BirdLife species factsheet for Rimatara Reed-warbler: http://datazone.birdlife.org/species/factsheet/rimatara-reed-warbler-acrocephalus-rimitarae
Rimatara Reed-warbler, Acrocephalus rimitarae, is endemic to the small island of Rimatara in the Austral Islands of French Polynesia, with a Minimum Convex Polygon Extent of Occurrence of only 10km2. The species occurs across the island, even in swamps and the central fern-covered hills (Thibault and Cibois 2006a,b). However, it may be dependent of wooded areas for at least part of the year as breeding predominantly occurs in forested areas as well as the undergrowth of coconut groves and mixed horticulture (Thibault and Cibois 2006a,b).
The fact that it may be dependent on such habitat for breeding may leave it vulnerable to habitat destruction. The construction of an airport destroyed over a third of native forest (Thibault and Cibois 2006a), and it was extended in 2015 (Blanvillain et al. 2015), although (after the initial build at least) suitable habitat remained (J. Millet in litt. 2007). A threat that encompasses the whole of the range of this species is from invasives. Feral cats have become increasingly common on the islands according to local people (G. Dutson in litt. 2016) and they are likely to cause some mortality to this species (Thibault and Cibois 2006a,b). Polynesian and Brown Rats (Rattus exulans and R. norvegicus respectively) are both present on the island too. The Common Myna, Acridotheres tristis, and Black Rat, R. rattus, are not yet present on the island but their potential introduction remains of high concern given the declines and extinctions these species potentially have been involved with (e.g. Thibault and Cibois 2006a,b), although other Pacific Acrocephalus species do coexist with R. rattus (Seitre and Seitre 1991). Thus, given the threats it 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).
The species is considered it to be abundant over much of Rimatara (P. Raust in litt. 2012, C. Blanvillain in litt. 2016 per G. Dutson in litt. 2016), with the population having been estimated several times over the years, using a variety of methods. 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 971 individuals (Blanvillain et al. 2015). 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).
The Rimatara Reed-warbler is currently listed as Vulnerable under criteria D1+2 on the basis of it occurring on just one small island with a potentially small population size (see BirdLife International 2017). This is because the species was tentatively considered to be stable, but expert opinion is that it should now be treated as undergoing a slow decline (C. Blanvillain in litt. 2016 per G. Dutson in litt. 2016). From this species would meet the threshold for Endangered under criterion C2a(ii). However, in the recent regional Red List for France, which included an assessment of the birds of French Polynesia (UICN France 2015), this species was listed as Critically Endangered under criteria B1ab(iii)+2ab(iii). Based on available information, though, our proposal differs from this in two respects. Firstly we do not include criterion B2 because, despite the species likely having a very small Area of Occupancy (AOO), the AOO not been calculated by BirdLife using the strict method required by IUCN (using 2km x 2km grid squares) (see IUCN Standards and Petitions Subcommittee 2016). Secondly, expert opinion is that the species is likely in decline, and so condition (v) under criterion B1b would be met in addition to condition (iii). Therefore, we propose for the species to be listed as Critically Endangered under criterion B1ab(iii,v), on the basis that the species has an extremely small Extent of Occurrence, confined to one very small island, where the population and area/quality of habitat is declining.
We welcome any comments regarding this proposed uplisting.
Albar, G.; Doukas, T.; Kape, J.; Gouni, A. 2009. Programme de conservation du Lori de Kuhl et suivi des effectifs de Rousserolles de Rimatara en 2009. Société d’Ornithologie de Polynésie, Tahiti, Polynésie Française.
BirdLife International. 2017. Species factsheet: Acrocephalus rimitarae. Downloaded from http://www.birdlife.org on 31/03/2017.
Blanvillain, C. 2002. Les oiseaux terrestres de Rimatara et des Australes. 2ème phase. Rapport de mission au Service de l’Equipement de Polynésie française et à la SOP. Société d’Ornithologie de Polynésie, Tahiti, Polynésie Française
Blanvillain, C.; Hatitio, G.; Mooroa, T.; Percheron, E.; Sulpice, H.; Brown, F. 2015. Bilan du programme biosécurisation de l’avifaune de Ua huka et de Rimatara 2015. Rapport final. Rapport SOP Manu à la DIREN et au HautCommissariat. Société d’Ornithologie de Polynésie, Tahiti, Polynésie Française.
Gouni, A. 2004. Etude de l’avifaune de Rimatara. 3ème phase. Société d’Ornithologie de Polynésie, Tahiti, Polynésie Française.
IUCN Standards and Petitions Subcommittee. 2016. Guidelines for using the IUCN Red List categories and criteria. Version 12. Prepared by the Standards and Petitions Subcommittee. Downloadable from http://www.iucnredlist.org/documents/RedListGuidelines.pdf
Raust, P.; Sanford, G. 2002. Etude de l’avifaune de Rimatara. 1ère phase. Société d’Ornithologie de Polynésie, Tahiti, Polynésie Française.
Seitre, R.; Seitre, J. 1991. Causes de disparition des oiseaux terrestres de Polynésie Française. South Pacific Regional Environment Programme, Nouméa.
Thibault, J-C.; Cibois, A. 2006a. The natural history and conservation of Acrocephalus rimitarae, the endemic reed-warbler of Rimitara Island, Oceania. Bulletin of the British Ornithologists’ Club 126(3): 201-207.
Thibault, J.-C.; Cibois, A. 2006b. Une situation favorable pour le Rupe de Makatea. Te Manu 54: 2-3.
UICN France; MNHN; SOP Manu. 2015. La Liste rouge des espèces menacées en France – Chapitre Oiseaux de Polynésie française. Paris, France.