Mass migrations have puzzled scientists for decades, and perhaps none more so than the North American monarch butterfly. Each fall, millions of the beautiful orange and black winged insects begin their journey from the eastern US and Canada. They travel over 2,000 miles to winter in specific groves of fir trees in the Michoacan Mountains in central Mexico, where they crowd together so densely that the air is filled with butterflies, monarch butterfly expert and University of Minnesota professor, Karen Oberhauser told USA Today.

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