Justification of Red List Category
Once among the rarest birds of the world, this species has been brought back from the brink of extinction. It has been downlisted to Vulnerable as the population has increased in recent years owing to intensive conservation efforts with no evidence of a continuing decline for the last five years. However, it still has a very small population and range and remains threatened by chance events such as cyclones and other stochastic factors that could drive it to qualify as Critically Endangered or even Extinct in a short time period. The survival of the species remains dependent on a continuation of intensive conservation efforts.
The population was estimated at 306 in August 2004, but a subsequent series of devastating cyclones in early 2005 lead to a decline, especially on Rarotonga. The total population began to increase in 2007 due to good recruitment on Atiu (where 30 birds were transferred in 2001-2003) and a stable population on Rarotonga. In 2011 the population was estimated at c.380 birds, including 69 yearlings (Robertson et al. 2011, H. Robertson in litt. 2011), suggesting a population of c.310 mature individuals. The number of mature individuals is now estimated at 500 (H. Robertson in litt. 2016).
The population has grown rapidly owing to intensive management, particularly predator control, and the transfer of 30 young birds to Atiu Island in 2001-2003. The population on Rarotonga declined as a result of five cyclones in one month in early 2005, and subsequent poor breeding in 2005-2006; however, the population has now been increasing since 2007, owing to intensive conservation action.
Pomarea dimidiata is endemic to Rarotonga, Cook Islands, where it is largely restricted to the Totokoitu, Turoa, western Avana, and Taipara Valleys. It was common until the middle of the 19th century, but thought to be extinct in the early 1900s (Robertson et al. 1994). A recovery plan initiated in 1987 improved breeding success and recruitment, resulting in the increase of 177 birds in 1998 to 308 in 2003 (H. Robertson in litt. 2007). In 2001, ten birds were translocated from Rarotonga to Atiu, 200 km north-east, and similar numbers, consisting of one to two-year-old birds, were translocated to Atiu in 2002 and in 2003 (Robertson et al 2006), and another 10 yearlings were moved from Rarotonga to Atiu in 2011. A census in February-March 2015 found a minimum of 156 birds on Atiu, including 117 adults (H. Robertson in litt. 2016).
It prefers steep-sided, wet, forested, small valleys sheltered from south-east trade winds in the headwaters of streams. It feeds mainly on small caterpillars, flies, beetles and bugs. Clutch-size is two. Usually only one brood is raised each year (McCormack and Künzle 1990, Sanders et al. 1995, Saul et al. 1998, E. Saul in litt. 1999). Before intensive predator control began, annual adult mortality was 24.3% and life expectancy was 3.6-6.0 years for males and 2.4 years for females. Since intensive management, annual mortality dropped significantly to 14.2% and life expectancy increased to 7.6 years between 1989 and 2006 (H. Robertson in litt. 2007); however as the population on Rarotonga has continued to grow, mean life expectancy has declined to 6.0 years in 1986-2015 because more birds have settled in unmanaged areas outside of the core management area (H. Robertson in litt. 2016). The species is capable of breeding at one year old, but more recently, it is rare for yearlings to breed (<5%), and most do not start breeding until 3-4 years old (H. Robertson in litt. 2016).
The highly localised distribution of the species leaves it vulnerable to cyclones, invasion of weeds and forest clearance. It continues to be threatened by black rat Rattus rattus and cats Felis catus. Predation by Long-tailed Cuckoo Urodynamis taitensis, a migrant from New Zealand, remains a possibility. The introduction of avian diseases could have a major impact, as could invasion by new predators (e.g. snakes and mongooses) (Robertson et al. 1994, H. Robertson verbally 1999, E. Saul in litt. 1999). Despite passing through a bottleneck of 29 birds, the genetic diversity in the species is moderately good and so inbreeding effects and loss of genetic diversity are not thought to be a threat (Chan et al. 2011).
Conservation Actions Underway
The valleys where the species survives (called the Takitumu Conservation Area) are managed by three landowning families who have developed an ecologically and commercially sustainable ecotourism venture (Keppel et al. 2012). Intensive rat control is carried out during the breeding season, including fortnightly poisoning (Robertson et al. 2009). An insurance population has been established on Atiu and is breeding well in a variety of habitats (Robertson et al. 2006, Saul et al. 2007). The survival of the species remains dependent on the continuation of such intensive conservation action.
Identification: 14 cm. Inquisitive, grey-and-white flycatcher. First year, orange plumage with yellow base to lower mandible, 2nd year, orange plumage with evenly steel-blue bill (in hand, appearing black in field), 3rd year, mixed grey-and-orange plumage, black bill, 4th year, grey and white plumage, black bill. Similar spp. None, the smallest landbird on Rarotonga, could only be confused as an adult (momentarily) with adult Rarotonga Starling Aplonis cinerascens. Voice Wide variety of discordant calls. Loud male territorial call given repeatedly during pre-breeding period, and onomatopoeically rendered as Kakerori, the bird's Maori name.
Text account compilers
Derhé, M., Mahood, S., Harding, M., O'Brien, A., Pilgrim, J., Shutes, S., Stattersfield, A., North, A.
O'Brien, M., Robertson, H., Saul, E., Ghestemme, T.
BirdLife International (2020) Species factsheet: Pomarea dimidiata. Downloaded from http://www.birdlife.org on 19/09/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 19/09/2020.