First it was a single orange-and-black butterfly fluttering over our heads. That single appearance turned into bunches of the colorful creatures, clinging to bushes and flowers near the steadily climbing trail.

Once we entered the woods, however, the air changed.

So many butterflies hovered over our heads that the air pulsated with their trembling wings. Like a cloud, the brilliant bunch of monarch butterflies stretched to the treetops.

There were so many, the quiet of the darkened forest was broken by the beating of their wings like the sound of a gentle rain.

I had come here to see the winter gathering of the monarchs, nestled at some 10,000 feet in the mountains west of Mexico City. Each year millions of them – researchers aren’t really sure how many – migrate from the United States and Canada, traversing as far as 3,400 miles to converge in giant colonies on these steep mountains.

My interest in them was piqued as a child in Ohio when I watched the majestic monarchs winging through our backyard. Although they were joined by many others of many colors, the monarchs were singularly beautiful, their wings of gold veined with black and touched with white dots.

As an adult, I’d seen the pictures of their annual gathering, when they left the cold of the North to congregate in the warmth of Mexico, and it had always been my dream to one day journey to this spot.

Over the years, my hope dimmed as pesticides and herbicides took their toll on the butterfly’s population as well as native milkweeds, the only plant on which monarchs will lay their eggs. In fact, in the last two decades, the monarch’s population has declined by 90 percent.

I had, in fact, largely forgotten my dream until one day this fall when I spotted a newspaper story that predicted an unusually large gathering this winter. Indeed, their population had begun recovering in 2014 as North Americans began taking serious steps to preserve them.

The story recounted how the area covered by the monarchs in the Mexican mountains was three times greater than last winter, a total of 10 acres, still down from the 44 acres covered by butterflies 20 years ago.

So it was that a friend and I hopped on a plane to head to Mexico City. Two days later we found ourselves west of the city at the foot of the pine-and-fir-covered mountains where the monarchs dwell.

We had chosen the sanctuary near El Rosario, the largest and most well-known of the butterfly reserves, as the spot we’d have our butterfly encounter. Butterflies here usually cover 1,500 trees.

It’s part of the much larger Monarch Butterfly Biosphere Reserve, a World Heritage Site that contains most of the eight to 12 over-wintering spots where the butterflies congregate. Here the butterflies cling to trees, hanging like golden garlands or bunched in dense clusters, the branches sagging under their weight.

It’s thought that the thickly bunched clusters protect the butterflies from the mountain’s chill air so they can return to the North with the coming of spring to lay their eggs. But during bright days – when the sun heats the atmosphere – the monarchs leave their bunches to transform the very air into a living cloudlike swirl.

The sites had only been discovered in the 1970s; before then no one knew where the monarchs went during the winters. The Transvolcanic Mountain Range is steep and thickly covered in trees; even the locals didn’t know the secrets they held.

In fact, one of the biosphere rangers, Juan DeJesus, grew up in the nearby town of El Rosario, yet never knew what existed in the pine forests above his village. Sure, the residents would sometimes see bunches of the bright butterflies in their backyards, but never thought they were part of a massive population wintering so close by.

“When I saw them the first time, I was so impressed,” he said. “It seemed like magic. To work here gives me even more respect for them.”

Visitors to the El Rosario reserve are given the choice of climbing a steep trail up to the wooded area where the monarchs reside, or mounting horses that will carry them up the hill. Since the altitude was already taking its toll on one of our party, we chose the horses.

After what seemed like a half-hour trek straight up the mountain, the horses and our walking guides arrived at a clearing where we were told to dismount. We weren’t even in the dense forest, but we could already see groups of monarchs drifting through the air.

Another short walk on foot took us deep into the trees and up to the very edge of the colony.

And this was where the magic really began.

Nearby, dozens of visitors stepped up to the roped-off area to peer at the enormous nearby colony of butterflies. The monarchs hung in huge clusters from the branches of dozens of trees and covered every bare spot on the tree trunks, turning the bark orange with their wings.

Each morning the rangers trek to the reserve before visitors arrive to rearrange the ropes to protect new areas to which the colony has moved. The rangers then post themselves in the main viewing area to ensure visitors speak only in whispers, so as not to frighten the creatures, causing them to waste precious energy they’d need to survive.

“Shush,” finger to lips, a ranger warns us when our laughter becomes too loud. The butterflies swirl over our heads.

We simply stood in this magically vibrating forest for nearly an hour, watching as the monarchs swooped around us in ever-increasing clouds as the sun rose higher.

“They’re moving because they’re looking for the sun,” the ranger, DeJesus, said in a whisper.

Just as impressive as this winter gathering is the monarch life cycle that brings every fourth generation of butterflies here.

When these weary sojourners return to the United States and Canada in the spring, they do so only to lay their eggs on milkweed then quickly die. Two more generations are born, lay eggs and die each summer before the fourth generation, born in late fall, takes flight toward Mexico. This generation lives longer than its predecessors but only long enough to over-winter and return.

We could have stayed all day in this spot, enjoying what is certainly one of the most spectacular displays in the natural world. Being surrounded by soaring monarchs was an experience that fed my wonder of nature as well as a desire to protect that which inspired the wonder.

But our horses were waiting. It was time to depart.

Today, the memories remain as photos saved on my cellphone. And the wonder that the monarchs inspired won’t ever depart.

Maybe someday I’ll return again to see this Mexican wonderland.

I know for sure that the flight of the monarchs is a sight that could never grow old.