Northern Marquesan Reed Warbler Acrocephalus percernis Scientific name definitions

18–22 cm. A large, bulky reed-warbler, bright yellow below, with greenish upperparts, medium-length, slightly curved bill, and yellow-tipped tail. Probably indistinguishable from allopatric A. mendanae in a field situation. Sexes similar. Juvenile poorly known; on Uahuka (race idae) has back hair-brown. Races differ only in measurements and in subtle shades of colour: nominate has size and structure of A. mendanae, but has rich citrine-yellow wash to supercilium, ear-coverts and neck-, throat- and breast-sides, which becomes less intense on flanks , belly and undertail-coverts, and upperparts are slightly greener with brighter yellowish fringes to feathers; race aquilonis has structure and plumage similar to race dido of A. mendanae, but has slightly smaller bill (mean 20 mm, versus 21·5 mm), marginally darker upperparts and paler underparts (less intense yellow); race <em>postremus</em> is similar to last-named race in size, structure and plumage, but has the upperparts slightly more olive- or greyish green (and less yellowish); and race <em>idae</em> is the smallest race with a short slender bill (mean 18·8 mm) and short wing (mean 89 mm) and tarsi (29·8 mm), versus 98 mm and 31·8 mm in nominate, and has the crown to mantle slightly paler than A. mendanae and the fringes to the greater wing-coverts and tertials slightly broader and warmer yellow.

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Editor's Note: Additional distribution information for this taxon can be found in the 'Subspecies' article above. In the future we will develop a range-wide distribution article.

Occupies most vegetation types, other than low, sparse grassy areas, but is most abundant in dense bushes, thickets, secondary forests and coconut groves, provided there is understorey, and is less so than in lowland forests or montane rain forests. Has successfully colonized large areas of invasive vegetation, especially Leucaena leucocephala on Nuku Hiva and Ua Huka, and Acacia farnesiana on Eiao. On islands also inhabited by endemic monarchs (Pomarea spp.), reed-warblers frequent drier, low-lying and more xerophytic areas than these species. Recorded from sea-level to c. 1100 m on Nuku Hiva and 750 m on Ua Huka, but to lower elevations on both other islands (whose summits are lower).

Diet presumably identical to that of A. mendanae.

Song of nominate race is a loud, comparatively long melodious varied phrase of whistles  , warbles, harsher churring notes and rasping scolds (duration c. 4–10 seconds, frequency range 1–5 kHz) , followed by a pause of similar duration before shifting to another slightly different phrase and so on; may continue for several minutes. Call a harsh grating “grrrah”. Subspecific differences poorly known, but race <em>idae</em> is slower with more churring notes (like calls) that run into short fluid sequences that last 1–2 seconds, whereas nominate has more fluty, oriole-like phrases, separated by shorter pauses. On Nuku Hiva, nominate race frequently performs a distinctive song flight up to c. 20 m above the canopy, but usually lower, parachuting downwards in long, slow glide to a perch close to the top of a bush or tree, often still singing (this behaviour is unknown in A. mendanae).

Nests, territorial behaviour and other aspects of breeding biology apparently very similar or identical to A. mendanae, but rigorous comparative studies are lacking and see Voice, above.

Not globally threatened (Least Concern). Restricted-range species: present in Marquesas Islands EBA. Nominate race is common throughout the island of Nuku Hiva, within an overall area of suitable habitat recently estimated at 250 km², it formerly being particularly common in indigenous Metrosideros collina forest on Toovii Plateau, but this area was cut in 1970s and replaced by exotic Pinus caribaea, which are not used by reed-warblers and these non-native trees now cover 25·5% of the island. Nevertheless, surveys of Nuku Hiva in 2004 and 2010, focused on Terre Déserte and Taiohae, recorded densities of up to one territory per 3·25 ha and estimated the total number of reed-warbler territories to be in the region of 7700. Race postremus is rare on island of Hatutu and has apparently always been so (even at the time of its discovery in 1922), with an estimated population of just 30–50 pairs in the 1970s. A follow-up survey considered that numbers were stable in 2007, but a marked decline was noted in Jun and Nov 2010, with just c. 15 individuals being found during the last-named month when a severe drought had caused complete defoliation of bushes and trees on the island. Race aquilonis, which is confined to Eiao, was believed to be endangered (even being speculated to be extinct at one time), having been rare when discovered in 1922 and was apparently confined to secondary scrub on the island’s central plateau in early 1970s. However, surveys in 1987 suggested a population of 100–200 pairs, whose habitat was being negatively affected by sheep overgrazing, but subsequently invasive trees have provided this race with additional suitable woodland. The area of favourable habitat was most recently estimated at 12 km², with a mean density of one territory per 3 ha and a total population of reed-warblers of c. 400 pairs in 2010. Race idae was abundant during surveys in the early 1970s of Ua Huka; subsequently, in 2006 and 2009, found c. 50 km² of suitable habitat within which density was estimated at one territory per 3 ha and an overall estimate of c. 1700 territories.