Monarch Butterfly : Migration

Nature of the Monarchs
 

The Monarch's migration is unique in nature, more like the migration habits of birds and whales. These creatures travel vast distances (up to three thousand miles round trip). They are the only butterfly species to make such a long trek, flying in large groups to the same winter grounds year after year, often to the very same tree.
 

How and why? Their homing system is still a mystery to the scientists and naturalists who've studied them for over two decades, and no one knows all the answers.
 

But it is estimated that some 200 million Monarchs begin their trek each fall from Canada and the upper Northern United States to points south. Beyond our heads and unknown to most there is a glorious highway of colorful butterflies each year soaring to places like the Rio Grande Valley and coastal areas of Texas where they often stop to rest before continuing their migration journey to their primary winter nesting grounds in Mexico.

An internal biological clock that times the 24-hour cycle of night and day allows the monarch butterfly to calculate its direction of flight when migrating either north or south, depending on the time of the year, scientists have discovered.

Monarch butterflies are famous for the journey they make each spring from their winter roosting sites in the mountain pine trees of Mexico to as far as the US-Canadian border and back again in autumn - an unparalleled migratory feat for such a small creature.
 

As the monarchs fly north in spring they breed several times during their summer journey. Four or five generations later, their offspring make the long journey home again, often landing in the same Mexican valley and even on the same tree that their great-great-grandparents left the previous winter.

Reference：

http://mexicolesstraveled.com/monarch.htm
         http://www.nzherald.co.nz/mexico/news/article.cfm?l_id=65&objectid=10485997