The complex systematics of the Acrocephalus of the Mariana Islands, western Pacific Takema Saitoh A , Alice Cibois B,D , Sayaka Kobayashi A , Eric Pasquet C and Jean-Claude Thibault C A Yamashina Institute for Ornithology, 115 Konoyama, Abiko, Chiba, 270-1145, Japan. B Natural History Museum of Geneva, Department of Mammalogy and Ornithology, CP 6434, CH-1211 Geneva 6, Switzerland. C Muséum National d’Histoire Naturelle, Département Systématique et Evolution, UMR7205 Origine, Structure et Evolution de la Biodiversité, 55 rue Buffon, and Service de Systématique Moléculaire, UMS2700-CNRS, 43 rue Cuvier, F-75005 Paris, France. D Corresponding author. Email: alice.cibois@ville-ge.ch Abstract. The Nightingale Reed-Warbler (Acrocephalus luscinius) is known from six islands of the Mariana Archipelago in the Pacific Ocean. A recent phylogeny of the reed-warblers of the Pacific islands suggested however that the species was polyphyletic, the result of at least three independent colonisations. We present here a complete phylogeny of the Mariana reed-warblers that includes two populations, from Alamagan and Aguiguan, not yet studied using molecular techniques. Both of these populations belong to the Pacific Acrocephalus radiation, with birds from Alamagan closely related to the Saipan population, and those from Aguiguan having unresolved relationships within the Micronesian clade. These results suggest that the Mariana Islands experienced multiple colonisations by reed-warblers. We use a combination of molecular phylogeny and biometry of museum specimens to propose a new species-level taxonomy for Acrocephalus of the Marianas. These results have conservation implications for the two remaining populations, on Alamagan and Saipan, which probably belong to the same taxon, Acrocephalus hiwae (Nightingale Reed-Warbler). Received 15 February 2012, accepted 25 June 2012, published online 19 September 2012 Introduction The reed-warblers (Acrocephalus) comprise more than 35 species distributed in Eurasia, Africa, Australasia and Oceania. In the Pacific Ocean, they are found from Australia and Micronesia, including the Mariana Islands, to the north-western Hawaiian Islands and eastern Polynesia (Kennerley and Pearson 2010). Located in northern Micronesia, the Mariana Archipelago (16  37 0 0 00 N, 145  37 0 0 00 E) comprises an arc of 15 islands: 10 younger volcanic islands, some of which are still active, in the north and five older islands with more complex geology, includ- ing uplifted coral limestone, in the south (Fig. 1). The islands are from 1 million to 43 million years old (Gillespie and Clague 2009). Their sizes vary from volcanoes just emerging above the sea with areas of a few square kilometres to the largest island of Guam (541 km 2 ). Reed-warblers are known from six islands in the Marianas, with museum specimens for populations from five islands – Pagan, Alamagan, Saipan, Aguiguan and Guam – but reed-warblers from the sixth island, Tinian, are known only from subfossil remains (Steadman 1999). The reed-warblers on Pagan, Aguiguan, Guam and Tinian are all extinct (see Discussion for details). A recent phylogeny (Cibois et al. 2011) of the reed-warblers of the Pacific islands hypothesised independent colonisations of Guam, Pagan and Saipan, whose populations had traditionally been considered conspecific based on morphological characters (Yamashina 1942; Baker 1951). According to the molecular phylogeny, the nominate form from Guam (traditionally A. luscinius luscinius (Quoy and Gaimard, 1830); Yamashina 1942; Watson et al. 1986; for spelling of specific name see David and Gosselin 2002) is not part of the Pacific radiation but belongs to a clade that groups the Pacific taxa with the Clamorous Reed-Warbler (A. stentoreus), the Oriental Reed-Warbler (A. orientalis) and the Great Reed-Warbler (A. arundinaceus), with no clear resolution among them. The reed-warblers from Saipan (formerly A. luscinius hiwae (Yamashina, 1942)) are basal in one of the two main clades of Pacific reed-warblers, and the birds from Pagan (formerly A. luscinius yamashinae (Takatsukasa, 1931)) belong to the other, which includes birds from Nauru, the Line Islands and the southern Marquesas. Thus, the Mariana Islands may have played a key role in the diversi- fication of the reed-warblers across the Pacific Ocean with both ancient and recent lineages from different sources. However, a complete picture of the biogeography of reed-warblers in the Marianas requires consideration of the remaining populations not yet studied using molecular techniques: those on Alamagan, classified under the nominate subspecies luscinius (Baker 1951; CSIRO PUBLISHING Emu, 2012, 112, 343–349 http://dx.doi.org/10.1071/MU12012 Journal compilation Ó BirdLife Australia 2012 www.publish.csiro.au/journals/emu