The black robin is an endearing ball of beaked fluff, found only in the Chatham Islands off the eastern coast of New Zealand. By 1980, there were just five of them left.

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.

Melanie Massaro from Charles Sturt University in Australia has now shown that rim-laying had a strong genetic basis. Under normal circumstances, natural selection would have quickly weeded out the alleles (versions of a gene) behind the behaviour, because any female who carried them would lay doomed eggs. By saving those eggs, the conservationists inadvertently gave the rim-layers a pass, turning their maladaptive behaviour into a neutral one. They allowed for “survival for the not-so-fit”.

Sure enough, the alleles for rim-laying spread through the growing robin population. The speed of this spread became clear when Massaro checked old records of the robins’ behaviour. Old Blue, the last fertile female as of 1980, laid eggs in the normal way. By 1984, there were five females and one laid a single rim egg. By 1989, there were 35 females, of whom 18 (more than half) were rim-layers.

You can see the spread for yourself in the family tree below, which shows the relationships between every breeding robin during the 1980s. Blue circles represent females that lay eggs in the right place; the rim-layers are in red. This is the sort of diagram you can make when a species only has a few dozen survivors, and you know all of them.

View Images Black robin pedigree, 1980-1989. Credit: Massaro et al, 2013. PLOS.

It’s possible that the robins increasingly laid eggs on rims as a response to their environment, but the pattern in the family tree suggested otherwise. It looked like rim-laying had a strong genetic basis. In fact, it was most probably caused by a dominant allele—a version of a gene that produces rim-laying even when a bird inherits just one copy from either of its parents.

If the team had continued to replace the rim eggs, you could imagine a future when the robins were entirely dependent on humans for their survival. They’d effectively be living in a wild zoo. Consider the domesticated silkworm—the moth that provides us with silk. Five thousand years of coddled breeding at the hands of humans have left these insects unable to fly or feed themselves as adults.

Thankfully, “the black robin just narrowly escaped such a fate,” Massaro wrote.

The team stopped repositioning the eggs in 1990, once robin numbers had bounced back to more promising levels. Natural selection started doing its thing and the proportion of rim-laying females has fallen from 50 percent to around 10 percent. The allele is still around though, hiding from natural selection in the bodies of males, and waiting to pass to another generation.

The goal of conservation isn’t just to save a species temporarily, but to create a wild population that can sustain itself without our help. The black robin shows just how difficult this goal can be. The team saved the bird, but their practices threatened to leave it incapable of breeding on its own. That would have been no use: conservation programmes are laborious and expensive, and can’t go on forever.

As the team write, “Conservation planning has to overcome this fundamental dilemma of rapidly increasing the size of severely endangered populations to avoid immediate extinction but without simultaneously increasing the frequency of detrimental alleles that are already present in the population.”

PS: As per usual, the acknowledgements of the paper detailed what each of the authors did, by their initials. It says who performed the experiments, analysed the data, contributed materials and tools, and wrote the paper. And rather delightfully, it ends with: “Led the team that saved the black robin from extinction: DM.” Well done, Don Merton. Well done.