Environmental group Green Power has released the latest results of the "Overwintering Danaids Survey", summarising data on local overwintering Danaid population of last winter (October 2025 to January 2026). A total of 1,693 overwintering Danaids were recorded across four survey locations: Deep Water Bay on Hong Kong Island, Siu Lang Shui in Tuen Mun, as well as Fan Lau and Shui Hau on Lantau Island. This represents a significant 3.9-fold increase compared to the 345 butterflies recorded in previous survey (Winter 2024), successfully reversing a four-year consecutive decline. The rebound was mainly driven by a dramatic increase in Danaids at Deep Water Bay. Recorded populations at four sites, in descending order, were: 1,408 in Deep Water Bay, 182 in Fan Lau, 84 in Siu Lang Shui, and 19 in Shui Hau.
Every autumn and winter, Danaids inhabiting colder northern regions migrate southward to warmer areas to cluster and spend the winter. Located in the subtropical region, Hong Kong is a key stopover along the Danaid migratory route. Green Power launched the "Overwintering Danaids Survey" in 2009 to continuously track the population of overwintering Danaids and monitor changes in their migration habits across Deep Water Bay, Siu Lang Shui, Fan Lau and Shui Hau.
"Crows" Susceptible to Environmental Changes; Numbers May Soar This Winter
Mr Matthew Sin, Chief Environmental Affairs Manager of Green Power, pointed out that the overall surge in overwintering Danaids was largely attributed to the population boost in Deep Water Bay. The latest survey recorded 1,408 Danaids at this site, which is the second-highest count in the 17-year history of the survey, trailing only the peak of 1,566 recorded in 2020.
Reviewing historical data for Deep Water Bay, mass aggregations exceeding 1,000 butterflies occurred three times: 1,122 in 2017; 1,566 in 2020; and 1,408 in the latest survey. In each instance, over 90% belonged to the "Crow" species (e.g. Blue-Spotted Crow, Common Indian Crow), while "Tigers" (e.g. Ceylon Blue Glassy Tiger, Common Tiger) accounted for less than 10%.
Matthew added that besides Deep Water Bay, Siu Lang Shui is also predominantly inhabited by "Crows". Its overwintering Danaid population has likewise shown wide fluctuations over the years, ranging from dozens to thousands. In contrast, Fan Lau and Shui Hau in South Lantau are dominated by "Tigers", where population fluctuations have been relatively minor, hovering between dozens and hundreds.
Matthew believes that the migration behaviour and site selection of "Crows" are more susceptible to annual macroclimatic conditions, wind directions, and environmental factors, leading to dramatic "ups and downs". He noted that global and international meteorological agencies have forecast a Super El Niño event to occur between October and December. Looking back at the last Super El Niño in the winter of 2015, we recorded 1,596 Danaids in Siu Lang Shui in Tuen Mun, which is its second-highest count on record. Matthew hopes to see another significant surge in overwintering Danaids at Siu Lang Shui this coming winter. He will closely monitor the impacts of this climate phenomenon on Hong Kong's overwintering Danaid population, and further explore the correlations between extreme climate events, Danaid migratory routes, and population trends.
