Large-billed Reed-warbler (Acrocephalus orinus) is an Old World warbler in the genus Acrocephalus. The species has been dubbed as "the world's least known bird". It was known from a single specimen collected in India in 1867 and rediscovered in the wild in Thailand in 2006. The identity of the bird caught in Thailand was established by matching DNA sequences extracted from feathers; the bird was released. After the rediscovery in the wild a second specimen was discovered amid Acrocephalus dumetorum specimens in the collections of the Natural History Museum at Tring.

Description Primary tip shape

This species has the upper plumage and visible portions of wings and tail olive-brown while the underside is pale creamy with the underwing and axillaries paler.

The length is about 5 inches with the tail being 2.3 inches and the wing 2.4 inch long. The tarsus is .85 inch while the bill from gape is .8 inch. The first primary measures .35 inch while the second is intermediate in length between the ninth and tenth. The closed tail appears graduated with the difference between the longest and shortest feathers being .4 inches. The type specimen was obtained in the Sutlej valley not far from Rampur.

The upper mandible is dark, but the cutting edges and entire lower mandible are pale. The tarsi, toes and claws appear pale brown. The hind claw is longer than in dumetorum. The tips of the tail feathers are pointed and more acutely lanceolate than in dumetorum or Acrocephalus concinens. The primary tips are broad and rather squarer. Recent observers note that it has a habit of fanning out its tail open as it forages.

The specimens from Afghanistan and Kazakhstan suggest that they breed in Central Asia and moult indicates that they migrate along the Himalayas to winter in northern India and Southeast Asia. Sequence variation points to a stable or shrinking population structure.

History

It was first collected by Allan Octavian Hume in the Sutlej Valley near Rampoor, Himachal Pradesh, India on 13 November 1867. This specimen (BMNH registration no. 1886.7.8. 1742) was first provisionally described as Phyllopneuste macrorhyncha (Hume, 1869) but the name was changed two years later to Acrocephalus macrorhynchus (Hume, 1871). H C Oberholser however pointed out in 1905 that this was unacceptable because a specimen from Egypt described by von Müller in 1853 as Calamoherpe macrorhyncha turned out to be Acrocephalus stentoreus; Acrocephalus macrorhynchus was abandoned in favour of A. orinus. The identity of the species was in question and until 2002 was considered as a synonym of the Clamorous Reed Warbler (Acrocephalus stentoreus). Some others considered it an aberrant Blyth's Reed Warbler. A recent re-check of the morphology and the mtDNA suggested that it was a distinct species.

Rediscovery

On March 27, 2006 a living specimen was caught at the Laem Phak Bia Environmental Research and Development Project in Phetchaburi, Thailand by ornithologist Philip Round of Mahidol University. The bird was ringed and two feathers were extracted; DNA from them was found to match the DNA of the 1867 specimen.

Based on the short and rounded wings, earlier studies had suggested that the species was likely to be a short-distant migrant or a resident. The rediscoveries of a second museum specimen from a different location and the wild specimen from Thailand suggest that this may not be so.

Some field identifications from West Bengal and central India were subsequently reported.