Migrating monarch butterflies are in "grave danger," according to a report that shows their colonies in Mexico now occupy the smallest area since records began in 1993.

The report is based on a survey of Mexico's Monarch Butterfly Biosphere Reserve done in December 2013. The butterflies, which spend the winter hibernating in the reserve's forest, occupied only 1.65 acres (0.6 hectare) in December 2013—a 44 percent drop from 2012, when the insects covered 2.76 acres (1.12 hectares) of land. The survey was conducted by the WWF-Telcel Alliance and Mexico's National Commission for Protected Areas.

Though monarchs are found in many parts of the world, the migratory monarch is the most thoroughly studied, since it's the group that's most