Nauru reed warbler

Taxonomy and systematics

Otto Finsch was the first naturalist to visit the island of Nauru, stopping for six hours on 24 July 1880 while travelling from the Marshall Islands to the Solomon Islands. His 1881 report included a warbler he initially identified as the Caroline reed warbler. By 1883 he considered it to be a new species, Calamoherpe rehsei. The generic name Calamoherpe is now recognised as a synonym of Acrocephalus, leading to the current binomial name. The generic name Acrocephalus derives from the Greek el, meaning "topmost", and el, meaning "head". The el part of the name may have been given through confusion with el, and taken to mean "sharp-pointed", referring to the angular head shape typical of this genus. The synonym Calamoherpe is from the Greek el, meaning "reed", and el, meaning "creeping thing". Since the original descriptions, little has been written about the species, and details about its ecology and behaviour are poorly known.

Though the Nauru reed warbler is generally accepted as a species, some authorities, such as H. E. Wolters in Die Vogelarte der Erde (1980) and Howard and Moore in A Complete Checklist of the Birds of the World (1991), have considered it a subspecies of Acrocephalus luscinius, the nightingale reed warbler. A 2011 DNA study has affirmed its status as a separate species. In the native Nauruan language, it is known as Itsirir.

A 2009 phylogenic study of the family Acrocephalidae did not include this species, and as recently as 2010 its relation with other members of the genus was unknown.

The pattern of colonisation of the Pacific islands and eventually Australia by the Acrocephalus warblers from Asia was complex, with multiple colonisations of even remote archipelagos. Although the Hawaiian islands were colonised about 2.3 million years ago, the other islands were reached much more recently, in the mid-Pleistocene (between 0.2–1.4 million years ago) or even later. The nearest other warblers geographically to Nauru are the Carolinian reed warbler and the nightingale reed warbler.

Description

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. The beak is long, thin and straight.

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. The bird measures up to 15 cm, with a wingspan of 6.7 to.

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 this species may be confused with any other. The Nauru reed warbler is slightly smaller than the Caroline reed warbler, which is also lighter in colour, with a more contrasting eyebrow. The nightingale reed warbler is substantially larger,

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. It is most common in the remains of forest found on the island's steep slopes. It is also readily observed in gardens and ruderal areas on the island's coast. In 1881, Finsch described the species as abundant, calling it "as common as the House-Sparrow in England." Biologist Donald Buden again found it widespread on the island in 2008.

The species is sedentary, meaning that the birds do not naturally leave Nauru. Banaba is the nearest island, and despite being similar to Nauru, it lacks any warblers. 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. which 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 they started to become feral.

Behaviour and ecology

Nauru reed warbler nests are cup-shaped and woven from grasses and twigs. They sometimes include Cassytha filiformis vine or Casuarina equisetifolia needles. The nests are bound to upright stems in a way typical of warblers.

Finsch described the warbler as insectivorous, feeding primarily on dragonflies. 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". Feral cats and wild rats in particular are potential threats to the Nauru reed warbler.

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." BirdLife International previously estimated that there were between 10,000 and 20,000 Nauru reed warblers, based on 1993 data, but Buden's 2006 estimate of 5,000 birds of all ages In 2008, however, Buden claimed that "habitat degradation and loss of native forest via mining operations has apparently had no major adverse effects on the population." Nauru reed warblers were observed to be common on the island, and flourishing in the scrubland left by mining.

Unlike other birds on the island, the species is not hunted, and is protected under Schedule 1 of Nauru's Wild Birds Preservation Ordinance 1937. For conservation purposes, the IUCN recommends regular surveys of the population and the establishment of a monitoring programme through training local people. It also proposed raising conservation awareness by increasing the profile of the bird.

Notes

Cited texts

References

1. BirdLife International. (2016). "''Acrocephalus rehsei''".
2. Buden 2008a, p. 8.
3. Jobling 2010, p. 84.
4. Jobling 2010, pp. 30–31.
5. Finsch named the species after [[Ernst Rehse]], a German ornithologist and collector and one of Finsch's travelling companions.Finsch 1883, p. 142.
6. Jobling 2010, p. 332.
7. Buden 2008a, p. 16.
8. (2010). "Reed and Bush Warblers". [[A & C Black]] ([[Bloomsbury Publishing]]).
9. (2003). "Nesting of the ''Acrocephalus'' warblers". Acta Zoologica Cracoviensia.
10. Dyrcz, Andrzej. (2006). "Family Sylviidae (Old World Warblers)". Lynx Editions.
11. (2009). "Multi-locus phylogeny of the family Acrocephalidae (Aves: Passeriformes) – The traditional taxonomy overthrown". [[Molecular Phylogenetics and Evolution]].
12. Saitoh, Takema. (2012). "The complex systematics of the ''Acrocephalus'' of the Mariana Islands, western Pacific". [[Emu (journal).
13. Cibois, Alice. (August 2024). "Charting the course of reed-warblers across the Pacific islands". [[Journal of Biogeography]].
14. The extinction of the ''annañ'' may have been due to hunting by cats,Spenneman 2006, p. 259.
15. Spenneman 2006, p. 260.
16. Stephen, Ernest. (1936). "Notes on Nauru". [[Oceania (journal).
17. Buden, Donald W.. (2008b). "First records of Odonata from the Republic of Nauru". Micronesica.
18. Buden 2008a, pp. 16–17.
19. Administrator of Nauru. "Wild Birds Preservation Ordinance 1937". Administration of Nauru.