They lived in a rocky outcrop about the size of a few city blocks. The precipitous cliffs kept them safe from the cats, stoats and rats that sailors had brought to the islands. But the high winds were too much for these small birds, and most of the survivors had died. With a single breeding pair left—Old Blue and Old Yellow—their future looked bleak.

Don Merton and a team of conservationists mounted a heroic effort to save them. They relocated the tiny population to larger islands and managed their reproduction over many years, transferring their eggs to foster parents for incubation. By 1989, there were 80 robins. By 1998, there were over 200. Once the world’s most endangered bird, the black robin became a flagship example of conservation success.

But it’s also an example of good intentions leading to unintended consequences.

In those early years, when the team was still carefully managing the birds, they noticed that many females laid their eggs on the rims of their nests, rather than the centre. Precarious positions aside, these “rim eggs” were never incubated and never hatched. With the species’ fate hanging in the balance, every egg was precious. The team repositioned the ones on the rims.

Without this move, it’s unclear if the species would have made its dramatic recovery. But it also saddled the robins with a difficult legacy.