Large-billed Reed Warbler

The 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. A breeding area was found in Afghanistan in 2009 and studies in 2011 pointed to its breeding in Kazakhstan and Tajikistan. One bird was found in the Baikka Wetland in Srimangal, Bangladesh on 7 December 2011.

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 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 A. dumetorum. The tips of the tail feathers are pointed and more acutely lanceolate than in A. dumetorum or A. 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.