According to a survey report by Green Power on Sunday, the number of Danaids overwintering in four sites across Hong Kong, including Deep Water Bay of Hong Kong Island, Siu Lang Shui of Tuen Mun, Fan Lau and Shui Hau of Lantau, was recorded as 982 between last October and this January, marking a 15 percent decline year-on-year.

Siu Lang Shui witnessed 601 Danaids and Fan Lau possessed 331. Shui Hau has only 50 butterflies visited, while Deep Water Bay saw no Danaids for the first time since 2009.

"The number of Danaids has also changed within the last decade. Siu Lang Shui once saw more than 1,000 butterflies in 2012 and 2015," said Helen Yau, Education & Project Manager of Green Power.

She added that Deep Water Bay originally had more than 1,000 butterflies in 2017 and 2020, and the ecological environment has mostly stayed the same. "[The sharp decrease in butterflies' number] may be due to human activities," she said.

Yau added the two new overwintering sites of Danaids in Lamma Island and Lantau, which were found last year, showed no activity records of the species anymore this year.

"Every year in autumn and winter, thousands of Danaids come to Hong Kong from the chilly regions in the north. Their habitats were easily impacted by weather and environment," Yau said.

She called for more natural woodland reserved to accommodate Danaid butterflies for overwintering.