The Nauru Reed Warbler (Acrocephalus rehsei) is a passerine bird endemic to the island of Nauru in the Pacific Ocean. It is a medium-sized warbler with dark brown upper-parts, and cream under-parts, with a long, thin beak. Details about the species are poorly known. It feeds on insects, and is found throughout the island, which has changed substantially in recent decades due to phosphate mining. It is one of only two native breeding land-birds on Nauru, the other being the Micronesian Pigeon, and is the only passerine found on the island. Although no details of its relationship with other warblers are known, other nearby warblers include the Carolinian Reed Warbler, with which it was initially confused, and the Nightingale Reed Warbler, with which it is sometimes considered conspecific. The Nauru Reed Warbler is threatened by introduced predators and habitat loss, and its small range means that it could be vulnerable to chance occurrences, such as tropical cyclones. Reports of a similar bird from other islands suggest that it may once have been found elsewhere, but was driven to localised extinction by introduced cats.

Taxonomy and systematics

Otto Finsch was the first naturalist to visit the island of Nauru, stopping for 6 hours on 24 July 1880 while travelling from the Marshall Islands to the Solomon Islands. In 1881, he reported on species he had observed, including a reed-warbler he identified as the Carolinian Reed Warbler. In 1883, he described it as a new species, Calamoherpe rehsei.^[2] Finsch named the species after Ernst Rehse, one of his travelling companions.^[3] Since Finsch's original descriptions, little has been written about the species,^[4] and details about it are poorly known.^[1][5]

Its relation with other members of the genus are unknown;^[5] a 2009 phylogenic study of the family Acrocephalidae did not include analysis of the species.^[6] The closest other warblers geographically are the Carolinian Reed Warbler and the Nightingale Reed Warbler.^[7][5] Though generally accepted as a species, some authorities, such as H. E. Wolters, R. Howard and A. Moore, consider it a subspecies of Acrocephalus luscinius, the Nightingale Reed Warbler.^[8] When recognised as a species, it is considered monotypic, meaning there are no recognised subspecies.^[5] It is known by the English common names Finsch's Reed-warbler, the Nauru Reed-warbler, and the Nauru Reed Warbler.^[1] In the native Nauruan language, it is known as Itsirir.^[4]

Description

The Nightingale Reed Warbler (pictured) is somewhat larger, though the two are sometimes considered conspecific.

The Nauru Reed Warbler is a medium-sized and warmly coloured reed warbler, with a relatively light build. The entirety of the upperparts are dark brown, with the rump and uppertail coverts slightly brighter than the tail and mantle. When closed, the wing is the same colour as the mantle, short and rounded. The wing does not reach the start of the tail feathers, which enhances the appearance of a long tail. Close inspection of the wing reveals darker centres to both the greater coverts and tertial feathers. Its face shows little contrast, as the ear coverts, crown, nape, chin and throat are all a similar shade of pale brown. The lores are a dark brown, and there is a pale, creamy supercilium, or "eyebrow", extending from the beak to the ear coverts, which are a cinnamon-brown, darkening and merging with the nape.^[5] The beak is long, thin and straight.^[5][9]

The underparts are much lighter, darkening towards the vent and undertail coverts. The chin is a dull cream, merging with the throat, which then browns towards the base. The centre of the breast is a dull brown-yellow, while the sides are a reddish-brown. The upper-mandible of the beak is dark grey with pink edges, while the lower mandible is pink, darkening towards to the tip. The legs and feet are dark grey. It measures up to 15 centimetres (5.9 in), with a wingspan of 67 to 72 millimetres (2.6 to 2.8 in).^[5]

The species exhibits no sexual dimorphism, and characteristics of the young are unknown. As the only passerine on the island, there is no chance that the species may be confused with another. It is slightly smaller than the Carolinian Islands Warbler, which is lighter in colour, with a more contrasting eyebrow. The Nightingale Warbler is substantially larger.^[5] The Oriental Reed Warbler has a duller colouration, with whiter underparts.^[10]

Distribution and habitat

The Nauru Reed Warbler is endemic to the island of Nauru, in the Pacific Ocean. It is one of only two indigenous land birds which breed on the island, the other being the Micronesian Pigeon. The warbler can be found throughout the island, thriving in the scrubland in areas previously used for phosphate mining, as well as the remaining patches of forest on the island's central plateau.^[4] It is most common in the remains of forest found on the island's steep slopes.^[1] It also commonly observed in gardens and ruderal areas on the island's coast.^[4] In 1881, Finsch described the species as abundant, calling it "as common as the House-Sparrow in England". Ornithologist Donald Buden again found it common on the island in 2008, observing that "Habitat degradation and loss of native forest via mining operations has apparently had no major adverse affects on the population."^[4]

The species is currently sedentary. Banaba is the nearest island to Nauru, and despite being similar, it lacks any warblers.^[5] However, it is possible that populations of the Nauru Reed Warbler existed on other islands until comparatively recently. On the Marshall Islands, traditional stories refer to a small bird, known variously as annañ, anang and annãng. This bird was considered the property of chieftains. Though no physical descriptions exist of the species, it has been described as butterfly-sized, pleasant-smelling and as living among rocks on the shores of north-western islands. Ethnographers Krämer and Nevermann reported that the bird became extinct or extirpated around 1880. Based on descriptions of birds seen on Jaluit, Paul Schnee hypothesised that the annañ may have been a Nauru Reed Warbler.^[7] The extinction of the annañ may have been due to loss of swampy habitats or hunting by introduced cats.^[11] Cats were introduced to the Marshall Islands by the Russian Otto von Kotzebue in 1817 to hunt rats. They then multiplied, before being spread by locals as pets, after which point they started to become feral.^[12]

Behaviour and ecology

External audio

Nauru Reed Warbler call, recorded by Donald Buden, 22 Dec 2006. Macaulay Library, Cornell Lab of Ornithology, Cornell University.

Nauru Reed Warbler nests are made up of woven grasses and twigs, and are cup-shaped. They sometimes include Cassytha filiformis vine or Casuarina equisetifolia needles.^[4] The nests are bound to upright stems in a way typical of warblers.^[8] Buden reported that the warblers nest in trees and shrubs at a height of 2 to 8 metres (6.6 to 26 ft). The species may also nest on the ground; the young in ground nests may be more vulnerable to predation by rats. Eggs have variously been reported in December and July, and ornithologist A. Pearson suggests that the species may nest all year round.^[4] The clutch size is unclear,^[5] with between two and three eggs reported.^[4] Incubation and fledgling periods are unknown.^[5] Pearson reported lower nesting than Buden, recording nests in bushes and undegrowth between 45 and 300 centimetres (18 and 120 in) from the floor, especially in forked braches of hibiscus and lime.^[8] Buden reports that the birds were more vocal in December than in March and April.^[4] The call has been described as similar to that of a Song Thrush or Common Blackbird.^[5]

Finsch described the warbler as insectivorous, feeding primarily on dragonflies.^[4] Six species of dragonfly have been observed on the island; Ischnura aurora, Anax guttatus, Diplacodes bipunctata, Pantala flavescens, Tholymis tillarga and Tramea transmarina.^[13] Buden did not observe the species feeding on dragonflies, but did see three different feeding habits. Most frequently, the birds were seen moving through trees and shrubs, catching prey on the foliage. Other birds were seen perched close to the floor, darting to the ground, and returning to the perch with prey. In open areas, the birds were observed moving across the ground, "occasionally grasping a presumed prey item".^[4] In coastal areas, it has been observed feeding in coconut trees.^[1] Potential predators for the birdlife of Nauru in general include feral cats and dogs, as well as the Polynesian rat and the Tanezumi rat.^[14] Feral cats and wild rats are potential threats to the Nauru Reed-warbler in particular.^[4]

Status

The IUCN lists the species as "Vulnerable", because "its very small range leaves it susceptible to chance events, such as cyclones and the introduction of alien predators."^[1] BirdLife International previously estimated that there were between 10,000 and 19,999 Nauru Reed Warblers, but, based on Buden's estimate of 5,000,^[4] this has been revised down.^[9][1] Unlike other birds on the island, the species is not hunted, and is protected under the Wild Birds Preservation Ordinance 1937.^[15] For conservation purposes, the IUCN recommends regular surveys of the population and the establishment of a monitoring programme, through training of people local to the species's range. Further, they recommend raising conservation awareness by increasing the profile of the bird.^[1]

Notes

Cited texts

• Buden, Donald W. (2008). "The birds of Nauru" (PDF). Notornis 55 (1): 8-19. http://notornis.osnz.org.nz/system/files/Notornis_55_1_8.pdf.
• Finsch, Otto (1883). "XIV.—On a new Reed-Warbler from the Island of Nawodo, or Pleasant Island, in the Western Pacific". Ibis 25 (2): 142-4. doi:10.1111/j.1474-919X.1883.tb05490.x. http://archive.org/stream/ibis511883brit#page/142/mode/2up.
• Spennemann, Dirk H. R. (2006). "Extinctions and extirpations in Marshall Islands avifauna since European contact; a review of historic evidence" (PDF). Micronesica 38 (2): 253-66. http://university.uog.edu/up/micronesica/abstracts_38/pdfs_38/Spennemann.pdf.