Every fall, monarchs migrate to a very small region in Mexico. They fly from across eastern North America to a region that's only 73 miles wide. The region is outlined with an orange rectangle on the map below.

Within the region, only 12 places have the habitat the butterflies need to survive for the winter. The region is outlined with an orange rectangle on the map below. Which 12 places do the monarchs choose to form their colonies?

• Click "Show/Hide Monarch Sanctuaries" below and see!

The butterflies in a colony crowd together so tightly there can be 15,000 butterflies on a single branch.

The fact that monarchs migrate to spectacular winter sanctuaries in Mexico was not known until 1975. A Canadian biologist, Dr. Fred Urquhart, had been searching for their destination for nearly 40 years. It took a network of volunteers and several hundred thousand tagged monarchs before the monarch's winter home in Mexico was finally found. National Geographic announced Dr. Urquhart's discovery in the August, 1976 issue of its magazine.

When scientists traveled to the region they found a landscape of volcanic mountains, rising to 10,000 feet in elevation. That's nearly two miles above sea level!

They found the monarchs clustered together in colonies in a forest of fir trees called "oyamel" ("o-ee-ya-mel").

Scientists later learned the oyamel forest is very rare in Mexico. Take a look! This vegetation map shows the only places where oyamel forests grow in Mexico.

Scientists were surprised to find the monarchs in such a cold place. Overnight temperatures often drop below freezing and sometimes it even snows! They wondered why monarchs would fly across the continent to a place that's so cold.

There are a few flowers in the forest, but not enough to feed millions and millions of monarch butterflies all winter. How could the monarchs survive for 5 months in a place with so little food?

Ever since the monarch's winter sanctuaries in Mexico were discovered in 1975, scientists have been trying to understand the monarch's basic biological needs. They are still learning about the unique conditions, or microclimate, of the oyamel forest and why it's critical for monarch survival.

The fact that the monarchs can survive for five winter months in Mexico is as amazing as their spectacular migration.

Learn more!

• Explore the monarch's winter habitat with Journey North's collection of slideshows and resources.

• Join us this winter with live reports from Mexico when Journey North begins again in February.