Fanning Reed Warbler (Acrocephalus aequinoctialis ssp. pistor)

The Fanning Reed Warbler was originally described in 1883 as a full species but was later recognized as a subspecies of the Kiribati Reed Warbler, which is endemic to the giant Kiritimati atoll in the Line Islands group of Kiribati.

The Fanning Reed Warbler appears to inhabit two atolls north of Kiritimati, namely Tabuaeran and Teranina, it was originally described from Tabuaeran which was called Fanning Island at that time.

The reed warbler is now extinct on Tabuaeran but still inhabits Teraina, where it may even be quite common.

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The taxon is just discussed here because the birds from Tabuaeran and Teraina differ from each other insomuch that the upperparts of birds in fresh plumage from Tabuaeran apparently showed broad white fringes, giving them the appearance of being dusted with flour, while birds from Teraina are not known to show such white fringes and thus more closely resemble the nominate from Kiritimati. 

In my opinion the birds from Teraina may in fact prove to be subspecifically distinct from those from Tabuaeran, making the ssp. pistor being extinct and leaving the Teraina birds unnamed. But this has apparently not yet been tested.

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References:

[1] David Pearson; Peter Kennerly: Reed and Bush Warblers. Christopher Helm 2009

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edited: 21.01.2019