Philip Round, manager of conservation projects at Mahidol University in Bangkok, said he stumbled across the large-billed reed warbler in March 2006 at the water treatment project in the province of Phetchaburi.
"I was really, really excited. I've known the existence of this bird for many years through reading and old references, but I've never, ever expected to find it here," the British researcher said.
The warbler, which was last seen in northwest India in 1867, was found in grass filter beds used for household sewage treatment on the coast of the Gulf of Thailand.
"When I caught this bird, I knew that something was very different. Its appearance was different from any other Thai reed warblers," Round said, adding the bird stood out because of its long beak and short wings.
Weighing just 9.5 grammes (0.3 ounces), the 14-centimetre (5.5-inch) long male bird was believed to be about one year old. Round took a couple of its tail feathers and sent them to Sweden's Lund University for DNA tests.
Within hours, the tests confirmed the bird was the large-billed reed warbler. Round said the rediscovery of the rare bird after more than a century raised hopes among ornithologists.
"We thought it was probably extinct, but now we have proved that the bird still exists. I believe more researchers will be looking for them in India, Nepal and even Burma," he said, referring to Thailand's neighbour Myanmar.Round said his findings would be published in the International Journal