The Monarch Butterfly Migration

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I’m not sure how I originally heard of the Monarch butterfly migration.  All I knew was that a bunch of monarchs cluster on trees in Mexico during the winter, and I thought it would be cool to photograph.  Well, I got so much more than I bargained for and learned a ton.  What a special experience!

Before I delve into the details, I have two major tips for seeing the monarch butterfly migration in Mexico. 

1. Unless you are fluent in Spanish, read the book Bicycling with Butterflies by Sara Dykman, as the guides don’t speak English.  While she pushes her agenda on occasion, she provides a wealth of knowledge about the monarchs I wish I had known prior to visiting.  I ended up listening to the entire book on my extended travels home from Mexico.  It made what we watched that much more fascinating.  I have a renewed appreciation of the monarch butterfly.
2. If you only have time to visit one Sanctuary, make sure it is El Rosario and go on a weekday.  DO NOT go to any sanctuary on a weekend.  They get way too crowded.

The Monarch Butterfly Migration to Mexico

I don’t even know where to start with this blog post.  I suppose I will begin with the monarch butterfly migration, itself.

The monarch butterfly migrates from Canada to Mexico every year beginning in September. The monarch butterfly is the only butterfly that migrates to Mexico and back and the monarch butterfly migration round trip can be up to 3,000 miles. They begin their flight south when the sun is 46 degrees to the horizon.  The monarch butterflies that migrate to Mexico are the great-great-grandchildren of the overwintering monarchs in Mexico from the previous year.  No monarch butterfly completes an entire roundtrip journey, but as a group they do complete a two-way migration.

Take a second to think about that.  The great-great-grandchildren (4^th generation) make it from Canada to Mexico without the guidance of a butterfly that has previously wintered in Mexico!  How can this be?  Below is the process of the monarch butterfly migration:

1. The monarch butterflies that will overwinter in central Mexico were born after mid-August in Canada.  They use both their site and their internal clock in their antennae to determine the position of the sun and the time of day, respectively.  They must store up enough body fat on the way south to get them through the winter in Mexico.  As a result they feed along the way, and wait for prevailing winds in order to glide from 13-89 miles daily.  If they beat their wings, their stored energy would be used up in 10 hours.  The overwintering monarch butterflies live the longest, 6-9 months.
2. After its two-month, 1,000-3,000 mile journey, the monarch butterfly rests in the high altitude forest of central Mexico.  They concentrate in a 3-hectare area of oyamel trees located in a bioreserve.  Up until mid-March when they prepare to leave, they live here in a semi-dormant state (more info on their time in Mexico may be found later).
3. In mid-March they will begin their migration to Texas.  They must time their arrival with the milkweed, as this in the ONLY plant the monarch caterpillar will eat.
4. The female monarch butterfly will lay from 300-500 eggs and usually die thereafter.  The males die after mating.  If there is enough milkweed, the female will lay only one egg per plant to provide the best chance for survival.  Only 10% make it to adulthood as the larvae is eaten by spiders, fire ants, and wasps.  If you live in the USA or Canada and want to help the monarch butterfly, plant milkweed! 
5. The monarch butterflies born in the south in April and May are the first generation of the reproductive butterflies which only live an average of 30 days.  The first generation of monarchs migrate north to the Great Lakes where the second generation is born in June. 
6. The third generation is born farther north in the USA as well as in Canada in July.  The monarch butterfly population peaks in July.
7. The fourth (and sometimes fifth) generation which are born after mid-August in Canada begin the migration to Mexico where they arrive on November 1, the Day of the Dead.
8. The process starts over!

While I just explained the entire migration process for 25-50 million monarch butterflies, not all of them migrate to Mexico. Some monarchs join established, resident butterfly colonies and don’t leave.  Additionally, most of the monarch butterflies west of the Rockies migrate to California rather than to Mexico.  Only the monarchs east of the Rockies go to Mexico.

The Monarch Butterfly in Mexico

Once the monarch butterfly has made it to central Mexico, it overwinters in one of 12 colonies that span over 3 hectares of oyamel trees which make up only 2% of the Mexico forest. To put it into perspective, 1 hectare is about the size of two football fields.  So, all the monarch butterflies that spread across 1 billion acres of breeding grounds in the USA and Canada are now concentrated into six football fields. 

The oyamel trees are a special kind of fir tree with large canopies that are very important to the survival of the monarch butterfly.  The tree trunks are 2.5 degrees warmer than the ground.  Since monarchs cannot fly when the temperature is below 55 degrees and cannot crawl when the temperature is below 41 degrees it is important for them to stay in clusters off the ground for warmth. It is estimated that one oyamel branch will hold up to 6,000 clustered monarch butterflies. 

Not only do the large canopies protect the monarch butterfly from winter elements and near freezing temperatures, they, also protect them from their predators, of which there are only three in Mexico (the black back oriole, the black headed Groesbeck, and the black eared mice). Uniquely, the monarch butterfly holds a toxin from the milkweed in its abdomen that induces vomiting in vertebrates if ingested, so they are safe from most predators.  That said, the mouse can eat close to 40 butterflies a night!

The Oyamel Tree and the Mexican Government

Because the oyamel trees are so important to the monarch butterfly habitat, in 1986 the Mexican government created a two-zone bioreserve on communal land owned by the peasant farmers (ejidos) without including the ejidos in the plan.  The government declared that trees within the nuclear zone could not be logged, and the buffer zone was subject to controlled logging. 

The peasant landowners were not adequately compensated for the logging limitations which caused fear and increased illegal logging.  Fortunately, the government revisited the decree in 2000 and included the ejidos in the process.  In the new agreement, the community is paid in exchange for not logging or limited logging within the nuclear or buffer zones, respectively.

Additionally, some community members are chosen to guide in the monarch butterfly sanctuaries.  This honor is bestowed to the oldest male in the family and it is passed down to the eldest son.  They guide every three years. 

Hopefully the oyamel trees in Mexico will be protected and the milkweed in the USA will be restored, otherwise the monarch butterfly migration will be threatened.  While the monarch butterfly won’t go extinct, the migration phenomenon could end.

The Monarch Butterfly Sanctuaries

Anyway, while I will write another post about the sanctuaries in Mexico, four of which are open to the public, I wanted to touch on some of the monarch butterfly behaviors while in Mexico in this post.

As mentioned, once the monarch butterflies arrive in Mexico, they seek shelter in oyamel trees in twelve known locations, though the area is very concentrated.

The monarch butterflies, while in Mexico, spend most of their time resting on the trunks and branches of the oyamel trees.  The trees, from a distance look like orange trees with thousands of butterflies on each branch.

However, when the temperature raises above 55 degrees, they fly to the water.  While their bellies may be full from feeding during the migration, they can die from dehydration.  At times, it looks like a mass exodus, like bats flying from a cave.

The monarch must be very careful to get back to their resting place before the temperature drops.  They are so careful, that when clouds appear in the sky, they hover around the trees.

During our visit in late February, the monarchs were very active on sunny days around 11am.  Many fed on flowery bushes.  Others mated. While others went for water.  Interestingly, the monarchs that mate or feed while in Mexico will likely not survive the migration.

They needed to store enough energy from the southern migration to overwinter in Mexico and return to Texas.  The female monarch butterfly will try to avoid male advances.  But the males swoop down, attach themselves to the female and fly upwards with them.  The mating ritual can last up to 16 hours.

While we were mesmerized by all the activity and shared the paths with hundreds of butterflies, still millions more rested in the distant trees.  The experience was truly remarkable!  Stay tuned for my final monarch butterfly post: How to Visit the Monarch Butterfly Sanctuaries.  ETB

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