What is the mountain of butterflies? How were they found?

After decades of theories about the migration patterns of hundreds of millions of monarch butterflies that dot the meadows of North America in summer, researchers finally discovered what they called “Mountain of Butterflies,” the monarch’s winter retreat, in Mexico 41 years ago today.

Every autumn, millions, maybe even a billion, of the iconic orange and black butterflies journey up to 3,000 miles south, from Canada, through the United States and to the high-altitude forests of Mexico’s Sierra Madre mountains. And that's where they spend their winters, clinging to the branches of the oyamel fir trees of central Mexico, packed in tight clusters to keep warm.

They blanket a small area of a forest reserve that has since been created to protect the trees from illegal logging, “colouring its trees orange and literally bending their branches under their collective weight,” according to the Monarch Butterfly Biosphere Reserve, established in 2008 in the forests of Michoacán, about 60 miles northwest of Mexico City.

It is said that when they fly en masse overhead, they block out the light, the sound of their collective wing flapping resonates through the sky like light rain.  

According to “No Way Home,” a book about the growing threats to the world’s migratory animals by Princeton University ecologist David S. Wilcove, the monarch’s winter home was discovered in 1975 thanks to efforts by Canadian entomologist Fred Urquhart, who had been trying to find it since the 1930s.

Dr. Urquhart was able to finally track the monarchs’ migration by marking their wings with a tiny adhesive tag that identified each one and instructed anyone who found it to notify him. By 1975, and with the help of a publicity campaign in Mexico that galvanized the public around his mission to find the winter home of the monarchs, Urquhart prevailed, wrote Prof. Wilcove.

Although, the "mountain of butterflies" was known to Mexican locals well before 1975, its role in the monarch migration was not well understood. 

Wilcove quotes Urquhart’s first impressions of the colony, which he likened to a “cathedral where one should converse in whispered tones for fear of breaking the enchantment.”

As we gazed in silence at the scene before us, the sun, which had been hidden behind a gray cloud, emerged and beamed a warm ray of golden light upon one of the great clusters of dormant butterflies. As if on cue from a director, the tree was transformed into a blaze of color as the butterflies spread their bright orange wings to the warm sunlight.”

In the decades since the monarch’s winter colony was discovered, winter migrations have declined dramatically along with the insect’s population, as the butterfly’s source of food and reproductive habitat, milkweed, has been lost due to pesticides use and rapid development.

But there are multi-national efforts underway, as The Christian Science Monitor has reported, to preserve the butterfly population, including a push in the United States and Canada to encourage farmers and citizens to plant more milkweed.

The Mexican government and international conservation groups are hopeful that the winged population this year could cover nearly 10 acres in central Mexico, three-to-four times the size of 2014, when there were an estimated 35 million butterflies.

It’s seen as good news. For two decades the winter migrations have declined dramatically. In 1996 the butterflies covered about 44 acres here and in neighboring Michoacán state and they numbered nearly one billion.

Today renewed multi-national efforts to preserve the farthest migrating butterfly population in the world seem to be working. So do efforts to aid the Monarch's habitat after it was degraded due to farming, development, extreme changes in temperatures, and excessive and often illegal logging in North America.