Justification of Red List category
In 1980, this robin had the smallest population of any bird species for which precise figures were known and it seemed doomed to extinction. Its spectacular recovery, following intensive management, is a renowned conservation success worldwide. The species now occurs on two islands with an overall stable population of c.300 mature individuals. It remains at risk from the accidental introduction of non-native predators that depleted its population initially, which could cause it to become Extinct or Critically Endangered in a short period if time. For this reason it is assessed as Vulnerable.
Counts are made pre-breeding (October-November) and post-breeding (March-April), with the former referring to mature individuals. Since November 2012, the pre-breeding numbers have not fallen below 250 mature individuals, with most recent counts of 289, 296 and 298 in Octobers 2018, 2019 and 2020 respectively (T. Thurley in litt. 2021). The majority of birds are on the island of Rangatira, where pre-breeding counts of 244, 263 and 264 were made in 2018, 2019 and 2020, while equivalent counts on the island of Mangere were 45 (15.6% of the total), 33 (11.1%) and 34 (11.4%). Post-breeding numbers ranged 342-369 between 2016 and 2021 (T. Thurley in litt. 2021).
Listed as Nationally Critical in New Zealand (Robertson et al. 2017, 2021), following the independent criteria set out in Townsend et al. (2008). Intensive conservation efforts boosted population sizes rapidly between 1980 and 1989 after the population was depleted to just one breeding female (Old Blue). After intervention ceased, population sizes increased naturally though at a slower rate. The population on Rangatira has increased steadily from c.180 mature individuals in October 2010, to c.260 in October 2020; on Mangere however, the population has declined slightly from a high of 51 in October 2013 to 34 in October 2020. Overall, the population is stable (T. Thurley in litt. 2021).
Petroica traversi is endemic to the Chatham Islands, New Zealand. It declined rapidly during the late 1800s, and by 1980 the population had fallen to five birds, comprising two males and three females. A convalescent pedigree confirms that all birds alive today are descendants from a single breeding pair (Old Blue and Old Yellow). Intensive management by the New Zealand Wildlife Service (later Department of Conservation) has resulted in a continuous increase in numbers: from seven adults pre-breeding in 1981 (12 post-breeding), to 93 in 1990 (128 birds post-breeding), 197 adults in 1998 (270 individuals post-breeding) (Kennedy 2009), and c.300 in 2020 (c.350 post-breeding) (T. Thurley in litt. 2021). The population is now restricted to Mangere (1 km2) and Rangatira (= South East, 2 km2) islands.
The species lives in low-altitude forest remnants. It is entirely insectivorous, and feeds on the forest floor and low branches like other Petroica. It is a very poor disperser such that it is unable to naturally recolonise other islands in the Chathams (Kennedy 2013). It usually lays two eggs, and re-lays if a clutch is lost. Young normally begin to breed at one year of age (Kennedy 2009). Survivorship under natural conditions between 1990 and 1998 indicates mean life-expectancy of 4.2 years for males, and 3.74 for females (both means are lower than for the intensively managed years, though not by much for males; Kennedy 2009). Some individuals may live up to 16 years.
The introduction of rats Rattus spp. and cats, following human settlement, extirpated the birds from all but Little Mangere Island (Butler and Merton 1992). Since Black Robins are insectivores feeding mainly on the ground and have not evolved any anti-predator behaviours, they cannot co-exist with invasive mammalian predators. Any accidental introduction of mammalian predators to the islands where the species currently survives may cause extinction on the island. Introduced Common Starlings Sturnus vulgaris, which now number over 1,000 pairs on Rangatira, are a serious threat to Black Robin reproduction (Massaro et al. 2013a). Black Robins nesting in cavities experienced a significantly higher rate of predation (36.33%) than open nesters (10.82%) (Massaro et al. 2013a). Nest height also influenced predation, with predation risk increasing from 4.88% for nests below 1 m to 31.89% for nests above 3 m (Massaro et al. 2013a). A potential future threat to this highly inbred species is the arrival of new pathogens.
Fire, catastrophic storm events and natural processes of forest recovery, exacerbated perhaps by climate change, are key extrinsic threats to habitat quality and extent. For example, severe storms in the winter of 2015, caused higher than normal winter mortality rates, especially for males in the Top Bush. The extensive loss of genetic diversity through genetic drift and chronic inbreeding since the severe population bottleneck in 1980, compromises reproductive output and juvenile survival (Massaro et al. 2013b, Kennedy et al. 2014, Weiser et al. 2016) and is threatening the long-term viability of this species.
Hybridisation with congeneric Chatham Island Tomtits P. macrocephala chathamensis remains a concern, although the probability of recurrence may be low. A recent paper has shown that there is no evidence of recent hybridisation with the sympatric Chatham Island Tomtit P. macrocephala chathamensis (Cubrinovska et al. 2016). The species remains susceptible to outright loss owing to stochastic events (E. S. Kennedy in litt. 2012).
Conservation Actions Underway
In 1976, following forest deterioration on Little Mangere Island, the seven surviving birds were relocated to Mangere Island. Prior to reintroduction, thousands of trees were planted to provide future habitat. In 1979, productivity failed to offset losses for the first time, and the population declined to five adults. In 1980-81, eggs and chicks were cross-fostered to the Chatham Island Warbler Gerygone albofrontata in order to induce Black Robin females to renest. Supplementary feeding commenced, along with protection of nests from seabirds and Common Starlings. The warblers proved unsuitable as foster-parents. In 1981-82, Old Blue’s eggs were cross-fostered to congeneric Tomtits P. macrocephala chathamensis on Rangatira Island. The chicks were returned to Mangere Island to assist future breeding there. Fostering to Tomtits proved successful, and in 1983 a permanent population of Black Robins was founded on Rangatira Island (E. S. Kennedy in litt. 2012). Intensive management ceased after 1989 (Heather and Robertson 1997). Annual monitoring of numbers, reproductive success and distribution within habitats continues in both island populations. Reforestation on both islands is on-going, and both island habitats are subject to strict quarantine measures to avoid introducing predators, pathogens and other threats. In 2014, 100 nest boxes were installed on Rangatira Island, to test whether a) nest boxes decrease starling predation on robin nests and b) the availability of nest boxes in the upper part of the bush on Rangatira Islands will increase densities of pairs in Top Bush. This study is ongoing (M. Massaro in litt. 2016). A recent study investigating genetic diversity and differentiation in the two island populations has shown that a) genetic diversity is very low in both populations and b) that due to strong genetic drift genetic differentiation between the two populations has taken place (Forsdick et al. 2017). The populations of both islands are subject to annual censuses to monitor population changes.
Conservation Actions Proposed
Continue to monitor population and demographic trends. Expand populations sizes through reforestation of Mangere Island and infill planting on Rangatira Island. Protect populations on Mangere and Rangatira Islands from biosecurity threats. Establish a third subpopulation within the Chatham Islands. Continue to work with landowners and Department of Conservation to provide safe habitat on Chatham Island.
15 cm. Small, pure black bird. Plumage of sexes alike, but female slightly smaller. Short, slender, black bill. Voice Male song simple phrase of 5-7 notes. Call notes are distinct and full.
Text account compilers
Aikman, H., Benstead, P., Houston, D., Kennedy, E., Khwaja, N., Mahood, S., Massaro, M., McClellan, R., Merton, D., O'Connor, S., Stringer, C., Symes, A., Taylor, J., Temple, H. & Thurley, T.
BirdLife International (2023) Species factsheet: Petroica traversi. Downloaded from http://datazone.birdlife.org/species/factsheet/black-robin-petroica-traversi on 26/09/2023. Recommended citation for factsheets for more than one species: BirdLife International (2023) IUCN Red List for birds. Downloaded from http://datazone.birdlife.org on 26/09/2023.