For thousands of years, millions of monarch butterflies have migrated from the northern U.S. and Canada to Mexico for the winter. But exactly how they know where to go is still a mystery.

What sounded like leaves rustling in the wind was the flickering wings of thousands of monarch butterflies as ... thousands ...

The Xerces Society for Invertebrate Conservation says, along the California coast, 4.5 million monarchs overwintered in the ...