The warbler is said to prefer a secluded lifestyle.
Scientists say they have for the first time discovered a breeding site for the world's least-known bird species.
Little is known about the large-billed reed warbler, but researchers have found a thriving flock of the birds in a remote corner of Afghanistan.
Robert Timmins from the US based Wildlife Conservation Society discovered them when he was conducting a survey in the area.
The tiny brown bird, first spotted in 1867, has not been seen since 2006.
Mr Timmins says one of the reasons why so little is known about these birds is that they are often mistaken for other types of warblers and they are a very private species.
"Reed warblers are very good at hiding and they don't like to be seen. They usually like to skulk in thick vegetation", he said.
Mr Timmins taped the rhythmic song of these reed warblers, and with colleagues used the recording to entice other members of the colony from their hiding places.
DNA analysis confirmed this was a distinct breeding population of the large-billed variety.
While the scientists are excited by the discovery, they are concerned about the long-term survival of these birds.
Ironically the ongoing war and the remoteness of their location have helped their survival - but according to Mr Timmins, this might change.
"We don't wish a war-like situation on anybody. But once peace comes and development starts, you really do have to think about what will happen to the natural environment," he said.
As well as a flood of new information about the large-billed reed warbler, this discovery will hearten researchers that species can survive in even the most difficult of environments - there may well be other birds out there that have been presumed extinct, but are in fact surviving in some remote corner.