Transcript: Monarch Migration

The story begins on March 21st -the spring equinox. Everywhere on the Earth gets exactly twelve hours of daylight and twelve hours of darkness.

For the northern hemisphere, this is the first day of spring.

In the Sierra Madre mountains of Mexico, the coming of spring means it's time for millions of monarch butterflies to prepare for the long journey north.

They came to just a few isolated groves from as far away as Canada -to escape the bitter northern winter. Why these particular groves -no-one knows.

As the day warms up, the butterflies begin to stir.

The sun's heat reaches those on the outside of the roost, so they're soon warm enough to fly but those on the inside are still cold after the chilly mountain night -and they fall from the trees like autumn leaves.

But, spread out on the forest floor, all the butterflies soon warm up -enough to take to the air.

They feed on nectar, building up strength for the journey ahead and they gather on puddles and seeps to drink.

And then -it's time to begin the long journey north.

As they leave the trees, the great blizzard of butterflies starts to spread out.

They leave the mountains behind and cross into the United States.