Rapid development on the mainland is stopping butterflies reaching Hong Kong, local experts say. In the case of one species, the milkweed butterfly, up to 45,000 would arrive each winter until 2002, but three years later there were fewer than 100.
Pun Sui-fai, an adviser at the Fung Yuen Butterfly Reserve in Tai Po, said although arrivals had risen this season, with about 1,000 of the butterflies identified in Deep Water Bay on the south of Hong Kong Island, and there had been a rarely seen chestnut tiger, there was a long way to go before numbers reached 2002 levels.
Development in nearby mainland cities was deterring the butterflies' migration to Hong Kong, he said. The rapid construction of roads and buildings in recent years had destroyed their habitats. 'They are forced to change routes, and can't make it to Hong Kong for winter.'
Dr Cheng Luk-ki, head of scientific research and conservation at NGO Green Power, agreed that the falling numbers could be due to changes in other places, although he said more studies of the butterflies' migration routes were needed to support the claim.
'In Hong Kong, physical habitats are not undergoing big changes, and there isn't a clear trend in the change of temperature,' Cheng said. 'So, looking at the city itself, it's difficult to conclude why there is such a drop in numbers.'
But that did not mean Hong Kong was doing enough to protect habitats for butterflies, Cheng said.
The conservation group was also seeing a decline in the number and quality of butterfly hot spots in the city.
'Some butterflies with higher conservation value rely on rare plants,' he said. 'And when a natural environment is destroyed, those rare plants go as well. It does not help even if you build a park there afterwards.'
Pun's colleague Colleen Chiu Yuk-lin said the group had kept a chestnut tiger found last week in Deep Water Bay identified by markings as having flown 2,500 kilometres from Japan, and was communicating with the Museum of Natural History in Osaka for further studies.
Both groups conduct butterfly-population surveys regularly and study butterfly routes.
They catch the insects, mark the date and location of their capture on their wings with oil-based pens, then release them.
Chiu said marking did not harm the butterflies, as their wings were protected by minute scales.
TIPS FOR HUNTERS
Top 10 places to spot butterflies
Fan Lau, Lantau Island
Shing Mun Reservoir, Tsuen Wan
Deep Water Bay
Wu Kau Tang and Lai Chi Wo, near Plover Cove Country Park
Tai Po Kau Nature Reserve
Pak Tam Chung, Sai Kung East Country Park
Fung Yuen Valley, Tai Po
Lung Kwu Tan, Tuen Mun
The Peak, in the area between Lugard Road and Harlech Road
Siu Lang Shui, Tuen Mun