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Animal Migration in a Climate of Change is a special four-part series that explores the way environmental change is affecting the natural movement of animals all around the world.
In the first programme, The Mexican Wave, the focus is on the Orange Monarch butterfly.
Each autumn in the pine forests of central Mexico, one of the greatest natural spectacles gathers pace, as tens of millions of beautiful orange Monarch butterflies pour into the trees to spend the winter months.
Some of them have migrated several thousand kilometres from their breeding grounds in North America and Canada, but although they've never been here before, they navigate south and find their way to an area not much bigger than the English county of Devon.
Although they coat the trees and when they take flight, look like an orange snowstorm, these butterflies are under threat at both ends of their journey.
The pine forest are being felled illegally by gangs of loggers, despite protection from the Mexican government.
Local people need timber and firewood to survive. Where the bitterflies breed in North America, their foodplant, the milkweed, is being killed by herbicides and agricultural practices, and the Monarch's migration is in jeopardy.
With location reports from the Mexican forests, Brett Westwood explores how steps are being taken to develop sustainable forestry to help preserve this magnificent insect.
First broadcast on 18 September 2008