How Many?
At least 18 trees were filled with butterflies last week when scientists visited the Sierra Chincua Sanctuary. Monarchs have been traveling to this region since August. Some will fly up to 3,000 miles to reach their winter home. How many monarchs will survive the trip? This week, learn how scientists estimate the number of monarchs overwintering in Mexico. How will this winter's population compare to the past?

Image courtesy of Filipe Martinez Mesa and Rocio Trevino