Environmental organisation Green Power has released the latest results of the "Overwintering Danaids Survey", summarising the data on the local overwintering Danaid population from last winter (October 2024 to January 2025). The survey covers four locations, including Deep Water Bay on Hong Kong Island, Siu Lang Shui in Tuen Mun, as well as Fan Lau and Shui Hau on Lantau Island. A total of 345 overwintering Danaids were recorded. Compared to the 451 butterflies recorded in the previous survey, this marks a 23% decline, making this the fourth consecutive year of decline. This is rarely seen.
Among the four surveyed sites, Fan Lau recorded the highest number of 217 overwintering Danaids, followed by Shui Hau with 59 Danaids. Siu Lang Shui and Deep Water Bay recorded 43 and 26 Danaids, respectively.
Huge 90% Drop in Crow Numbers
Mr Matthew Sin, Chief Environmental Affairs Manager at Green Power, noted that fluctuations in the number of overwintering Danaids were normal, but a decline for four consecutive years would be quite unusual. He explained that overwintering Danaids are mainly categorised into the "Crows" and "Tigers" species. Among them, the Crows, such as the Blue-Spotted Crow (Euploea midamus) and the Common Indian Crow (Euploea core) have suffered a staggering decline, with their overall numbers plummeting by 93% over the past five years. Deep Water Bay and Siu Lang Shui, both traditionally dominated by the Crows, have seen their numbers fall sharply, from over a thousand overwintering Danaids at their peak to just a few dozen in the latest survey. In contrast, Fan Lau and Shui Hau, where Tigers are the majority, have had relatively stable numbers over the past five years. In the latest survey, Fan Lau recorded an increase of nearly 20% in overwintering Danaids compared to the previous survey.
Weakening of the Northeast Monsoon may be a Contributing Factor
Matthew explained that the number of Overwintering Danaids in Hong Kong is influenced by a variety of factors. As the migration of Danaids spans multiple regions, any environmental changes along their route, such as urban development or farmland expansion, can destroy habitats, reduce living space, and ultimately lead to a smaller migrating population.
In addition, Hong Kong's autumn weather also plays a role in affecting the overwintering Danaids. Overwintering Danaids migrate from colder northern regions to spend the winter in Hong Kong each autumn. They gather in local areas near rivers and streams with water sources to overwinter. Looking at Hong Kong's recent autumn temperatures, it has been getting warmer since 2021. For example, the autumn high temperatures from 2021 to 2024 repeatedly broke records. The average highest temperatures in September to November of those four years ranked among the top three warmest in the past 20 years. The autumn of 2024 was the hottest on record, with an average temperature of 26.5 degrees Celsius. Matthew pointed out that Hong Kong's autumn and winter seasons are typically cooled by the cold air brought by the northeast monsoon. The increasing warmth reflects a weakening of the northeast monsoon, which could reduce the trigger for the Danaids to migrate south for the winter. At the same time, Danaids tend to cluster together to keep warm in strong cold air. Hence, the weakening of the northeast monsoon could cause the overwintering Danaids in Hong Kong to become more scattered, leading to a decrease in the number of butterflies recorded in the survey.
About Overwintering Danaids Survey
The survey, conducted by Green Power, began in 2009 and covers four locations—Deep Water Bay on Hong Kong Island, Siu Lang Shui in Tuen Mun, and Fan Lau and Shui Hau on Lantau Island. The survey begins in October and ends in January of the following year. It documents the number, types, and behaviours of the overwintering Danaids. Every autumn and winter, tens of thousands of Danaids migrate southward from the colder regions to Hong Kong. Currently, their migratory route is still unclear. Apart from Hong Kong, overwintering Danaids have also been found in Japan, Taiwan, and Hainan. It is likely that these various regions are on the same migratory routes of the Danaids.