The Black Robins or Chatham Island Robin Petroica traversi is an endangered bird from the Chatham Islands off the east coast of New Zealand. It is closely related to the New Zealand Robin.
Petroica traversi is a sparrow-sized bird. Its plumage is almost entirely brownish-black, unlike its mainland counterparts, and as it evolved in the absence of mammalian predators, its flight capacity is somewhat reduced. In the early 1980s only five Black Robins survived but were saved from extinction by Don Merton and his team, and by “Old Blue”, the last remaining female.
All of the surviving black robins are descended from “Old Blue”, unfortunately giving little genetic variation among the population and creating an extreme population bottleneck.
Interestingly, this seems to have caused no inbreeding problems, leading to speculation that the species has passed through several such population reductions in its evolutionary past and thus losing any alleles that could cause deleterious inbreeding effects.
It was generally assumed that the minimum viable population protecting from inbreeding depression was around 50 individuals, but this is now known to be an inexact average, with the actual numbers being below 10 in rapidly-reproducing small-island species such as the Black Robin, to several hundred in long-lived continental species with a wide distribution (such as elephants or tigers).
The species is still endangered, but now numbers around 250 individuals in a single population on tiny Little Mangere Island. Ongoing restoration of habitat and eradication of introduced predators is being undertaken so that the population of this and other endangered Chatham endemics can be spread to several populations, decreasing the risk of extinction by natural disasters or similar stochastic events.