Once a year millions of a special long-living generation of Monarchs migrate from the north-eastern United States and southern Canada to the Oyamel fir forests of Mexico, where they survive the winter. But then, as soon as they feel the first warmth of spring they all set out on the return journey through the Rio Grande and Texas, procreating on the way, until the second and third generations arrive back at the Great Lakes to start the cycle all over again.

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