The Migration to Mexico — South by Southwest

People in the US and southern Canada east of the Rockies have long been aware of monarchs in the summer and knew they flew south in the fall. But no one knew where they went for the winter. Local Mexicans living in the mountains west of Mexico City knew where they wintered, but they had no idea where they went for the summer. It was Kenneth C. Brugger and his wife Catalina (sometimes referred to as Cathy) who brought together the two halves of the same story on January 2, 1975. For such a big discovery, that wasn’t very long ago. In fact, it was the same year that Saturday Night Live premiered on NBC and Bruce Springsteen released his album, Born to Run.

Although the Bruggers found the overwintering site, the discovery was made possible by the pioneering work of Frederick Urquhart, a professor of zoology in Canada. In the early 1940s he and his wife Norah tagged thousands of monarchs, affixing a tiny label to the wing of each, reading “Send to Zoology University of Toronto Canada.” In 1952 he appealed for volunteers to assist with the tagging and over the next 20-odd years thousands of people participated. In January 1975 these efforts paid off when Ken and Catalina Brugger called to tell Urquhart they’d found millions of monarchs high in the mountains about 75 miles (120 km) west of Mexico City— south of the Tropic of Cancer. In 1976 the Urquharts were able to see the spectacular sight for themselves. Here is how Dr. Urquhart poetically described the experience in an article he wrote for National Geographic magazine in August 1976.

“I gazed in amazement at the sight. Butterflies— millions upon millions of monarch butterflies! They clung in tightly packed masses to every branch and trunk of the tall, gray-green oyamel trees. They swirled through the air like autumn leaves and carpeted the ground in their flaming myriads on this Mexican mountainside.”

The Overwintering Sites

Because their overwintering sites are essential to the survival of the migration, in 1986 a presidential decree created the Monarch Butterfly Special Biosphere Reserve covering almost 40,000 acres (16,110 hectares). Of these, 11,097 acres (4,491 hectares) were designated as the “core” with no commercial forest activities allowed. The remaining 28,711 acres (11,619 hectares) were designated as a buffer zone. In 2000 the reserve was enlarged with another decree. This enlarged reserve, called the Monarch Butterfly Biosphere Reserve covers almost 140,000 acres (56,259 hectares) with 33,487 acres (13,552 hectares) of them in the core and slightly over 105,000 (42,707) of them in the buffer. For comparison’s sake, Manhattan Island in New York City has just over 14,ooo acres, a distance of over 2300 miles (32oo km).

Located in the Transverse Neovolcanic Belt of mountains across central Mexico, the biosphere includes a number of communities straddling the border of two Mexican states; Mexico and Michoacan. Within this biosphere the population of monarchs varies widely from year to year, and is difficult to estimate. However, one knowledgeable estimate is that all colonies combined have recently contained as few as 23 million and as many as 176 million monarchs from as far away as southern Canada.

Within the biosphere, monarchs cluster in oyamel fir trees (Abies religiosa) located at elevations between 9,000 and 12,000 feet (2,750 to 3,657 meters). (For comparison’s sake Mt. Washington is a little over 6,000 feet and Pike’s Peak a little over 14,000.) Over the years, hundreds of colonies have been discovered in areas spread over 12 mountains and new sites certainly remain to be discovered. Although many sites appear one year and disappear the next, perhaps in connection with the size of the arriving population, permanent colonies form each year inside the biosphere at Sierra Chincua, Sierra El Campanario (El Rosario), and Cerro Pelon. Outside of the reserves they form each year at San Andres, Palomas, and Piedra Herrada. For the monarchs these mountains are home for an average of 154 days, beginning with their arrival in November and their departure in March.

Overwintering sites must meet some very specific conditions including temperatures moderate enough so the monarchs don’t freeze, but cold enough so they are semi-dormant and don’t burn so much energy they consume all of their stored fat (lipid) reserves. When they gather into clusters, they prefer to do so in mature trees over 65 feet (20 meters) tall with no openings in the canopy above. They roost in these trees halfway between the ground and the canopy because this layer of the forest has the most stable temperatures throughout the day—cooler in the daytime and warmer at night. So many cluster on the large tree trunks it’s thought these trunks help regulate the monarch’s temperature since the trunks are warmer at night and cooler in the day than the surrounding area. Areas above the canopy, under openings in the canopy, and near the forest floor are subject to more extremes. The trees they select are generally on steep slopes (23–26°), at high elevations above 9,500 feet (2,890 meters), close to streams or at their heads (less than 1,300 feet/400 meters), and facing south or southwest. As the season progresses and the stream heads move down the arroyos, the monarchs follow them.

Even with ideal condition, many monarchs don’t survive the winter. The main causes of mortality at the overwintering sites are birds, mice, winter storms, low temperatures and depletion of lipid reserves. Birds can sometimes devour 40% or so of the monarchs in a colony although the toll is usually less. Although birds are known to avoid monarchs because of the toxins they ingest from eating milkweed when caterpillars, this doesn’t seem to dissuade black-backed orioles and grosbeaks. The weather that brings on the highest mortality, and even catastrophic losses of monarchs, is a storm that begins with rain, followed by winds, then snow and freezing temperatures. The perfect storm of such weather occurred in 2002 and it’s estimated that 80% of the monarchs perished that year.

Eligio Garcia-Serrano and others have written that overwintering monarchs go through four phases in the sanctuaries— arrival, colony establishment, colony movement and dispersal.

• During the arrival phase in October and November the monarchs engage in active flights and form small clusters that may or may not be in the same location as the final colonies.
• In the colony establishment phase from December to February— the coldest months— their flights become less frequent and the colonies become more stable, although under certain conditions a colony may move. The monarchs roost mostly on oyamel trees, but occasionally in other types of trees, especially as spring approaches and they move down the mountain into mixed forests.
• In the colony movement phase during February and March the monarchs become more active and seek water in streams in and around the colonies. During this phase some colonies may divide or disappear entirely.
• In the dispersal phase during March and April, the migration north begins with the monarchs engaging in widespread mating as they move first toward the edge of the forest, then to more northerly gullies, flying when the days are clear and the winds are favorable.

Visiting the Reserve

Of the hundreds of colonies located on 12 or more mountains, only a few are open to the public. These include El Rosario and Sierra Chincua in the state of Michoacan; and Cerro Pellon, La Mesa and the recently opened Piedra Herrada in the neighboring state of Mexico. You can visit these reserves during the overwintering season from November to March.

Visits to the monarch butterfly sanctuaries require walking about two miles at altitudes up to 10,660 feet (3,249 meters) where the air is thin and breathing is difficult. The trails lead over rough terrain with steep inclines, and can be considered strenuous, especially for people who are less active. To make the trip easier, horses are readily available so you can ride rather than walk. Generally, trips later in the season tend to be less strenuous since the butterflies move down the mountain, shortening the hike or horseback ride. Be sure to wear good hiking shoes or boots.

Tours can be arranged through a number of travel agencies and organizations but you can also go it alone from Morelia or Mexico City, the two closest cities with airports. Day trips are possible from these cities but require up to eight hours in a bus. Check with the local tourist office for information and advice. When staying overnight, two popular towns to stay in near the biosphere are Angangueo and the much larger Zitacuaro. In either town it’s easy to find a guide who’ll not only take you to the monarchs, but will explain things along the way.

The easiest sanctuary to visit is El Rosario, located about 45 minutes from the old mining town of Angangueo. With about 50,000 residents, an old silver mine and the historic Church of the Inmaculada Concepcion and San Simon, Angangueo is located at an altitude of 8,000 feet (2,438 meters) in the mountains close to the eastern border of the State of Michoacan.