How do Monarch butterflies migrate?

Scientists still don’t understand how these distinctive orange-and-black-winged butterflies – which are common in gardens across North America – travel thousands of kilometers to winter in the mountains of Central Mexico. And no one knows why tens of millions of Monarch butterflies converge on only a few sites in these mountains.

Visiting a Monarch winter roost is like walking into an enchanted forest. Tree trunks, branches, and leaves are blanketed in hibernating butterflies. In March, when the weather gets warmer, the butterflies embark northward to lay their eggs on milkweed plants in the southern United States. Having fulfilled their reproductive purpose, the Monarchs die.

But their children, grandchildren, and great-grandchildren keep fanning north, to wherever there are milkweeds, the only plant their caterpillars can eat.

The lives of summer Monarchs are fleeting – just two to five weeks long. And the final generation of the year is born in early fall. Sensing the approaching winter, Monarchs migrate to the same central Mexico wintering grounds of their great-great grandparents – only to return the following spring to seed a new year of butterflies.

October 1, 2009

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