Butterfly numbers at the city's leading habitat bounced back this winter after hitting record lows in the previous two years, but environmentalists say global warming may be causing the migratory creatures to stay for shorter periods.
Environmental group Green Power conducted a survey that found 600 butterflies, mostly of the blue-spotted crow variety, living in Siu Lan Shui, a wooded former landfill site near Tuen Mun that is considered the most important wild habitat for butterflies in Hong Kong.
The figure was nearly a tenfold improvement on the 65 butterflies found the previous winter and the low of 41 in the year before that, which led to fears among environmentalists that the habitat might no longer be sustaining the creatures or that something had happened to dramatically reduce the numbers of this species.
'We were worried that the habitat was already dead and feared that no butterfly would return. The latest finding is a big boost for us,' the group's senior environmental affairs manager, Matthew Sin Ka-wah, said
Sin said butterfly surveys had a long history in Hong Kong, starting in around 1999 when the blue-spotted crow was seen in the area.
In 2004 and 2005, around 40,000 butterflies were found to be spending the winter in Siu Lan Shui, a site of special scientific interest.
But mystery still surrounds the resurgence of the butterfly this year, and most of the specimens had disappeared from the site by late December, a month or two earlier than in previous years.
No carcasses were found, so it seemed unlikely that the butterflies had died. But why they had left so early and where had they gone were still unknown.
Sin said one theory was that a shift in the climate had reduced the amount of time the butterflies needed to spend in their winter habitat.
'This butterfly is very sensitive to temperature and it is very likely their disappearance or early departure is related to that,' he said.
Another mystery is where the butterflies spend the rest of the year.
Research, Sin said, was almost non-existent and difficult to carry out, although the butterflies, part of the Danaidae, or milkweed, family, are believed to favour sites further north for breeding.