How the Mexican Monarch Butterfly Overwintering Sanctuaries Were Discovered: The 40-Year Citizen Science Mystery

The Great Entomological Mystery of the 1930s

Where did millions of autumn monarchs vanish each year?

In the 1930s, naturalists across the northern United States and southern Canada observed an annual spectacle: every August and September, millions of Monarch butterflies (Danaus plexippus) gathered along lake shores and flew determinedly southwest. By November, every monarch had vanished from the temperate zone.

Unlike birds, whose migration paths were established through banding, insects were considered too small and fragile to track over long distances. Most entomologists assumed monarchs simply died off in frost-free southern states or scattered harmlessly throughout Central America without congregating in any single location.

Inventing the Adhesive Wing Tag & The Insect Migration Association

Overcoming technical failure to build the first continental citizen science network

Dr. Fred Urquhart of the Royal Ontario Museum and the University of Toronto began experimenting with butterfly tags in 1937. Initial attempts—rubber-stamping wings, applying dye, clipping metal clips—all failed because the labels were too heavy or rubbed off the delicate wing scales.

In 1952, Fred and his wife Norah perfected a lightweight, pressure-sensitive, waterproof paper tag coated with a specialized adhesive that bonded securely around the monarch’s leading forewing margin without tearing the membrane or disrupting aerodynamics. Each tag was printed with a unique alphanumeric serial code and the instruction: "Send to Insect Migration Association, University of Toronto, Canada."

The Urquharts mobilized an army of over 3,000 volunteer citizen scientists—schoolteachers, 4-H clubs, amateur naturalists, and families—who tagged more than 300,000 butterflies over the next 25 years. Returns began plotting a distinct directional funnel moving southwest through Texas toward the Mexican border.

Catalina Aguado Trail & Kenneth Brugger in the Sierra Madre

The Mexican expedition that solved the missing geographical link

Once tagged monarchs crossed the Rio Grande into northern Mexico, the trail went cold. In 1972, Norah Urquhart placed an advertisement in Mexican newspapers seeking English-speaking field researchers.

Kenneth Brugger, an American textiles engineer living in Mexico City, answered the call. In 1973, he was joined by 24-year-old Mexican native Catalina Aguado (later Catalina Trail). Driving a modified campervan and traveling by horseback into remote villages in Michoacán and the State of Mexico, Aguado’s cultural insight and fluent Spanish allowed her to interview local indigenous farmers who spoke of "las palomas" (doves) filling the sacred mountaintop fir forests every year around Day of the Dead (Día de los Muertos).

January 9, 1975: Entering the Oyamel Sanctuary

Stepping into a cathedral of fir trees carpeted in orange and black

On the afternoon of January 9, 1975, on the slopes of Cerro Pelón at 10,200 feet above sea level, Aguado and Brugger climbed into a microclimatic grove of sacred Oyamel firs (Abies religiosa).

Before them stood tens of millions of monarchs, densely packed from the forest floor to the high canopy. Entire tree boughs bowed under the collective weight of hundreds of thousands of dormant butterflies. When the midday sun broke through the mountain mist, the canopy erupted into a fluttering golden cloud producing a soft rushing sound like gentle rain.

Tag #PS397 & The August 1976 National Geographic Cover

The proof that linked a Minnesota schoolroom to a Mexican mountain

In February 1976, Dr. Fred and Norah Urquhart visited the overwintering site at Sierra Chincua. While standing in the grove, a dead monarch on a low branch caught Fred’s eye. Glued to its right forewing was a clean white tag: #PS397.

Checking their master ledgers in Toronto, the Urquharts confirmed that butterfly #PS397 had been tagged on September 5, 1975, by high school student Dean Boen and his teacher Jim Gilbert in Chaska, Minnesota—over 2,000 miles away. This single recovery provided conclusive empirical proof of transcontinental migration.

The discovery became an international sensation when featured on the cover of National Geographic in August 1976, photographed with Catalina Trail sitting amidst thousands of roosting butterflies.

The Biosphere Reserve Legacy & Modern Stewardship

How historical discovery transformed into global international conservation

Following the discovery, pioneering ecologist Dr. Lincoln Brower led scientific campaigns to protect the fragile oyamel forests from illegal commercial logging, establishing that the dense forest canopy acts as an essential thermal blanket and umbrella preventing freezing winter rain from killing the dormant butterflies.

In 1986, the Mexican government established the initial protected reserves, expanding them in 2000 into the 138,000-acre UNESCO Monarch Butterfly Biosphere Reserve (Reserva de la Biosfera Mariposa Monarca). Today, international citizen science continues through Monarch Watch, Journey North, and local community-led ecotourism.