The number of butterflies spending the winter at a key Tuen Mun site fell to a six-year low this season, possibly due to a changing climate and disruptions to migratory patterns, a study has found. According to environmental group Green Power, which conducted the study between November and last month, the number of danaidae butterflies at Siu Lang Shui - a wooded former landfill site near Butterfly Beach - dropped over 80 per cent this winter. From the 230 recorded in 2013-14, the number fell to just 41 this winter, the lowest since the green group's first survey in the winter of 2009-10. Danaidae, or milkweed butterflies, are a common subspecies. They include the crow, tiger and monarch butterflies. The group's senior environmental affairs manager, Matthew Sin Kar-wah, said numbers fluctuated from year to year. They hit a high of 5,469 in 2012-13, before dropping again this winter. It is not known what caused the sudden surge in 2012, nor the reasons for this winter's drop. "When everyone was disappointed about the low numbers, it suddenly jumped back up in 2012 and we all thought it was a recovery," said Sin. "That is why we can't say for sure whether [this year's] drop indicates a good or bad trend." Two other major sites - Deep Water Bay on Hong Kong Island and Fan Lau on Lantau Island - also saw major drops. The group suspects two factors are at play. One was a relatively warmer climate in East Asia, which may have deterred the butterflies from flying further to escape harsh winters. Another possible reason was a change in the environment of stopover points along migratory routes. Rapid urban development on the southern mainland may have altered or even destroyed natural habitats, disrupting migratory patterns, Sin said.