The bird specialists were speaking in Bangkok on the sidelines of the World Conservation Union meeting, attended by at least 6,000 officials, scientists, executives and environmentalists from around the world.
The conference, held every four years, outlines new threats to plant and animal species.
As the meeting opened Wednesday, the union issued its Red List of species threatened with extinction.
It warned that wildlife exploitation and habitat destruction could kill off more than 15,500 plant and animal species unless urgent steps are taken. The Red List said 332 bird species in Asia are threatened, 117 of them in Indonesia, the highest number of any country in the world.
"In Indonesia, the major threat is the clearance of lowland forest and lower elevation hill forests for oil-plant development, pulp and paper plantations, agriculture and commercial logging, exploitation of tropical hardwoods," Richard Grimmett, the head of the Asia division of the conservation group BirdLife International, said at a news conference.
Other threats include development in the Inner Gulf of Thailand, where plans are under way to build a roadway bridge over mud flats and inshore waters.
The inner gulf is "a vital stopover site for birds on migration from East Asia to Australasia," Grimmett said. Conservationists estimate that species like the sky-blue Caerulean Paradise-flycatcher and the Sangihe Shrike-thrush, are unique to the tropical islands of southeast Asia and cannot be found anywhere else.
Grimmett said that while few ordinary people would lament the passing of the Sangihe Shrike-thrush, the decline of even the smallest of bird species suggested the wider environment is in trouble. "Like amphibians, birds are excellent indicators of wider biodiversity loss," he said. "When they're in danger, you can be sure that the environment as a whole is suffering."