Monarch Butterflies mass along a Oyamel fir tree in the Sierra Pellon mountain at the Monarch Butterfly Biosphere Reserve in Sierra Pellon central Mexico in Michoacan State. Each year hundreds of millions Monarch butterflies mass migrate from the U.S. and Canada to forests in the volcanic highlands of central Mexico. North American monarchs are the only butterflies that make such a massive journey—up to 3,000 miles (4,828 kilometers).
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