The Annual Migration

The monarch butterfly, called Danaus plexippus by scientists, is a magnificent animal. Not only is it beautiful, but many millions of them also perform an amazing annual migration from their summer breeding grounds as far north as southern Canada, to the Pacific coast of California or a small mountainous area in Mexico where they spend their winter before mating and flying north in the spring. These migrations of millions of monarchs, much like migrations made by hawks and other birds, are one of nature’s wonders. What makes this migration possible is the unexplained way the last summer generation born in the north lives 8–9 months, long enough to make the journey all the way to their overwintering sites and at least part way back. Other, non-migrating, generations of monarchs, like those we see in gardens and meadows all summer, live a much shorter life of 2–5 weeks, long enough to mate and lay eggs for the next generation.

Using migration alone to differentiate them, monarchs belong to one of three distinct groups — eastern migrating, western migrating and non-migrating.

The Eastern Migrating Monarchs

The eastern population, the largest of the three groups, includes all monarchs that summer east of the Rocky Mountains. This is the group that migrates each fall to a small number of overwintering sites in central Mexico to avoid freezing and starvation as the temperature falls and the summer growing season comes to an end. What they migrate to is a microclimate that meets their needs for vegetation, temperature, humidity and water. Usually this is a grove of trees that shelter them from the rain, wind and cold but keeps them cold enough to reduce their energy consumption to the point where they can survive the winter on their stored reserves.

No one knows exactly how many butterflies are involved in the southward migration each year, but it’s certainly many millions, and in some years hundreds of millions, enough to weigh down the branches of the fir trees in their overwintering area. It’s been estimated that as many as 20,000 may hang from a single bough. What is known is that the number involved and the timing of the peak migration varies widely from year to year. In the spring, the same generation that migrated to Mexico mates in their overwintering area and then starts the movement back north looking for emerging milkweed on which to lay their eggs, and newly flowering plants on which to feed.

The Western Migrating Monarchs

The western population includes all monarchs that summer west of the Rocky Mountains in the United States and southern Canada. This group migrates to overwintering sites along the Pacific coast of California with the largest concentrations between Santa Cruz in the north and Santa Barbara in the south. Some have been seen crossing the border into Mexico but nothing is yet known about this aspect of the migration. Like the monarchs in Mexico, the western monarchs mate in late winter and then expand their range north and east into their spring and summer breeding ranges.

The Non-Migrating Monarchs

Not all monarchs migrate. South American or tropical monarchs are located in a number of northern South American countries. There are also isolated overwintering populations of monarchs in Southern California, Florida, Texas and the Caribbean as well as in Africa, New Zealand, and other countries around the world. These non-migrating monarchs remain in the same place and mate throughout the year when milkweed is available. Unlike the migrating monarchs, these monarchs don’t have a long-lived generation.

A Day in the Life of a Migrating Monarch

As the days shorten, the temperature fluctuates more, and both milkweed and other flowering plants start to die-off for the season, the last monarch generation of the year responds to unknown urges to fly south or southwest. When the winds become favorable those in the east start on their annual migration that takes some of them over 2000 miles (almost 3,500 kilometers) to a mountain range in central Mexico.

When winds are coming from the south or southwest, or are strong in any direction, they fly low or not at all. Instead, they take the opportunity to build up their energy reserves for the trip and winter ahead by feeding on flowering fall nectar plants such as asters and goldenrods. On these days they can be found where there are flowering plants in low lying areas protected from the winds, often near streams or other water sources with nearby trees. When it rains they seek shelter to avoid being knocked to the ground by rain drops that can weigh as much or more than they do.

When the winds are favorable — light, and from the north or northeast, often preceding cold fronts — they migrate in waves flying only in daylight. How many do this on any given day is influenced by cloud cover and temperature as well as the winds with fewer flying on cloudy, colder days. When the skies are clear and the winds are light and in a favorable direction, they fly high into the sky, often riding thermals as hawks do, until they are out of sight unless you have binoculars and the patience to look. Using the prevailing winds as tailwinds, they flap and glide in a south or southwesterly direction making course corrections as they go. How far they travel in a day depends mostly on the strength and direction of the winds they ride. Some estimates put it as high as 400 miles on a really favorable day.

Migrating monarchs, at least those headed to Mexico, tend to follow the same path taken by other butterflies earlier and later in the migration and often from year to year. Like a large river system, tributaries of butterflies periodically merge into the main group, steadily increasing the southern flow. These well established highways in the sky are called flyways.

After flying all morning, monarchs descend from the heights to spend the afternoon feeding. One thing they don’t do during the migration is mate. When born, the migrating generation is in a state called "reproductive diapause," or arrested sexual development. They postpone mating and laying eggs until their diapause ends in the late winter or early spring of the next year just before starting the return trip north.

As sunset approaches, monarchs stop feeding and look for an overnight roosting area. Some of these overnight roosts are minor mimics of what you’ll see in the overwintering areas— tree branches covered with dozens or hundreds of hanging butterflies. These are known as "butterfly trees" and in rare cases the butterflies return to the same trees year after year. These gatherings on the migration are called "roosts," "aggregations" or "bivouacs." One of the more amazing things is that after a long day of flying the monarchs don’t just land in a tree and immediately go to sleep. Instead they often end the day by flying around chasing each other, as if engaged in a form of play. In the morning, the rising sun warms the clustered butterflies and they leave the roost to feed in the fields or catch the winds to resume the journey.

A Day in the Life of an Overwintering Monarch

When monarchs finally reach their overwintering sites, they gather into clusters attached to tree limbs. The reasons for this clustering are unknown but it could be for warmth from shared body heat or less wind. More likely it’s a way to reduce, if not avoid predation. Birds, voles and mice consume millions of monarchs on the migration and especially at their overwintering sites. Clusters can reduce the toll in three ways. First, there is less chance that any individual monarch will be picked off when it’s in a large cluster. Second, when startled by a predator or even by a passing cloud, an entire cluster releases from the bough and drops toward the ground in an explosion of colorful, wing-flapping butterflies referred to as a cloud bomb. Much like schools of fish, the sheer numbers and sudden confusion when they take flight overwhelm predators and ensure they come up empty-handed. Third, when the butterflies hang with their wings closed over their back to display their dull undersides, they are quite well camouflaged. You can walk by a tree with these drab clusters hanging down and easily mistake them for moss or dead boughs.

 

Periodically the warm sun will strike a cluster of monarchs and they spread their wings so the cluster turns from drab to bright orange.

For much of the winter the cold keeps monarchs in a semi-dormant state that reduces their energy requirements to the point where they don’t need to eat, although they will occasionally fly off for water. It is this semi-dormancy, along with their sexual diapause, that’s believed to contribute to their long life compared to other generations.

This dormancy isn’t total and at times, especially as winter draws to a close, they take flight, sometimes in the millions. People who have seen these mass flights report being awestruck by the shear number of monarchs and the sound of millions of beating wings.

How Monarchs Navigate

How do monarchs know which way to head to get back to a tiny area, sometimes even a specific tree over 2000 (3500 kilometers) miles away? An area and a tree they have never seen before! As with so many aspects of their migration, no one knows for sure, but scientists are closing in on answers. Other migrating animals are known to use one or more of a variety of techniques — celestial navigation using the sun, moon and stars, landmarks, polarized light, or infrared perception. One leading theory is that monarchs use a combination of an internal clock and the angle of the sun to stay on a specific course — south or southwest in the fall and north or northeast in the spring. However, since they are known to fly on cloudy days when the sun is obscured, it’s suspected they also use a built-in magnetic compass or polarized light that penetrates the clouds, much as some birds do.