Every fall, monarch butterflies embark on their famous migration, one of the most daunting in the animal kingdom. From the United States and Canada, they fly 3,000 miles to overwinter among the oyamel fir groves of central Mexico, in the exact place that their parents did the year before.
Over the past decade, however, the number of monarchs that make it to Mexico has been shrinking, and scientists are racing to figure out why.
