Each team member settled at the allocated spot, clutching a large white net, ready to swoop. They didn't have to wait long. Without warning, vast clouds of butterflies woke with a spectacular burst of energy. The experts have recorded as many as 10,000 butterflies in the area - mostly of the blue-spotted crow variety.
These butterflies take a break here during their migration every winter, from November to January. They behave similarly to monarch butterflies, which travel 4,000 kilometres from Canada to Mexico to mate and breed.
But unlike the monarchs, we know little about these butterflies.
"We don't know where they come from and where they will head," says project officer Colleen Chiu Yuk-lin.
That's why, starting in 2010, Fung Yuen launched a yearly project to tag at least 10 per cent of the butterflies found in Siu Lang Shui. This year, they have trained a team of 20 students from secondary schools and universities.
Volunteers use an oil-based marker to gently write on the lower left wing of each butterfly when and where it was found. For example, 1120 HKS108 means it's the 108th butterfly recorded in Siu Lang Shui in Hong Kong on November 20. They also record each butterfly's gender, size and wing quality on a tally sheet before releasing it.
If the butterfly is captured again by the group's network of partners in Asia, including China, Taiwan and Japan, Fung Yuen can establish their trail.
Last October, a chestnut butterfly tagged 83 days earlier in Hong Kong was discovered in Wakayama in Japan - 2,500km away. Chiu says this species migrates within Japan and to Taiwan, so it was a pleasant surprise to know some travel longer distances. "This proves tagging works," she says.
Most migratory butterflies gather in Siu Lang Shui, but some are also found in Deep Water Bay, Fau Lau (the south-westernmost tip of Lantau Island) and Shing Mun Reservoir. Nicknamed Butterfly Valley, Siu Lang Shui is a landfill-turned-conservation site covered with Taiwan acacia, brush box and eucalyptus. Fung Yuen volunteers need to obtain a special permit from the Environmental Protection Department to enter the zone.
Chiu says the butterflies prefer Siu Lang Shui because the woods offer shelter from predators and unforgiving winds.
What's worrying is that their numbers have plunged from a record of more than 45,000. Chiu fears that property development in New Territories West and on the mainland have blocked the butterflies' migration routes, but hopes Fung Yuen's work will mean a brighter future.
"With more information coming from the study, we hope we can argue for the butterfly's case and protect areas around Siu Lang Shui from development," Chiu says.
An expert tries to capture butterflies. Photo: Jonathan Wong