Data Zone
Justification of Red List category
This species is listed as Critically Endangered because it has an extremely small range on one island, it is considered to be undergoing a decline as a result of invasive species, and it is threatened by the invasion of Black Rats and Common Myna.
Population justification
The species is considered to be abundant over much of Rimatara (P. Raust in litt. 2012, C. Blanvillain in litt. 2016 per G. Dutson in litt. 2016). Various population estimates have been made which differ widely depending on methods used and the season in which the surveys took place. Applying a correction to account for differences in methodology would lead to higher estimates from the transect surveys (Blanvillain et al. 2015). There was a 35% difference in the numbers recorded in May and September 2017, with the lower number recorded in May, possibly due to the birds behaving more secretively in winter (SOP-Manu 2019).
Point count observations in 2002 gave a population estimate of 3,851 birds (Blanvillain 2002), but transect surveys in the same year estimated 740 individuals (Raust and Sanford 2002). In 2004, transect surveys gave an estimate of 675 individuals (Gouni 2004) and territory mapping techniques used by Thibault and Cibois (2006a) gave estimates of 1,777-2,567 breeding birds (roughly 2,665-3,850 individuals), with a population density estimate of just over 3 birds per hectare in good habitat (Thibault and Cibois 2006a). Transect surveys in 2009 produced a population estimate of 677 individuals (Albar et al. 2009).
The most recent surveys were carried out in autumn-winter 2017, following the transect methodology used by previous surveys, with the addition of the use distance sampling. Without applying distance sampling, the resulting population estimate was 1,475 individuals in May 2017 and 958 individuals in September (SOP-Manu 2019). Following the application of distance sampling software, the population size was estimated at 2,225 (1,780 - 2,781) individuals in May 2017 and 1,444 (1,022 - 2,040) individuals in September 2017 (SOP-Manu 2019). This roughly equates to 681 - 1,854 mature individuals, here placed in the band 600-1,900 mature individuals.
Trend justification
Although the differences in survey methods used make it difficult to determine trends, the most recent estimates are lower than the estimate obtained by point count suveys in 2002 (3,851 individuals; Blanvillain 2002) and the estimated number of individuals derived from territory mapping in 2004 (roughly 2,665-3,850 individuals; Thibault & Cibois 2006a), so the population is considered to be declining (SOP-Manu 2019). Based on these figures, a reduction of 30-49% over the past ten years is suspected, and is assumed to be continuing into the future at a similar rate.
Acrocephalus rimitarae is endemic to Rimatara in the Austral Islands, French Polynesia, and is found throughout the island in a wide variety of habitats.
The species is found throughout the island, including in horticultural areas, coconut plantations, feo forest, settlements and swamps (Thibault and Cibois 2006a,b). Breeding however mainly occurs in wooded areas such as the undergrowth of coconut groves, mixed horticulture, coastal forests and natural forest on limestone substrates (Thibault and Cibois 2006a,b).
Habitat destruction is a concern; the construction of an airport destroyed over a third of remaining native forest (Thibault and Cibois 2006a) and it was extended in 2015 (Blanvillain et al. 2015) Half of the feo forests disappeared in Rimatara in less than 12 years (SOP-Manu 2019). However, the species is more frequently recorded in horticultural areas (Albar et al. 2009, J. C. Thibeau in litt. 2019). Feral cats are likely to cause some mortality (Thibault and Cibois 2006a,b) and have become increasingly common on the island according to local people (G. Dutson in litt. 2016). The introduction of alien species, particularly Common Myna Acridotheris tristis, is a grave concern given the species's very small range and the declines it has caused in populations of other Pacific Acrocephalus species (Thibault and Cibois 2006a,b). Polynesian rat Rattus exulans and brown rat R. norvegicus are present on the island, but not black rat R. rattus (McCormack and Künzle 1996, P. Raust in litt. 2012). The latter has been implicated in the decline and extinction of many birds on oceanic islands; its arrival might be a cause for concern, although other Pacific Acrocephalus species coexist with it (Seitre and Seitre 1991).
Conservation Actions Underway
Population surveys and assessment of the threats have been conducted (Thibault and Cibois 2006a,b, Albar et al. 2009, SOP-Manu 2019). Biosecurity measures to prevent the introduction of black rats R. rattus to Rimatara are in place at the wharf, managed by the agriculture officer with support from SOP Manu (Albar et al. 2009, Blanvillain 2019). A plan has been put in place for the response to detection of R. rattus, a rat detector dog has been brought in and an inspector has been trained on how to detect and respond to invasive rats (Blanvillain 2019). Posters have been distributed and workshops held with local communities to raise awareness of the threat of rat invasion to the species (Blanvillain 2019). A conservation organisation, Rima'ura, has been established on the island and trained to carry out bird surveys (Blanvillain 2019, SOP-Manu 2019). Awareness-raising activities have been carried out in schools in Rimatara (Albar et al. 2009).
17 cm. Large warbler with relatively short bill. Adult olive-brown above, yellowish-white below, with dark streak through eye and pale supercilium. White feathers variably and often asymmetrically scattered among darker feathers, often producing large blotches. Voice Loud chack-chack or high-pitched chirp. No song reported.
Text account compilers
Wheatley, H.
Contributors
Dutson, G., Millett, J., Raust, P., Thibault, J.-C., Westrip, J.R.S., Pilgrim, J., Mahood, S., Stattersfield, A., Derhé, M., O'Brien, A. & Shutes, S.
Recommended citation
BirdLife International (2024) Species factsheet: Rimatara Reed-warbler Acrocephalus rimitarae. Downloaded from
https://datazone.birdlife.org/species/factsheet/rimatara-reed-warbler-acrocephalus-rimitarae on 22/12/2024.
Recommended citation for factsheets for more than one species: BirdLife International (2024) IUCN Red List for birds. Downloaded from
https://datazone.birdlife.org/species/search on 22/12/2024.