Butterfly experts are baffled by the surge in seasonal visitors to the city's leading natural habitat and are calling for more volunteers to help them tag the insects.
On Wednesday, volunteers from Tai Po Environmental Association, which manages the nearby Fung Yuen Butterfly Reserve, found more than 10,000 butterflies - mostly of the blue spotted crow variety - in Siu Lang Shui, a wooded former landfill site near Butterfly Beach at Tuen Mun, while they were conducting an annual survey.
The numbers of migrating butterflies hit record lows in the previous two years and this year's figure was a vast improvement on the 600 butterflies found at the same time last year. But it was still only a fifth of the 45,000 butterflies that wintered at the site in 2002 and 2003.
The green group said it was a mystery why the number of butterflies rose this year as Hong Kong was just one of the rest stops on the insects' winter migration route. It is not always possible to identify butterflies' country of origin, as many species occur throughout Asia.
"As we don't know about where these butterflies came from, we are not sure whether their numbers fell or rose due to changes in their specific habitat of origin," said Pun Sui-fai, the association's adviser.
Pun said that unlike Japan and Taiwan, Hong Kong lacked detailed studies of the migration routes of butterflies, a very important measure that helped conserve the delicate species.
"We have to know more about their migration routes, so we can ensure any prospective development won't block migration," Pun said.
Every year, the association recruits and trains about 40 volunteers from the science and environment fields at the city's universities to tag and conduct surveys of butterflies. It needs more volunteers this year.
"We need as many as possible, especially when we have so many butterflies coming in this winter," Pun said.
To tag a butterfly, researchers must catch the insect, mark the date and location of its capture on its left wing with an oil-based pen, and then release it. They hope to tag at least 1,000 at Siu Lang Shui this month.
On December 31 last year, researchers trapping butterflies at Deep Water Bay on Hong Kong Island found a chestnut tiger with a code written on its wings. Checks revealed the butterfly had been tagged in Osaka, Japan, and had flown for 83 days, covering an astonishing 2,500 kilometres.