In 2006, not long after the large-billed reed warbler was confirmed as a separate species, a keen-eyed bird ringer in Thailand captured a live bird that proved to be the second known individual, with a few others found there subsequently. This demonstrated that not only did the species survive, but that it could be found over 3,000 kilometres from its original site. Then a second, previously overlooked, specimen was found in the Natural History Museum bird collection. It had been collected in northern India in 1869.
Following this, a team led by Swedish ornithologist Lars Svensson turned up no fewer than 10 further museum specimens, including 3 more here at the Natural History Museum, 5 in the American Museum of Natural History and 2 in the University of Michigan Museum of Zoology. All were confirmed by DNA analysis and had been collected between 1879 and 1937 in countries from Kazakhstan and Afghanistan in the north, to Burma in the south.
These previously overlooked museum specimens helped confirm that the large-billed reed warbler was a migrant - breeding somewhere in south-central Asia and migrating around the western end of the Himalayas and then south-east across northern India, with some wintering as far south as Burma and Thailand.
These results have facilitated targeted field research, with the first breeding populations now discovered in mountain valleys in Afghanistan and Tajikistan in 2008 and 2009.
Acrocephalus orinus is known to breed in mountain valleys of north-east Afghanistan and south-east Tajikistan at altitudes of approximately 2,000–3,200 metres, and probably more widely in the general region.
The birds inhabit relatively tall (over 2m) dense riparian bushland composed of various shrubs, in particular sea buckthorn (Hippophae sp.).
Individuals on wintering grounds in Thailand have been trapped in grass filter beds of a water treatment site and in tall grass or reeds in alluvial flood plains.