In the next few months, the beating of fragile fiery orange and black wings will transport the monarch butterfly south. But the number of monarch butterflies (Danaus plexippus) reaching their final destination has steadily declined, dropping to its lowest level in two decades last winter, according to a recent survey.

The insect’s journey begins in late summer and August, when monarchs fly from Canada and the Northeastern U.S. to highly selective overwintering sites in Mexico. Individually weighing less than a paperclip, monarch butterflies employ an inherited compass to make the longest insect migration in the world, flying up to 4000 kilometers (2,485 miles) to reach their final destination by November.