Justification of Red List Category
This species is listed as Critically Endangered because it has an extremely small range on one island and is considered to be undergoing a decline as a result of habitat destruction and invasive species.
Various population estimates have been made which differ widely depending on methods used. Point count observations in 2002 gave a population estimate of 5,000 birds (Blanvillain 2002). Territory mapping techniques used by Thibault and Cibois (2006a) gave estimates of 1,777-2,567 breeding birds in 2004 (roughly 2,665-3,850 individuals), and transects gave estimates of 740 (Raust and Sanford 2002), 675 (Gouini 2004) and 670 (Albar et al. 2009), though applying a correction to account for differences in methodology would lead to higher estimates from these (Blanvillain et al. 2015). The most recent estimate generated is 1,780-2,781 individuals (C. Blanvillain in litt. 2017). Taking into account corrections (see Blanvillain et al. 2015), the range of estimates fall into the range of approximately 900-3,850 individuals, which roughly equates to 600-2,600 mature individuals (G. Dutson in litt. 2016).
Given the threats the species may be facing, and the continuing degradation of its habitat, it is considered to be undergoing a slow ongoing decline (C. Blanvillain in litt. 2016 per G. Dutson in litt. 2016). Population estimates made using similar methodologies in 2006 and 2017 suggest a reduction (Thibault and Cibois 2006a; C. Blanvillain in litt. 2017).
Acrocephalus rimatarae is endemic to Rimatara in the Austral Islands, French Polynesia. In 1989, it was found to be common and widespread at lower elevations (Seitre and Seitre 1991), and in 2004 population density was estimated at just over 3 birds per hectare in good habitat (Thibault and Cibois 2006). Population estimates vary widely depending on the methods used: the most recent estimate was 1,780-2,781 individuals (C. Blanvillain in litt. 2017), but other estimates have been much lower: 740 (2002) or 675 (2004) (Thibault and Cibois 2006). Recent observers consider it to be abundant over much of the island (P. Raust in litt. 2012; C. Blanvillain in litt. 2016 per G. Dutson in litt. 2016).
The species is found throughout the island, even in swamps and the central fern-covered hills (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).
Since the species requires forest for nesting, habitat destruction is a concern; the recent construction of an airport destroyed over a third of remaining native forest (Thibault and Cibois 2006) and it was extended in 2015 (Blanvillain et al. 2015), although suitable breeding habitat remains, after the initial build at least (J. Millett in litt. 2007). 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
A population survey and assessment of the threats have been conducted (Thibault and Cibois 2006a,b). 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.
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
Stattersfield, A., Wheatley, H., Westrip, J., O'Brien, A., Pilgrim, J., Mahood, S., Derhé, M., Shutes, S.
Dutson, G., Millett, J. & Raust, P.
BirdLife International (2020) Species factsheet: Acrocephalus rimitarae. Downloaded from http://www.birdlife.org on 25/09/2020. Recommended citation for factsheets for more than one species: BirdLife International (2020) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 25/09/2020.