Weather, it turns out, also presented Norah and Fred with a challenge. One of their first problems was to find a tag suitable for a featherweight insect that would neither impede its ability to fly nor be washed off by rain. After many years of experimentation, they found a solution: labels coated with pressure-sensitive adhesive.

Having found the right label, Norah and Fred began to look for volunteers to assist with tagging butterflies. When their project began in 1952, it was with just 12 volunteers, but by the time of the discovery in 1976, thousands more people across North America had taken part. Each label bore a unique number and a message: “Send to Zoology University Toronto Canada.” As sightings were reported, Fred and Norah would push pins into a large map in their red-brick Scarborough home, each pin marking the last known location of a tagged butterfly.

With each new report, excitement grew. Amongst other things, they learned that a butterfly could travel more than 100 kilometres a day. Over time, a pattern emerged on the map: a roughly northeast to southwest migration across the midwestern United States, through Texas and into Mexico.

From 1973 on, the Urquharts were aided in their search for the butterflies’ final destination by Mexico-based naturalists Ken Brugger and Catalina Aguado (now Catalina Trail), who spotted a notice the couple had posted in a Mexican paper. On the evening of January 9, 1975, Ken telephoned the Urquharts from Mexico. “We have located the colony!” he said. “Millions of monarchs — in evergreens beside a mountain clearing.”

***

After making repairs to the boat, we sailed southeast, hugging the coastline. The steep sea cliffs and towering redwoods of California’s Lost Coast were replaced by imposing volcanic plugs rising above the turquoise waters of the Pacific. As the weather improved, so did our spirits. We had just arrived at our first port of call in Mexico and were celebrating when we learned that Robin’s father (Fred and Norah’s son) Doug had suddenly passed away from a heart attack at the age of 68.

It was a shock. Doug had always been vigorously active, with an inexhaustible energy that fuelled adventures from dog-sledding in the northern wilderness to paddling an outrigger canoe around Fiji. Just days before his death, he’d been biking through the Sonoran desert. He was also relatively young. The idea that something as small as a dislodged fragment of plaque could topple such a force of nature seemed cruel and incongruent. Stunned, we flew to be with family in the weeks that followed.

After 40 years of searching, Fred had begun to worry that he might not live to find the monarch wintering grounds. Even as he climbed to see them for the first time at the age of 65, he wondered if he was up to it. “At 10,000 feet … our hearts pounded, and our feet felt leaden,” he later wrote in National Geographic. “The rather macabre thought occurred to me: Suppose the strain proved too much? It would be the ultimate irony to have come this far and then never witness what we’d waited so long to see.”

Their efforts were rewarded when they reached an airy fir tree grove and came upon a breathtaking scene beyond their wildest dreams. “[Butterflies] clung in tightly packed masses to every branch and trunk of the tall, grey-green Oyamel trees. They swirled through the air like autumn leaves and carpeted the ground in their flaming myriads.”

Fred’s reverie was broken when a butterfly-laden branch suddenly snapped and fell to the ground. As he stooped to inspect the branch, he spotted, there amidst the quivering orange mass, a white tag. Here was butterfly “PS 397,” tagged by two young boys and their teacher in Chaska, Minnesota. This was enormously lucky. In the year that had passed since the discovery of the grove, Catalina and Ken had tirelessly scoured the hills for a tagged monarch, but their efforts had proved fruitless. Now, Fred held in the palm of his hand the answer to his boyhood question and definitive proof of the monarch butterflies’ remarkable migration.

***

We left our boat near Puerto Vallarta and headed inland by car to the Monarch Butterfly Biosphere Reserve, a UNESCO World Heritage Site, which protects 56,259 hectares of forest. To reach the Cerro Pelón grove, some 2,700 metres above sea level, is a two-hour, dust-choked hike with over 300 metres of elevation gain. The altitude, combined with the dust and the scent of pine, made for a heady experience. Even the light seemed to trickle like sap, spilling through the canopy and pooling on the forest floor. As we gained elevation, we stopped more frequently. On one of these breaks, I caught the first flicker of orange out of the corner of my eye. As we continued to climb, butterflies began to appear in waves — half a dozen, a dozen, two dozen — stoking our anticipation until we ascended the last ridge at noon and entered a clearing. There, we saw hundreds of monarchs cresting the summit, their wings ablaze in the midday sun.