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Millions of monarch butterflies spend the winter at Sierra Chincua in Mexico. This is one of five wintering roosts for monarchs, where the cool mountain climate slows their metabolism enough for them to overwinter before migrating northward in the spring. Threats to this world heritage site comes both from logging and the lack of habitat in the U.S. when the animals move northward. Without plenty of milkweed plants, the animals can't continue their life cycle.