THE birdmen of Koh Yai So brandish automatic weapons and bootleg whisky, guardians of an exotic treasure that has brought violent death to their lonely ocean atoll near Thailand. Deep inside the caves that pockmark the tiny outcrop like crustaceans on a precious pearl, thousands of sparrows produce the precious ingredient for bird's nest soup - the Chinese delicacy so prized that men are willing to die for it. ''It is rare and everyone wants to get their hands on it. People have been killed trying to steal it, and they will keep coming,'' said a police officer who guards the island. Colonies of sparrows nesting on Koh Yai So and other atolls nearby secrete saliva on their nests during the breeding cycle, leaving behind the sweet mixture used to make the soup. It is so scarce that a billion-dollar trade has grown around the remote islands off Thailand's southeast coast, and a bizarre and little publicised guerilla war rages between the concession owners and poachers. One night, two months ago, a boatload of armed intruders slipped through the bamboo latticework of fishing nets strung across the mouth of the Songhla Lagoon and into the Gulf of Thailand, heading for the sparrow islands. When they passed white warning signs at the edge of the exclusion zone around the islands, the watch guards opened fire, killing 10 people on board. The main concession on the government-owned islands has been held by the same firm for four decades and is automatically renewed every five years. Its revenue is a close secret, but local businessmen estimate the contract is worth as much as 7.5 billion baht (HK$2.3 billion) annually, with a percentage going into state coffers. The lure of an industry, whose only real overhead is patience, has proven irresistible for rival firms. ''We always suspected there are some powerful (business) interests behind this,'' said the police officer. There were seven recorded attacks on the islands last year. Villagers in Chumphon and Phattalung provinces, where most of the nests are found, say dozens of people have died in other unpublicised clashes while trying to seize bird's nest shipments. On Koh Yai So, guards in sarongs with high-powered rifles peer out to sea through binoculars from tin-sheet huts riddled with bullet holes that are tethered to the rock by chains. A permanent police division armed with machine guns - probably, the only security force in the world assigned to guard sparrows - warns by radio of long-tail boats approaching the islands and ambushes by poachers aboard them. There are rumours of sophisticated radar devices hidden among the coconut palms and dense jungle foliage that crawls down the island's rugged cliffs to the sea. ''Now is the worst time, during the breeding seasons, when they know the nests are being harvested. In such times, this can be a very dangerous place,'' said another police officer. Koh Yai So in Thailand's Phattalung province is one of the largest of three dozen islands where the sparrows breed, in a belt running from Chumphon in the north to Krabi and Phangna in the west. The birds build their nests high in the rocks, gaining entry through small openings hidden by clinging ferns. They disappear during the day, returning in huge fluttering clouds in early evening to cement small twigs with their saliva in readiness for the egg-laying. Watching is the harvesting company, which has had to balance its economic needs with a careful pattern to enforce selective breeding. The birds will not lay their eggs if the nests are removed - and the sparrows would be wiped out if they couldn't nest, literally killing off the golden egg of bird's nest soup. To overcome this dilemma, three fixed extraction periods are allowed from the first week of April each year, with a four-month layoff from July to October to allow egg-hatching. The deposited saliva must be removed quickly before it deteriorates, and big teams of labourers are brought in from the mainland to work around the clock. Toiling under armed guard with only flashlights for protection, they crawl semi-naked through narrow black tunnels infested with poisonous snakes and wild monkeys that feed off the birds' eggs. The tunnels, often little more than gullies burrowed out by rainwater, climb up to caverns at the top of the island where tens of thousands of birds cling to tiny crevices in the floor, ceiling and walls. After being pulled out, the nests are taken by boat to Pak Pa Yuen, a port near the gulf town of Phattalung, and then to the regional capital of Hat Yai, where they are dried and processed into soup. A kilo of dried bird's nest fetches 30,000 to 40,000 baht (about HK$9,375 to $12,500) in Hat Yai. By the time it has been made into soup, a small bowl will cost 500 baht (HK$155) in southern Thailand and three or four times that much in Hongkong, the main overseas market. Breaking decades of silence on the lucrative concession, the Thai-Hongkong Kanjanapas family recently revealed it has been the mystery owner of the largest operation since the mid-50s. Mongkol Kanjanapas, patriarch of the Bangkok Land real estate empire, went public on the harvesting for the first time this week in an effort to end the violence surrounding the islands. ''The secrecy has created a worse problem with robbers, who go there because they think this (bird's nest) will give them power, it will make them strong. That is how big a business this is,'' he said in an interview with a Thai-language newspaper. ''They want to take the nests because it is worth more than gold.'' A family company has concessions in five coastal provinces, with three other firms holding minor contracts for islands in a further two provinces. The profits are so enormous that they are classified, and only a handful of company insiders know how much it makes each year. Even the exact number of islands has never been disclosed, though there are believed to be at least 50 contributing nests. ''Each season makes about 45 million to 50 million baht per island, or a total of 1.5 billion baht : and there are three a year,'' said a Pak Pa Yuen-based employee of the bird's nest company. ''The price is high because there is so little and it is such a difficult process extracting it. Even the processing is difficult, and not many people can do it,'' she said. With 50 islands producing roughly similar crops, combined revenue could be as high as 7.5 billion baht. The government gets little more than 15 million baht a year in tax. Hongkong Chinese discovered the source of the soup several years ago and are now the fastest-growing group of tourists in Hat Yai, combing the city's seedy red light district for premium samples of the delicacy. Apart from its rarity, the bird's nest is sought for its supposed health qualities and as an aid for virility. Quality declines between the first extraction period, when the nests are pure white and the second, when they have turned yellow. By the time October comes around, they will be bright red. Such is the mystique - and cost - of bird's nest soup that a predictable parallel market of cheap fake products has also sprung up. ''Some people use jelly instead or mix it with the real bird's nest. Customers often don't know the difference,'' said the company employee. The fake soup costs less than $10 at street stalls in Bangkok, and some of it is believed to find its way to Hongkong, Taiwan, Malaysia and Singapore, the main markets in Asia for the bird's nest. On isolated Koh Yai So, the villagers who guard the priceless sparrows never get to see the results of their labour. Paid 2,000 baht a month to keep intruders away, they are allowed visits to an inhabited island nearby every two weeks but cannot stay overnight as a security measure. They are virtually self-sufficient, spending their days making potent raw whisky from tree roots and a form of tobacco from reeds growing in the rocks. The villagers feed on wild mangoes and rice brought over by the police. ''We get paid 40 baht for every snake we kill, as they feed on sparrow eggs and reduce the harvest. There isn't much else to do out here,'' said Blurm, 46, who has spent two years on the barren rock. Another guard hasn't left the sparrow islands in a decade, braving winter mists and summer monsoons for a tiny bird that inspires a poetic respect from the superstitious islanders. Koh Yai So means Old Woman Island, a wry acknowledgement of the intense breeding cycle that takes place in its bowels. Each cave and hill on Koh Yai So also has a name, with Tham Kha Khwaay - Buffalo Leg - the largest nesting haven. ''The sparrows only come to our island because of its caves. They never go on land in their lifetimes, but they could find another place if we disturb them,'' said Blurm. ''It is our duty to protect the sparrows. To us, they are the only visitors who are welcome here.''