The Black robin or Chantham island robin (Petroica traversi) closely related to the New Zealand robin (P. australis) is an endangered bird found in the Chatham Islands to the east coast of New Zealand. Walter Buller described it in 1872. The binomial commemorates the New Zealand botanist Henry H. Travers (1844-1928). It is a sparrow sized bird measuring 14-15 cm (5.5-6 in). It is entirely brownish black with a black bill and brownish black yellow soled feet. Its flight capacity is reduced since it evolved in the absence of mammalian predators. In 1871 it became extinct on the main Island of Chatham, being restricted to Little Mangere Island thereafter.
There are now almost 250 black robins but before in 1980 only five Black Robins survived on Little Mangere Island but were saved from extinction by Don Merton and his Wildlife Service team, and by "Old Blue", the last remaining fertile female. The remaining birds were moved to Mangere Island. They increased the annual output of Old Blue (and later other females) by removing the first clutch over every year and placing the eggs in the nest of the Chatham race of the Tomtit, a technique known as cross-fostering. The Tomtits raised the first brood, and the Black Robins, having lost their eggs, relaid and raised another brood.
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 260 individuals in populations on Mangere Island and South East 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.