Cerro Pelon Colony on the Move

Ellen Sharp
Mar 2 · 4 min read
Joel Moreno films the river of monarchs flying down through La Cañada on Cerro Pelon on February 29, 2020.

The Season in Review

We spotted the first monarchs flying over Macheros and Cerro Pelon on October 31. They formed a colony above El Llano de Tres Gobernadores on November 10 and the sanctuaries opened to the public on November 16. There were many gloriously sunny days in December, as the monarchs continued to trickle in, with a noticeably large arrival as late as December 18. On Christmas day, the colony began moving from its original roost to a lower spot just a few hundred meters above El Llano, where some butterflies remain clustered to this day.

Monarchs filling La Cañada and surrounding visitors on Cerro Pelon, February 26, 2020.

Colony Movement Slowed by Bad Weather

Late January saw warmer temperatures and the monarchs started leaving their second resting place to move down the mountain through La Cañada, a ravine that connects the El Llano meadow to lower, nectar-filled slopes in the Ejido El Capulin, State of Mexico, side of the sanctuary. Cloudy skies and a cold snap grounded this population movement in early February. On February 5, rain drizzled all day and it felt like it might snow. Joel remarked that when he was a kid it always used to snow on this date.

That night, we awoke to wind and rain pelting our house around 1 AM. Up on the mountain, some monarchs were blown out of their trees and to the ground. On February 6, temperatures at dawn hit a low of 0.6°C (33°F). The skies cleared soon after and the temperature average rose to 15°C (59°F). The Butterflies & Their People arborists reported that almost all of the downed butterflies warmed up, dried off and rejoined the colony. After this brief intermission, the monarchs returned to the task of relocating their roosts.

The Beginnings of a Long, Staggered Departure

On February 16, the forest guardians observed “apareamiento masivo,” or a massive amount of mating. This day was noticeably warmer than previous days, and the workers think that the remigration north began in earnest on this date. Going forward, the remaining roosts started looking just a little bit less dense every day.

Number 1 indicates where the monarchs formed their first colony in November and December, 2019. By late December this colony shifted to location number 2, just above El Llano de Tres Gobernadores. In late January the colony began moving to its third location(s), shifting between several sites in La Cañada. The lowest number 3 marks the location of La Costera.

Forest workers consider this season an unusual one. For one thing, they have seen little to no frost in the mornings. Without frost, the flowers live longer on the mountain, and we’ve had fewer days of seeing the butterflies flying around Macheros looking for nectar this season than we did last season.

Horses and their handlers resting in La Costera on February 15, 2020.

For another, once the monarchs started their third movement down the mountain, they never seemed to settle in one place for very long. They lit upon La Costera for a few days in late January. La Costera is a usual resting place for them before they depart in March. (If you’d ridden up to El Llano, it’s where we usually stop to rest by some benches before starting the final steep ascent.) But this season, the monarchs kept flitting about La Cañada, seemingly undecided about where they wanted to settle.

Temporary roost alongside the trail up to El Llano on February 15, 2020.

A small group made their home for a while in several trees right next to the steep trail that connects La Costera to El Llano, where they could be seen from January 28 until February 18. On the 18th, they moved to another spot closer to la Costera before fluttering back to la Costera itself.

Since these movements started, there has been a lot of flight activity every day. The monarchs are dispersed across many trees instead of clustered tightly. “They’re not painting the trees anymore,” CEPANAF ranger Pato Moreno remarked. “It’s been a really unpredictable season.”

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