We recently received a request for more information about the Purple Milkweed Butterfly, which was the subject of a video covering their annual mass migration in Taiwan. As the winter monsoon moves down from the Northeast, thousands of Purple Milkweed Butterflies make a mass migration to south Taiwan, where they shelter in the warm valleys at the foot of Dawu Mountain. Spectators have described the spectacle as like watching a purple river coming around the mountain. Scattered with white dots over glistening blue and purple wings, the irridescent colour continually changes hue in the sunlight.
Milkweed butterflies are so-called because their larvae feed on milkweed plants, which cause the butterflies to be distasteful and toxic, an effective deterrent against predators. They are also known as Crow butterflies and the migrating purple crow butterflies are classified as members of the Euploea genus (Euploeini).
Kelly Her has written a comprehensive article about efforts to safeguard the seasonal migration and she can be contacted at mail.gio.gov.tw (usual mail address format, kelly@mail.gio.etc.etc). There is also a Purple Butterfly Valley Conservation Society in Maolin, Taiwan.
Last week I made some posts about butterfly migration and posted a video on the migration of the Monarch Butterfly to and from Mexico. Here is a short video about the migration of the Purple Milkweed Butterfly in Taiwan.
Two species of butterfly have been identified which take part in mass migration. Tens of thousands of Monarch butterflies travel about 5,000km from Canada and the US to over-winter in Mexico. From November to March, the hibernation point at Mariposa Monarca (Special Biosphere Reserve) encourages visitors to follow trails through the forests where the butterfly spends the winter months. In Taiwan, when the winter monsoon comes down from the Northeast, tens of thousands of Purple Milkweed Butterflies make a migration south to take shelter in the warm valleys at the foot of Dawu Mountain. Come the spring, they return to their breeding ground in the north which involves passing over some 600m of busy freeway and this year, the Taiwanese authorities are to close one lane of a major highway to protect them. The authorities are also installing special netting and ultra violet lights, in an effort to raise the flight path of the butterflies, and thereby avoid them being sucked into the down-draught created by the cars.
In Taiwan, when the winter monsoon comes down from the Northeast, tens of thousands of 
