Monarchs butterflies – The Great Migration – from San Francisco to the Mexican border

The fourth and final generation of Monarchs for the year emerge from their pupae around late summer. External natural cues, perhaps the waning length of the day and cooler temperatures, cause a change in their bodies; their reproductive organs are immature, they are in a state of reproductive diapause. Unlike their parents that lived for only 2 to 5 weeks as butterflies, these fall adults will have a much longer lifespan, about 8 months.

Two Monarchs butterflies

West of the Rockies, Monarchs butterflies from southern British Columbia and northern United States travel to wintering sites scattered along the coast of southern California, from San Francisco to the Mexican border.

Monarchs butterflies - The Great Migration - from San Francisco to the Mexican border 1Monarchs east of the Rockies, however, undertake the most spectacular migration of any insect, traveling a distance of 1,000 to 3,000 miles (1,600 to 4,800 kilometers) to their winter destination in the mountains of central Mexico.

During their southward journey, the Monarchs feed on nectar. Not only does the nectar fuel their long flight, it is also converted to fat reserves. This fat will sustain them during their winter hibernation, and fuel their journey north in the following spring. During their fall migration, Monarchs are often found at night clustered in large groups, from a few hundreds to a few thousands, a phenomena that puzzles scientists.

The Monarchs butterflies, on average, travel about 50 miles each day. Some tagged butterflies have been clocked at 80 miles per day. Yet butterflies are generally slow fliers. They do, however, get a lift from northward winds, and when conditions are right, the butterflies also hitch a ride on columns of rising air, called thermals, that carry them to high altitudes. Glider pilots flying two-thirds of a mile above the surface have reported gliding alongside migrating Monarchs.

A Quiet Winter

Eastern Monarchs reach the Texas-Mexico border in late September, arriving at their central Mexico wintering sites around late October to early December. Before 1975, scientists had no idea where these butterflies disappeared to. Imagine their surprise at finding the entire eastern population of Monarchs converged in about 14 sites, all in Oyamel fir forests, within an area only 300 square miles, high in the Transverse Neovolcanic Belt of central Mexico.

How You can Help the Monarchs

Monarchs butterflies - The Great Migration - from San Francisco to the Mexican border 2Project Monarch Watch has many suggestions on how you can help the Monarch butterflies. For instance, there’s information available on how to plant a butterfly and milkweed garden. You could support Monarch conservation programs in Mexico, and even participate in Monarch monitoring and tagging research programs that help scientists understand the migration of these amazing butterflies.

Visitors to these winter roosts will witness one of nature’s most amazing spectacles: Monarch butterflies, in the millions, blanket the tree trunks, branches, and fir needles of the forest trees. The high altitude Oyamel fir forests are a great place for hibernating butterflies. In this cool climate, their metabolic rate is lowered, allowing the Monarchs to conserve precious energy reserves for their northward migration in spring. The fir trees protect the butterflies, from above and around, from the direct assaults of natural forces like wind, snow, and hail. The fog and clouds that frequently envelop their mountain retreat serve as a source of moisture. On warm days, the butterflies will actually leave their roost, flying to nearby streams for a drink of water.

On the west coast, Monarchs butterflies spend the winter at sites scattered along the coast from San Francisco to the Mexican border. Many congregate on an unlikely species of tree, the introduced Australian Eucalyptus, brought to California in the 1850s for landscaping and as windbreaks. Fortunately, the arrival of the Eucalyptus coincided with the loss of the Monarchs’ original roosting trees, the Monterey Pine and Monterey Cypress, once widespread along the coast, but subsequently felled to supply the growing demand for wood.

In March, as the weather becomes warmer, the Monarchs butterflies become more and more restless. They begin to mate in mid February and start moving to lower elevations, then begin their northward journey to begin a new population of monarchs for that year.

Can the Cycle be Broken?

There are many weak links in the chain of survival for Monarchs.

Natural population fluctuations do occur. This is mainly due to adverse weather conditions. A heavy snowstorm, although uncommon in the mountains where the eastern Monarchs butterflies hibernate, could potentially wipe out a sizable fraction of their population. For instance, a snowstorm in 1995 killed 5 to 7 million Monarchs. Fortunately, enough butterflies survived to maintain the continuation of the species.

Monarchs butterflies - The Great Migration - from San Francisco to the Mexican border 3The main source of Monarchs’ problems, however, is humans. Their wintering sites in the mountain forests of central Mexico is vulnerable to logging, as well as clearing of land for grazing livestock. By removing trees at and near the roosting sites, a natural buffer against snow, rain, and wind is stripped away, leaving the fragile butterflies at the mercy of raw natural forces. Although some sites have been declared protected by the Mexican government, legal complications in land management make conservation difficult to enforce.

In California, the western Monarchs’ wintering sites often coincide with prime coastal real estate, making these areas vulnerable to development. Furthermore, the prime roosting trees of hibernating monarchs, the Australian eucalyptus, is a non-native tree that is being actively eradicated.

Monarchs face problems at their breeding grounds as well. Milkweeds, the only type of plants that Monarch caterpillars can eat, are considered weeds. They often fall victim to pesticides, be it in a field of crops, a garden, or along a highway. Monarchs can also be directly killed by insect-killing pesticides, if they happen to be at the wrong place at the wrong time.

This fall, when you see the Monarchs butterflies flitting in the air, don’t be deceived by their apparent carefree demeanor. They are in fact purposefully moving towards a faraway destination, guided by extraordinary homing instincts that still remain a mystery to scientists. No one fully understands why every fourth generation of eastern Monarchs undertake their extraordinary journey, at great peril, to winter in a very specific location. No one knows how they find their way to the same mountain retreats, year after year, to the very sites where their great-great grandparents spent the previous winter. And how does the fragile non-breeding Monarch survive 8 months in rigorous travel and frigid semi dormancy, compared to the short-lived breeding butterflies? Monarchs are certainly among the most beautiful of nature’s mysteries.

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