A total of 13 Danaid species have been recorded in Hong Kong, all of which have the habit of overwintering in groups. From October to November each year, local Danaid populations rapidly increase. Many come from regions north of Hong Kong, migrating south in October to find a warm overwintering site. They return to the north in February to March the following year, or may reproduce in the overwintering site.
In Hong Kong, there are about 10 known overwintering sites for Danaids. These sites are often densely wooded, providing habitats and shelter from wind and rain. Water supplies such as perennial streams are also present at the overwintering sites. Siu Lang Shui, Tuen Mun, is the largest known overwintering site for Danaids in Hong Kong; during the peak period in recent years, as many as 40,000 overwintering Danaids were recorded.
In Asia, overwintering Danaids are found in Hainan, Taiwan and Japan besides Hong Kong. Yet their migratory routes and patterns are still largely unknown to us.
In 2009, we launched the Overwintering Danaids Survey to collect more data on the habits, distribution and routes of the overwintering Danaids. In recent years, we have discovered new overwintering sites from time to time. We hope that our effort can provide important data to safeguard the overwintering Danaids and their habitats.