The Winter of Monarchs

Monarchs (Danaus plexipus) are iconic butterflies.  They are the most charismatic of lepidopterans and they have captured the imagination of North Americans.  First, they are brilliantly colored and patterned; orange with black stripes and white spots.  Second, their caterpillars eat and thrive on toxic milkweed plants (Asclepius sp.).  Finally, they undertake the most epic of insect migrations.  Each year several generations of Monarch Butterflies do a serial migration from Central Mexico to the Northern United States and Southern Canada and back. 

A Monarch Butterfly (Danaus plexipus) resting on the trunk of an Oyamel (Abies religiosa) tree
at the El Rosario Sanctuary in Michoacan, Mexico

We have seen all the stages of the Monarch life cycle from egg to caterpillar to adult, throughout the US.  We have also seen small parts of their migration.  In September Monarchs pass south through North Carolina.  The migration is more dramatic at gaps along the Blue Ridge Parkway with dozens of butterflies steaming through a mountain gap in an hour. 

In the mid-1970s, after decades of searching, researchers announced that millions of Monarch Butterflies spend the winter high in the fir forests of the Trans-Mexican Volcanic Belt west of Mexico City.  This winter, Diane and I went with another naturalist couple to Mexico see this final piece if the Monarch story.  To close the circle on the Monarch life cycle.

After spending a day enjoying the cultural splendors of Mexico City we piled into a 15-passenger van with our driver and guide for the three hour drive to the mountain town of Angangueo in the state of Michoacán.  This former mining boom town is now ground zero for Monarch viewing.  Our first stop was the Sierra Chincua Monarch Sanctuary north of Angangueo.  After paying our fees, we mounted horses and rode them about halfway to the butterfly colony.  A guide accompanies every group that ascends the mountain.  This arrangement keeps visitors on the trails and provides income for people living in the area.  

Oyamel trees in the Sierra Chincua Sanctcuary, Michocan, Mexico

As we gained altitude, we entered the fog that shrouded the mountaintops and the temperature dropped. Leaving the horses, we continued to climb on foot through a moist montane forest.  Large trees including pines and broad-leafed species gave way to Oyamel, the Sacred Fir (Abies religiosa).  The understory of the forest had a variety of sage species in flower, a yellow composite and abundant mosses and lichens.  Finally, at over 10,000 we reached the butterflies.  They coated the Oyamel trunks and hung in pendulous bunches from limbs.  The temperature was in the low 50s and the humidity was 100%.  These conditions allow the Monarchs to live for months during the winter rather than the weeks typical of the butterflies in the breeding season.  We saw many butterflies through the mist and they were all resting and conserving their energy.

We visited another Monarch Sanctuary near Angangueo called El Rosario.  El Rosario was the most developed of the sanctuaries.  Shops, food stands, souvenir vendors and an impressive gate with a butterfly theme welcomed us.  Even the restrooms had butterflies enameled in the sink.  

El Rosario Sanctuary gate, Michoacan, Mexico

Butterfly sink at El Rosario Sanctuary.

Rather than ride the horses up the mountain we decided to hike.  Our guide, Pepe, was knowledgeable and very enthusiastic.  He learned we were also interested in birds and pointed out many along the trail including the impressive Red Warbler (Cardellina ruber).  This tiny bird is bright red with silver cheek patches and is abundant in the forests where the Monarchs winter.  There were also many hummingbirds including the Mexican Violetear (Colibri thalassinus).  

Red Warbler (Cardellina ruber).  El Rosario Sanctuary, Michoacan, Mexico.

Mexican Violetear (Colibri thalassinus).  

As we reached the tall Oyamels we could see the large bunches of Monarchs hanging from limbs and dead butterflies littered the ground.  Some had probably died of natural causes but some were missing their abdomen.  This is a sign the butterflies were eaten by birds.  Avian predators usually avoid Monarchs because the milkweed toxins ingested by the caterpillars are retained in the adults after metamorphosis.  But, two species of birds in these mountains, Black-backed Orioles (Icterus abeillei) and Black-headed Grosbeaks (Pheucticus melanocephalus) eat monarchs.   

Monarch Butterflies on Oyamel trunks and limbs.
El Rosario Sanctuary, Michoacan, Mexico.

Dead Monarchs, El Rosario Sanctuary, Michocan, Mexico.

Not all is rosy for the Monarchs or the people who work to offer them sanctuary.  The number of Monarchs wintering in these mountains has been in a long-term decline.  The usual suspects are to blame; habitat loss, pesticides, climate change.  People that defend monarchs are also at risk.  The director of the El Rosario Sanctuary was murdered days before we arrived.  A former guide at the Sanctuary was killed while we were in Mexico.  The poverty and economic interests to convert forest to agricultural land are pressuring the wintering grounds of the Monarchs.  There is some good news, wintering Monarch numbers have increased in the last few years, perhaps because of milkweed plantings in the US and Canada.

Monarchs resting on Oyamel trunks and flying. 
Pierda Herrada Sanctuary, Mexico, Mexico.

While it was impressive to see hundreds of thousands of Monarch Butterflies resting on tree trunks and branches, we had hoped to see them flying.  The weather had not cooperated so far. However, we had one more sanctuary to visit.  In Mexico State, Piedra Herrada Sanctuary is in the mountains above Valle de Bravo.  We hiked to the butterfly roosts at this sanctuary and it turned out to be the longest and steepest walk.  After two hours on the trail and numerous stops to enjoy the plants, birds and to breathe, we made it to the Monarchs.  The day was warm and sunny.  The forest in this sanctuary was more open and diverse than the first two we visited and the butterflies were flying.  

Monarchs feeding on a yellow composite.  Piedra Herrada, Mexico, Mexico.

Monarchs were flying high and low, landing on people, sipping nectar from the abundant flowers and putting on a fantastic show.  However, this was not a show for our benefit.  When their internal body temperature get high enough the butterflies must feed on nectar to replenish their energy supplies.  They need the energy because in late February the Monarchs begin to reproduce and begin their migration north.  The Monarchs leave their winter mountain refuges, lay eggs on milkweed plants in Northern Mexico and the Southern US and die.   Their offspring continue the migration and complete the circle. 

Monarchs on Oyamel branches, Piedra Herrada Sanctuary, Mexico, Mexico.