Nightingale Reed Warbler

Acrocephalus luscinia

Description

The Acrocephalus luscinia, (nightingale reed warbler) is a slender, pale yellowish-buff bird. Below it is pale yellow buff, and above it is rufous brown or grayish olive brown. The bill is horn colored on the upper mandible, more yellowish on the lower mandible, and rather long and slender. Feet are light gray. The various subspecies vary from about 6-8 in (15-20 cm) in length. The female resembles the male but is slightly smaller.

A. luscinia was described from Guam in 1831 as Thryothorus luscinius, but has since undergone a variety of taxonomic treatments. The two major distinct populations include the Mariana Island and Caroline Island populations. Each of these have been known by several different genera. Generic synonyms for the Mariana population include: Thryothorus, Hybristes, Tatare, and Conopoderas. Synonyms for the Caroline Island population include: Sylvia, Eparnetes, Tatare, Calamodyta, Calamoherpe, and Conopoderas. All of these names have now been preempted by the genus Acrocephalus, the genus assigned to reed warblers found throughout the old world and thought to be the ancestral stock of the Pacific island populations.

Species names applied have been nearly as varied as the generic names assigned, and include syrinx, orientalis, mariannae, stentoreus, and yamashinae. These have now been generally lumped into the single species A. luscinia.

Populations of the nightingale reed warbler within the Marianas have been assigned to various subspecies. These are: A. l. luscinia on Guam, Saipan, and Almagan; A. l. yamashinae on Pagan; and A. l. nijoi on Agiguan. Populations in the Caroline Islands all belong to the same subspecies, A. l. syrinx. The single isolated population on Nauru is known as A. l. rehsei. Common names for the nightingale reed warbler include: the ga' ga' karisu (Marianas), the limwedi (Ponape), the lichok (Truk), and the nightingale reed warbler.

Behavior

Nesting has been recorded all months of the year except January and March, which indicates active nesting throughout the year. The normal clutch is two eggs. Eggs are white, occasionally washed with a greenish tint or slight buff, and are spotted with lavender, chestnut, and black. What is believed to be a courtship display has been observed on Truk and Ponape Islands. In both cases one bird (presumably the female) remained quietly on a perch while the male sang constantly and flew back and forth repeatedly to the female. The song given was a soft warble, not the loud song typically heard. The male flew only 12-16 in (30-40 cm) away on each back-and-forth trip, and each time he approached the female, she would parry his approach with her bill.

The nightingale reed warbler is nonmigratory and no other information is available on local movement patterns.

The nightingale reed warbler feeds primarily on insects, which are picked or gleaned from the foliage. Marshall listed food items from the Saipan population as lizards, snails, spiders, and insects. He reported the species gathered its prey from the ground. Coultas, in his observations of the bird at Ponape, relates that he was able to locate the warbler by listening for the "snapping of the mandibles as the bird is catching food".

During U. S. Fish and Wildlife Service (FWS) surveys, the species was found to forage primarily in dense brushy or grassy vegetation near the ground, in freshly mowed grass of the antenna field on Moen, and in the upper canopy as well. On Truk, an individual was observed following and foraging behind bristle-thighed curlews (Numenius tahitiensis ). As the curlews uncovered clumps of mowed grass with a flick of their long bill, the warbler would eagerly take exposed prey.

The species is diurnal and no information is available on other seasonal activities.

Habitat

The various Pacific island populations have adapted to different habitat types. On Guam and apparently Pagan, the nightingale reed warbler was restricted to wetland habitats, while populations elsewhere range more extensively into brush lands and forest. On Guam the species was restricted to cane thickets and adjacent areas in and near fresh and brackish water marshes. The small population on Pagan was reported to reside near wetland vegetation surrounding small ponds.

On Saipan, the species occurs in all types of forest habitat as well as the dense vegetation around wetlands. The bird prefers the dense and varied vegetation surrounding a wetland, rather than the monotypic stands of Phragmites or Scirpus within the marsh. Outside of wetlands the nightingale reed warbler utilizes both mixed as well as the monotypic stands of introduced Leucaena leucocephala that grow over much of the island.

On Agiguan there are no wetlands, and the species is found in second growth forests that are heavily grazed by feral goats. The understory is almost completely open due to excessive grazing, and birds forage above the grazing line in the dense foliage of the trees.

In the Caroline Islands, the nightingale reed warbler is typically an edge species, but it is highly adaptable and can be found in a variety of habitats, from mature native forests of upper elevations to scrubby open fields of the lowlands. Small populations can even be found on tiny offshore islets. The species is most fond of semi-open, lowland scrub, particularly near heavily overgrown taro patches and other wetlands bordered by rank stands of Phragmites. The nightingale reed warbler commonly utilizes mixed growth at the edge of clearings. Though not normally found in large stands of man-grove, the bird frequents mangrove edges, especially where there is an abundance of rank grass or other herbaceous cover nearby. The bird is not adverse to human habitation and is commonly found within villages and towns; garbage dumps are favorite habitats. Though normally found in dense vegetation near the ground, the species also utilizes the middle and even the upper canopy levels of larger trees.

In the Caroline Islands the nightingale reed warbler is found on amazingly small offshore atolls and islets. Vegetation on the atolls consists primarily of native strand bushes, trees, and planted coconuts.

On Ponape, the species was considered to be a bird of small bushes, open country, and grasslands, but not of true forest or high elevations. It was reported that the species was common in open country or trees bordering grasslands, but it was limited to grass or fern areas which have contrasting growth, with clumps of cane or numerous bushes and grasses. The bird was not found in short grass or fern openings. Generally, the species does not use the extensive forests of the interior, though a few can be found at these upper elevation forests. Most individuals in these interior forests were found in dense Pandanus or hibiscus thickets near a stream, or in broadleaf forests, but not in the extensive montane palm forests.

On Truk, the nightingale reed warbler is found in Phragmites swamps, dense vegetation, gardens, taro patches, and second growth forests. It is believed that the species utilized the forests of Truk because the woods are dissected by small openings. On Moen, birds can often be found in the short mowed grass of the antenna field.

Distribution

Historically, the nightingale reed warbler has occurred within the Mariana Islands, the Caroline Islands, and on the single island of Nauru. In the Caroline Islands the species has been recorded on Kosrae, Ponape, Truk, and other outer islands of Yap. In the Marianas the species has been recorded on Guam, Agiguan, Saipan, Alamagan, and Pagan.

Presently, the species occurs in the Marianas and has been recorded on Guam, Agiguan, Saipan, Alamagan, and Pagan. The population on Guam is extinct, and the population on Pagan may be extinct due to volcanic eruption. The population on Alamagan is estimated at 350-1,000 pairs.

In the Caroline Islands the species has been reported on Kosrae, Ponape, Truk, and certain outer islands of Yap. The nightingale reed warbler does not occur on Kosrae now, and the original record is questionable. On Ponape the species occurs on the main island of Ponape, Ant Atoll, and Nukuoro Atoll. On Truk, the bird is found on virtually all of the vegetated islands within the Truk Lagoon and on the outer atolls of Namoluk and Lukunor. The species' presence on Yap is based on its occurrence on the outer islands of Woleai and Lamotrek.

Threats

The original listing of the nightingale reed warbler is based on the status of the Nauru Island subspecies, A. l. rehsei. This subspecies is restricted to the single small island of Nauru, which has undergone considerable destruction by phosphate mining and by military operations. Because of these threats the population on Nauru was thought to be endangered. This population is apparently still extant.

Conservation and Recovery

The Guam population is now considered to be extinct, though the same subspecies is still found commonly on Saipan. The wetland habitat in which the nightingale reed warbler was once found on Guam (Agana Marsh) should be preserved. It may eventually be possible to trans-locate birds from Saipan to Guam, though further studies are needed on Guam before such an attempt is made. The population on Agiguan may be suffering from habitat destruction from excessive browsing by feral goats, and control of goats is recommended on this island. Likewise, the population on Pagan (which may be extinct) may be suffering from habitat loss due to the grazing of feral ungulates. Control or removal of these feral animals would allow vegetation to recover. On Nauru the primary threat to the species is thought to be destruction of habitat by phosphate mining. Controlling mining to reduce losses of habitat is recommended.

There is no Federally owned or managed land on any of the islands on which the nightingale reed warbler occurs with the exception of Saipan. Here the National Park Service operates the War in the Pacific National Historic Park, and the species lives on some of the land managed for this park.

Contact

U. S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Building
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

Reference

U.S. Fish and Wildlife Service. 1998. "Recovery Plan for the Nightingale Reed-warbler, Acrocephalus luscinia. Portland, Oregon.