Pacific reed warblers reveal at least two cryptic species

Nihoa Millerbird, found on the Hawaiian island of Laysan, is one of two sister subspecies on the chain. Photo: R Kohley (commons.wikimedia.org).
Nihoa Millerbird, found on the Hawaiian island of Laysan, is one of two sister subspecies on the chain. Photo: R Kohley (commons.wikimedia.org).

A near-comprehensive look at the large reed warblers of the Pacific has revealed at least two new species and a complex colonisation history.

Many of the myriad islands of Polynesia in the eastern Pacific have their own species of reed warbler Acrocephalus, often reaching the size of the Palearctic Great Reed Warbler. While all are related to a certain extent, a revealing way of discovering their likely ancestry and colonisation history is to analyse their genetic make-up. This might also show that some species need splitting and some need shifting to other genera to help the taxonomy make evolutionary sense.

A French, Swiss and American team sampled three different mitochondrial genes from all the known Pacific reed warblers (bar two subspecies of Nightingale Reed-warbler A luscinius on the island of Alamagan, for which no sample was available) to discover this history.

As well as discovering that the forms found on the Hawaiian, Marianas and Marquesas island chains nearly all share a common ancestor, the authors also recommend at least two to three splits, owing to the within-species lack of commonality in A luscinius.  The forms on Nauru (A rehsei) and the Caroline Islands (A syrinx) have had their previously recommended specific status reaffirmed.

The team estimate that reed warblers reached the Hawaiian chain about 2.3 million years ago, but the other Polynesian colonisations were more recent. These probably coincided with changes in the climate and ocean currents in the mid Pleistocene, between 0.2-1.4 million years ago, though some may have been even more recent. Only birds on Guam, the oldest of the extant islands in the region, appear to have colonised separately from the other forms.

More unexpected was the apparent lack of a logical 'stepping stone' process of colonisation, with birds moving from island to island. Genetic distances between each form showed that the most likely scenario was  that two lineages diverged from one common ancestor, which then went on to colonise the islands in two clades, one eastward and southward to Tuamotu and into Australia, the other with a more northerly bent, spreading eastward through Hawaii to the Pitcairn and Cook Islands. In the Marquesas group, where the two clades meet, there are two different representative species from the two clades endemic to islands only 30 miles apart, at the eastern end of these routes.

A recent survey found that the Marianas had lost reed-warblers from four out of the six islands, meaning at least two subspecies have disappeared. As the genetic analysis has found that the forms on Guam, Saipan and Mariana Island itself (nominate, A l hiwea and A l yamashinae respectively) are probably all good species, this loss now appears even more heavy; a third A l nijoi may also be divergent.  The team suspect that more species will be found once all the Asian oceanic and coastal large reed warbler forms are analysed.    

Cibois, A, Thibault, J-C and Pasquet, E. 2011. Molecular and morphological analysis of Pacific reed-warbler specimens of dubious origins, including Acrocephalus luscinius astrolabii. Bulletin of the British Ornithologists’ Club 131: 32-40.
Cibois, A, Beadell, J S, Graves, G R, Pasquet, E, Slikas, B, Sonsthagen, S A, Thibault, J-C and Fleischer, R C. 2011. Charting the course of reed-warblers across the Pacific islands. Journal of Biogeography doi:10.1111/j.1365-2699.2011.02542.x.  
Marshall, A P, Amidon, F A, Camp, R J and Radley, P M. 2011.  Nightingale Reed-warbler surveys in the Marian Archipelago. Oral presentation abstract: http://www.birdmeetings.org/aou2011/viewabstract2.asp?AbstractID=6479