Environmental group Green Power found a swarm of overwintering Danaids with an estimated 2,300 butterflies, something of a rarity for the sheer size of the aggregation in recent times, in southern Lantau yesterday (9 Dec). The overwintering site is in a valley of mixed woods of about 1,000 square metres wide in the country park. More than 90% of the aggregating Danaids were the Crows including Blue-spotted Crow (Euploea midamus), Common Indian Crow (Euploea core) and Striped Blue Crow (Euploea mulciber). A few Tiger butterflies were sighted, such as Blue Tiger (Tirumala limniace), Dark Blue Tiger (Tirumala septentrionis) and the very rare Chocolate Tiger (Parantica swinhoei).
Lo Wing-fung, Environmental Affairs Manager of Green Power, remarked that Danaids, known for long-distance migrations, migrate to Hong Kong from colder regions in the north every autumn and winter. Some of these migrating butterflies choose to stay in Hong Kong for the winter, while others take Hong Kong as a midway rest stop. They cluster together for warmth at night or on cold days. Several local and international groups have been tagging them to better understand the migratory routes of Danaids. Green Power started Overwintering Danaids Survey in 2009.
Lo pointed out that Lantau Island in the southwest of Hong Kong has stayed mostly natural with plenty of well-sheltered valleys suitable for Danaids to spend the winter. But Fan Lau and Shui Hau on Lantau Island, the two prominent overwintering sites for Danaids, have recorded diminishing numbers of Danaid butterflies for the last two years. Return of overwintering Danaids on Lantau Island this year is nothing less than a spectacle.
Lo noticed that Danaids overwinter in Hong Kong annually at different sites depending on a number of factors including the changes in weather, wind and the environment. In 2015 and 2022, Green Power discovered two new overwintering sites respectively on Lantau Island only to find swarms of Danaids disappeared the following years. So, the green group is yet to ascertain the discovery this time belongs to a single-year situation but will keep monitoring to shed light on the mystery of their migratory behaviour.
