Justification of Red List Category
This species has a small population and is therefore listed as Vulnerable.
The population was estimated to number a few hundred individuals by Monnet et al. (1993). Surveys in 2017-18 recorded a minimum of 181 breeding territories and estimated at least 372 adults, noting that they were unable to access all valleys where the species was previously recorded (Lazzari et al. 2018, SOP Manu 2019). The population size is placed in the band 250-999 mature individuals.
Surveys have been carried out in 39 valleys during 1986-91 (Monnet et al. 1993) and during 2017-18 (Lazzari et al. 2018, SOP-Manu 2019). The 1986-91 surveys recorded the species in 12 valleys and estimated the population to number a few hundred individuals (Monnet et al. 1993). The 2017-18 surveys detected the species in 19 valleys, including several valleys where the species was previously reported to be absent, and estimated a population size of at least 372 adults (Lazzari et al. 2018, SOP-Manu 2019). Due to access limitations, the 2017-18 survey did not cover one of the valleys previously surveyed, but all resurveyed valleys that were previously occupied were still occupied (SOP-Manu 2019). The 2017-2018 survey appeared to show a territorial expansion, but differences in survey methodologies, including the use of playback in the later survey, mean that the trend is uncertain and is precautionarily hypothesised to be stable (SOP-Manu 2019).
Acrocephalus caffer is found on Tahiti in the Society Islands, French Polynesia. The species is absent from the Tahiti peninsula (Tahiti Iti) and has been rare and local throughout the 20th century, being recorded in six valleys (out of 14 visited) during the period 1920-1923, 12 (out of 39 visited) during 1986-1991, and 19 (out of 39 visited) during 2017-2018 (Lazzari et al. 2018, SOP Manu 2019).
It occurs in bamboo thickets and second growth forests in river valleys and hillsides to 700 m. It feeds on insects but also takes lizards, small fish, crayfish, snails and nectar (Pratt et al. 1987, Thibault 1988). It is thought to breed exclusively in bamboo thickets (P. Raust in litt. 2007).
The development of hydro-electricity opened up the interior of the island with new roads and tracks (P. Raust in litt. 2007). This increased access has lead to a considerable increase in the exploitation of bamboo as well as invasion by the neotropical weed Miconia and an increase in tourists in four-wheel drive vehicles. These factors have modified the habitat considerably, and have caused a loss of breeding habitat, as well as causing disturbance to birds (P. Raust in litt. 1999, 2007). The impact of habitat degradation and destruction is not known.
The introduction of feral cats Felis catus, rats Rattus spp. (A. Gouni in litt. 2012) and many alien bird species, including the aggressive Common Myna Acridotheres tristis, and the Red-vented Bulbul Pycnonotus cafer, may also contribute to its rarity (Thibault 1988, Seitre and Seitre 1991, Lazzari et al. 2018), although they do not currently appear to be leading to declines (SOP-Manu 2019). Several introduced pathogens that have been detected on Tahiti, including avian pox and avian malaria (Plamnodium relictum), also may pose a threat (SOP-Manu 2019, C. Blanvillain in litt. 2020). P. relictum is suspected to have contributed to declines in other French Polynesian warbler species (C. Blanvillain in litt. 2020), although the related Northern Marquesan Reed-Warbler (A. percernis) is still numerous on Nuku Hiva despite P. relictum being detected in their blood (Beadell et al. 2006).
The current spread of Little Fire Ant Wasmannia auropunctata over the past 10-15 years in several of the reed-warbler's range valleys, including Papenoo valley where the largest population concentration is found, may threaten the species (Bouysseroux et al. 2018, SOP-Manu 2019, C. Blanvillain in litt. 2020). Little Fire Ant was first officially recorded on Tahiti in 2004, but it is likely to have been present on the island at least a decade earlier (Bossin and Padovani 2010, Bousseyroux et al. 2018) and it continues to expand its range on the island (SPREP 2014). Studies have shown that colonisation of areas of Tahiti by Little Fire Ants has led to a reduction in invertebrate diversity and may have caused Tahiti Monarchs to abandon their territories (Bouysseroux et al. 2018, SOP-Manu 2019).
Conservation Actions Underway
Between 2005 and 2009, treatment and monitoring initiatives were carried out in the Mahina Commune to combat the invasion of W. auropunctata (SPREP 2014).
Conservation and Research Actions Proposed
Monitor the population, particularly at sites colonised by Wasmannia auropunctata (C. Blanvillain in litt. 2020). Investigate the severity of the threat caused by A. tristis, W. auropunctata and other predators.
Control the exploitation of bamboo. Protect important sites from habitat clearance through road and dam building and degradation by off-road vehicles. Consider control of predators at sites known to be important to the species. Increase public awareness about methods to limit the spread of W. auropunctata (SPREP 2014). Control W. auropunctata, particularly at newly established and small infestations (SPREP 2014).
19 cm. Large, long-billed warbler with two colour-morphs. Most birds pale yellow, mottled with brownish-olive above. Dark morph all dark olive-brown. Similar spp. Tahiti Monarch Pomarea nigra blacker than dark morph, with pale blue, short bill; occur in different valleys. Voice Call a harsh churrr. Song a lively and varied series of whistles, churrs, and warbles; often long sustained. Hints Shy, skulking bird most easily located by its voice.
Text account compilers
Blanvillain, C., Cibois, A., Derhé, M., Gouni, A., Khwaja, N., Mahood, S., Martin, R., O'Brien, M., Raust, P., Shutes, S. & Stattersfield, A.
BirdLife International (2022) Species factsheet: Acrocephalus caffer. Downloaded from http://www.birdlife.org on 05/10/2022. Recommended citation for factsheets for more than one species: BirdLife International (2022) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 05/10/2022.