Acrocephalus rehsei

Otto Finsch was the first naturalist to visit Nauru . He traveled from the Marshall Islands to the Solomon Islands and landed on the island on July 24, 1880 for six hours. His report for 1881 included the reed, originally identified as Acrocephalus syrinx . In 1883, Otto Finsch concluded that this was a new species, Calamoherpe rehsei ^[1] . The generic name Calamoherpe is currently considered as a synonym for the genus Acrocephalus ^[3] . Acrocephalus comes from the Greek words akros ("upper") and kephale ("head"). The first part of the generic name, akros , characterizes the pointed shape of the bird's head and is probably due to confusion with acutus ^[4] . The species epithet rehsei is given in honor of Ernst Reze , a German ornithologist and collector and one of Finsch's fellow travelers ^{[5] [6]} . Since the description of Otto Finsham, Acrocephalus rehsei has rarely been the subject of research ^[1] , so many aspects of its ecology and lifestyle remain unknown ^[7] .

Although Acrocephalus rehsei is generally regarded as a separate species, as evidenced by recent DNA studies ^[8] , some authorities, such as Hans Walters in Die Vogelarte der Erde (1980) and Richard Howard and Alik Moore in A Complete Checklist of the Birds of the World ”(1991) consider it a subspecies of Acrocephalus luscinius ^[2] . The population of Acrocephalus rehsei is not divided into subspecies ^[9] .

An analysis of mitochondrial DNA in 2011 showed that Acrocephalus rehsei forms a clade with Acrocephalus australis , Polynesian reed ( Acrocephalus aequinoctialis ), Acrocephalus mendanae and now extinct Acrocephalus yamashinae from the island of Pagan . The latter species is the closest cousin of Acrocephalus rehsei ^{[8] [10]} .

Acrocephalus rehsei is a medium-sized reed with a light build. The upper part of the body is dark brown, the tail and the tartar are lighter in color. The plumage on the stomach is lighter than on the back. The central part of the chest is dull brown-yellow, reddish-brown on the sides. The face contrasts little with the overall color. Ear coverts, crown, nape, chin and throat are pale brown. The bridle is dark brown. From the beak to the ear feathers, a creamy “eyebrow” or supercilium extends. The beak is long, thin and straight ^{[9] [7]} . The beak is dark gray with pink edges; the mandible is pink, darkening towards the tip. Metatarsus and feet are dark gray. The wings are short, rounded, do not reach the tail feathers, which visually increases the length of the tail ^[9] .

The body length is about 15 cm, the wingspan is 6.7–7.2 cm. Sexual dimorphism is not pronounced. The structural and color features of young individuals remain unknown ^{[9] [7]} .

Acrocephalus rehsei is the only representative of the Passeriformes inhabiting Nauru ^[7] , and therefore it cannot be confused with other species ^[9] .

The colonization of the Pacific islands and Australia by representatives of the genus of real reeds ( Acrocephalus ) was complex, with numerous populations of remote archipelagos. Although the Hawaiian islands were colonized 2.3 million years ago, other islands were reached by birds much later, in the middle of the Pleistocene (0.2-1.4 million years ago) or even later ^[8] . Species of Acrocephalus - Acrocephalus syrinx geographically closest to Acrocephalus rehsei from the Caroline archipelago and the extinct Acrocephalus luscinius from the island of Guam ^{[9] [11]} .

Acrocephalus rehsei is one of two, along with the Micronesian fruit-eating pigeon ( Ducula oceanica ), endemic to the Pacific island of Nauru . It occurs throughout the island, lives in shrubbery growing in the area of ​​developed phosphorite quarries, as well as in preserved forests on the central plateau ^[1] . The bird is most numerous in forest fragments covering steep slopes ^[7] ; also observed in gardens and wastelands along the coast. Otto Finsch in 1881 characterized Acrocephalus rehsei as a numerous species, "as common as the house sparrow in England." Biologist Donald Boudin in 2008 confirmed this observation ^[1] .

Acrocephalus rehsei is a sedentary bird that never leaves the island of Nauru. On the island of Banaba , closest to Nauru and very geologically similar to it, any reeds are completely absent ^[9] . There is a hypothesis that populations of Acrocephalus rehsei previously existed on other islands until relatively recently. In the Marshall Islands, there are tales of a bird called annañ , anang and annãng , which was considered the property of the leaders. There is no physical description of these birds, it is only known that they were the size of a butterfly and inhabited the rocky northwestern islands. Ethnologists Kremer and Nevermann reported that the bird died out around 1880. Based on descriptions of individuals seen on Jaluit Atoll, Paul Schnee suggested that annañ is Acrocephalus rehsei . Its extinction on the Marshall Islands could have been caused by the hunting of feral cats brought to the archipelago to destroy rodents ^[11] .

The nests of Acrocephalus rehsei are bowl-shaped and are constructed from grasses and twigs. As a building material, filamentous cassite ( Cassytha filiformis ) and horsetail casuarina ( Casuarina equisetifolia ) are sometimes used ^[1] . The nest is located on trees or shrubs at a height of 2 to 8 meters or on the ground, in the latter case, the chicks become vulnerable to attack by rats. David Pearson reported nests at a height of 45-300 cm from the ground, located in the branches of hibiscus and linden ^[2] . Eggs were found in December and July, which may indicate year-round reproduction ^[1] . In the clutch there are two or three eggs ^{[1] [9]} . The duration of incubation and the age of the plumage of the chicks are unknown ^[9] . Donald Boudin noted that birds were more noisy in December than in March and April ^[1] . The voice is described as similar to the voices of the songbird and blackbird and the black-and-white fantail ^[9] . Acrocephalus rehsei is sung at any time of the day ^[12] .

The main food of Acrocephalus rehsei are various insects, mainly dragonflies ^[1] . Six species of dragonflies are distributed on Nauru: Anax guttatus , Diplacodes bipunctata , Ischnura aurora , Pantala flavescens , Tholymis tillarga and Tramea transmarina ^[13] . Donald Boudin described three features of catching insects. Most often, birds move through trees and shrubs, catching insects on the foliage. In the second case, Acrocephalus rehsei sat close to the ground, rushed down quickly and then returned to the branches with caught prey. In open areas, birds were observed moving on the ground, occasionally grabbing the alleged prey ^[1] . In coastal areas, they prey on insects in the crowns of coconut palms ^[7] .

In the IUCN Red List, Acrocephalus rehsei is classified as a vulnerable species ( English Vulnerable ) due to its limited range and, as a consequence, its susceptibility to random environmental influences, such as cyclones and predator introductions ^[7] . BirdLife International , based on 1993 data, estimated the population to be 10–20 thousand individuals, but Donald Boudin believes that the population includes 5000 birds of all ages ^[1] , which is equivalent to 3000 adult individuals ^[7] . He also noted that habitat degradation and deforestation caused by phosphate mining did not appear to have a serious adverse effect on Acrocephalus rehsei . Moreover, shrubs grow on the site of the developed phosphorite deposits, and reeds were able to occupy the regenerating regions ^{[1] [7]} .

Unlike other Nauru birds, Acrocephalus rehsei is not hunted and is protected by the Nauru Wild Bird Conservation Act of 1937 ^[1] . In order to maintain the population at the current level, IUCN recommends conducting a regular population survey and establishing a monitoring program by training local people ^[7] . The main threat to Acrocephalus rehsei today is the predation of wild cats, dogs and rats ^[1] .

• Jobling James A. The Helm Dictionary of Scientific Bird Names . - A&C Black, 2010 .-- 432 p. - ISBN 978-1-4081-2501-4 .
• Kennerley Peter, Pearson David. Reed and Bush Warblers. - A & C Black (Bloomsbury Publishing), 2010 .-- P. 463-464. - 712 p. - ISBN 978-0-7136-6022-7 .