BIRDS' Nest Soup could soon become a rarer treat as conservationists work to save the swiftlets whose saliva is the traditional delicacy's main ingredient. A multi-million-dollar industry has grown around nimble collectors who scramble up bamboo poles to harvest 20 million nests a year from caves around Southeast Asia. Most of the nests come through Hong Kong, which is the global hub for the tiny saucers of saliva extracted from the nests. The dish they provide then sells in restaurants for $150 to $400 a bowl. However, this industry and widespread smuggling are driving the birds to extinction. Conservationists are moving to bring in controls and prevent over-harvesting. A 35-page document on the trade will go before the United Nations' Convention on International Trade in Endangered Species in Florida in November. Entitled International Trade in Swiftlet Nests, with special reference to Hong Kong, it gives a full rundown on the effects of the industry. If the UN meeting approves moves sponsored by the Italian Government to have the swiftlets' nests ''listed'', all collectors and traders will have to be licensed. David Melville, joint author of the report and executive director of the World Wide Fund For Nature, said he was staggered by the trade. ''We reckon for 1990 there were 19 million nests in the global trade, but most were meant for Hong Kong,'' he said. The industry was more valuable than the ivory trade in the 1980s. Mr Melville said only three of the 30 species of swiftlet produced the majority of nests harvested in the main distribution countries of Thailand, Malaysia, Singapore, Indonesia, Burma and Vietnam. In the report he said: ''Harvesting pressure in many areas has increased and may be continuous throughout the year, with eggs and chicks being destroyed at the time of collection. This has resulted in marked declines in some populations of swiftlets and extinction of others.'' The report, which the Hong Kong Government is already using as a guide, says nests worth $1.09 billion came into Hong Kong between 1989 and 1991, the latest years for which figures are available. At the same time, nests worth $80.7 million were re-exported, mainly to Japan, China and the United States, after being packaged by the five large companies that control the market. The Agricultural and Fisheries Department, which will be responsible for enforcing licences, said Hong Kong traders were reluctant to reveal import details, even though they might not have a future. ''If it is approved we will have our own ordinance and they will be required to apply for licences,'' Department officer Dr So Ping-man said. ''But before we issue any licences we will ask then to provide a licence from the exporting country.''