature, “no single fact can be considered in isolation.”

When the first European colonists made landfall on New Zealand’s shores in Humboldt’s lifetime, the cats and rats that descended from their ships began decimating the native population of black robins — sparrow-sized birds with yellow-soled feet that had evolved without mammalian predators, mate for life in monogamous pairs, and raise only two chicks per year in cuplike nests close to the ground.

Bird by bird, claw by claw, there were only seven survivors within a century.

Desperate to encourage the survivors to breed, conservationists moved them to Mangere Island, where twenty thousand trees were planted just to provide a hospitable habitat for the robins. But they would not pair — mysterious are the ways of even a bird’s heart, for it is all a single mystery.

Two of the seven died.

Among the five survivors there was a sole female capable of laying fertile eggs — a robin so aged that she came to be known as Old Blue. At eight, she had outlived the average black robin twofold. With the survival of the species resting on Old Blue’s near flightless wings, scientists thought that if her offspring were raised by surrogate parents, she would be able to lay more eggs.

Warblers were the first designated foster parents, but they failed to feed the chicks enough.

Tomtits were tried next, but they were too successful as foster parents — the black robin chicks grew up perceiving themselves as tomtits and wanted to mate only with other tomtits.

Finally, the chicks were returned to Old Blue, in whose care they thrived as black robins.

A single mother brought a whole species back from the brink of extinction.

Old Blue lived to be fourteen and raised eleven chicks. All the black robins in the world today, numbering around 250, are fractal emissaries of her genes — a winged reminder that immensities of harm can be undone by a single act of tenacious tenderness.