I found another interesting account that I had overlooked previously in ‘Te Manu: Bulletin de la Société d’Ornithologie de Polynésie. Nr 25. December 1998’.:
“De notre correspondant Yves GENTILLOMME à Rurutu
Lors du passage de Jean-Yves Meyer sur l’île de Rurutu, Yves Gentillomme lui a signalé avoir entendu le chant facilement reconnaissable d’une fauvette (Acrocephalus sp.). Il n’y a pas de description de fauvettes à Rurutu dans la littérature mais on en trouve une sur l’île voisine de Rimatara: Acrocephalus rimitarae (autrefois considérée comme une sous espèce de Acrocephalus vaughani de Pitcairn) – cf TE MANU n°23. Il s’agit peut être d’un oiseaux en provenance de cette île. Il existe une autre observation de fauvette aux Australes sur l’île de Raivavae. Cette observation ancienne n’a jamais été renouvelée.” [1]
translation:
“From our correspondent Yves GENTILLOMME in Rurutu
During Jean-Yves Meyer’s visit to Rurutu Island, Yves Gentillomme told him that he heard the easily recognizable song of a warbler (Acrocephalus sp.). There is no description of warblers in Rurutu in the literature but there is one on the neighboring island of Rimatara: Acrocephalus rimitarae (formerly considered as a subspecies of Acrocephalus vaughani from Pitcairn) – see TE MANU n°23. It may be a bird from this island. There is another sighting of a warbler in the Austral Islands on Raivavae Island. This ancient observation has never been repeated.“
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Today the island of Rurutu does not harbor an endemic reed-warbler species but it may well have done so in former times; the neighboring island of Rimatara, however, still is home to an endemic form of reed-warbler, the Rimatara Reed-Warbler (Acrocephalus rimitarae (Murphy and Mathews)), and the bird heard singing on Rurutu in the late 1990s might indeed have been such a bird.
The Rimatara Reed-Warbler may be in the state of expanding its distributional area, or there may just be some stray birds appearing on neigboring islands from time to time.
There is, however, a very little possibility that there is an endemic Rurutu Reed-Warbler that has survived into the late 1990s.
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References:
[1] Te Manu: Bulletin de la Société d’Ornithologie de Polynésie. Nr 25. December 1998
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edited: 17.03.2020