Butterfly's amazing flight from Japan to Hong Kong

On December 31 last year, researchers trapping butterflies at Deep Water Bay on Hong Kong Island found a chestnut tiger with writing on its wings. In fine marker pen were numbers, letters and characters, including a code: "YSK541".

It was a thrilling find. The team from the Tai Po Environmental Association Fung Yuen Butterfly Reserve had caught hundreds of butterflies and only this one bore such writing. Checking information from elsewhere, they found it had been marked in Osaka, Japan, on October 10.

In the 83 days since, it had flown an astonishing 2,500 kilometres - the longest known journey of any butterfly species, after North America's monarch.

Monarchs are migrants that have been studied extensively. Some breed as far north as Canada, from where adults born in summer fly to Mexico, joining huge congregations. Early in spring, they leave for the southern United States to breed and die. Butterflies are born over the summer, with some moving north until members of the fourth or fifth generation reach Canada.

So do any of our local butterflies migrate in similar ways? No one knows, though there are several migratory species of tigers and crows in the danaid family.

The best information on Asian butterfly migrants is from Taiwan, where tens of thousands of purple crows travel to the south of the island for winter and north again in spring.

Some chestnut tigers marked in Taiwan have reached Japan in summer and others marked in Japan have made the reverse journey in autumn.