Justification of Red List Category
This species qualifies as Vulnerable as it is only found on one small island, where it remains at risk from the accidental introduction of alien species, especially mammalian predators.
In 1987, it was found to be abundant throughout the island, with an estimated total population of c.10,800 (adjusted to 13,000 when using the correct island area), and available habitat apparently saturated (Graves 1992). In 1991-1992 surveys estimated the population at 9,500 (11,000 using corrected island area) individuals and in both 2003 and 2011, there was no striking change evident (M. Brooke in litt. 2007, 2012), and the population was assumed to be stable. In 2015, the population at 25 point count locations was estimated to be 180 (95% CI 147 - 224) individuals, and the trend from 2011 to 2015 was estimated to be increasing substantially (mean 0.191, 95% CI 0.153 - 0.231) after the temporary relief from rat predation following the (failed) eradication attempt in 2011. If we assume that warblers counted at point count locations ranged up to 100 m from the location, and that 73% of the island is suitable habitat, the global estimate for the species in 2015 would be 7194 (95%CI 5875 - 8593) individuals. Graves (1992) estimated the population at 10,800 individuals, equating to c.7,200 mature individuals, and thus similar to Graves' (1992) extrapolations which were based on an entirely different census method.
There are no new data on population trends; however, the species's entire range is within a World Heritage Site and despite limited predation by Pacific rats Rattus exulans it is thought to be stable.
Acrocephalus taiti is endemic to Henderson, an uninhabited island in the Pitcairn Islands (to UK).
This is a forest species, typically found in family groups, foraging at all levels, and has a varied diet, which is known to include several species of land snails, ants, flies, beetles, cockroaches, large wasps, seeds and fruit pulp (Graves 1992). It has a well-defined breeding period from late August to early January, nesting in a wide variety of tree species in the lower canopy of forest. The species breeds either in pairs or trios, commonly of unrelated birds, probably a consequence of the island's stable habitat and climax forest, where young birds may be more able to secure a nesting territory when belonging to a trio than a pair (Brooke and Hartley 1995). Clutch-size is two to three and, once hatched, most chicks fledge. Predation by introduced Pacific rats (Rattus exulans) is the main cause of nest failure (Brooke and Hartley 1995).
Although Pacific rat R. exulans is common throughout the island, and has been observed in the upper branches of trees, the warbler appears able to coexist with it (Graves 1992). However, the temporary recovery of A. vaughani on Pitcairn Island, after the removal of feral cats and reduction of rats, indicates that rats probably suppress the population (B. Bell pers. comm. 1999). In August 2011, a rat eradication operation was carried out on Henderson Island to and although this eradication failed, there was a temporary increase in the A. taiti population between 2011 - 2015 (A. Bond & S. Oppel, in litt. 2016). The accidental introduction of other predators (especially other Rattus species) and diseases remain a threat as the island, although isolated, is visited by the passengers and crew of passing ships (Graves 1992).
Conservation Actions Underway
In 1988, Henderson was designated a World Heritage Site. Following a feasibility study (Brooke and Towns 2008) a rat eradication attempt was carried out on Henderson Island in August 2011; this failed and rat numbers had recovered to pre-eradication levels by 2013 (Churchyard et al. 2013). A follow-up monitoring expedition in 2015 indicated that the A. taiti population had increased from 2011-2015 (A. Bond and S. Oppel in litt. 2016).
17 cm. Large warbler with relatively short bill. Adult olive-brown above, white below with slight yellowish tinge. Mottled olive-brown and white crown, rest of head white with dark streak through eye. White feathers variably and often asymmetrically scattered among darker feathers. Some individuals are nearly all white. Voice No song. Call a harsh, short note.
Text account compilers
Mahood, S., O'Brien, A., Shutes, S., Stattersfield, A., Derhé, M., Stringer, C.
Bond, A., Bell, B., Oppel, S., Hall, J., Brooke, M.
BirdLife International (2019) Species factsheet: Acrocephalus taiti. Downloaded from http://www.birdlife.org on 22/11/2019. Recommended citation for factsheets for more than one species: BirdLife International (2019) IUCN Red List for birds. Downloaded from http://www.birdlife.org on 22/11/2019.