Black Robin

Black robins are territorial. Males will patrol and defend their areas. Females have been known to chase away other females. They make short flights from branch to branch and do not fly long distances.

There are now around 250 black robins, but in 1980 only five survived on Little Mangere Island. They 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. The team increased the annual output of Old Blue (and later other females) by removing the first clutch 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. Many females laid eggs on the rims of nests where the eggs could not survive without help. Human conservationists pushed the eggs back into the nests where they were incubated and hatched successfully. The maladaptive gene causing this behaviour spread until over 50% of females laid rim eggs. Humans stopped pushing eggs back in time to prevent the gene spreading to all birds which could have made the birds dependent on humans indefinitely. After human intervention stopped, rim laying became less frequent, but 9% of birds still laid rim eggs as of 2011. Conservationists have faced some criticism that they may inadvertently do harm, if they allow organisms with deleterious traits to survive and perpetuate what is maladaptive. All of the surviving black robins are descended from "Old Blue", giving little genetic variation among the population and creating the most extreme population bottleneck possible. However, this does not seem to have caused inbreeding problems, leading to speculation that the species has passed through several such population reductions in its evolutionary past, and has lost 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). Nest predation by introduced common starlings is the most common cause of nesting failure for black robins on Rangatira Island, with almost 21% of nests failing due to starlings. Incidences of failure caused by starlings are more common when black robins nest in cavities compared to open nests. The species is still endangered, but now numbers around 250 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.