The Migration of the Monarchs

The Migration of the Monarchs

The monarch butterfly ranks among the greatest wanderers of all time. When preparing for a migration, these butterflies collect in tremendous swarms until the air seems filled with butterflies.

Throughout the summer, when the flowers are in bloom in southern Canada and the northern United States, the monarch flit gracefully about as solitary individuals in gardens. But in September they congregate by the hundreds of thousands. The growing hordes become so great that tree branches actually bend under their weight. The insects are assembling for their migration southward, one of the most extensive and remarkable mass movements of any living creature.

The migration starts late in September. The butterflies travel en masse, spanning a continent and covering a distance of, in some cases, nearly 3,000 miles. In migration the monarchs generally fly at heights of about 300 feet, at cruising speeds of about eleven miles per hour. Flying doggedly through all kinds of weather, these fragile insects, whose weight is about that of a tiny bird feather, may travel as far as eighty miles in a single day.

A rather amazing feature of the autumn migration is that the butterflies always gather on certain trees along the way. These special "butterfly trees" are host to the monarchs year after year, serving as an overnight stopover. Why they select the same trees every year, so far from the point of origin, is as yet unknown.

The monarch fly as if guided by a compass; if they meet a solid obstruction, they rise above it, never changing direction as they wing toward their goal. As they are joined by several flights along their journey, they seem to swell to unbelievable numbers. One of the greatest witnessed migrations occurred over Texas in 1921.

At that time an immense swarm of monarchs, well over 250 miles long, passed over the Lone Star State. Scientists estimated that at any given sector along the migration route at least one million butterflies passed during every minute of daylight — and this lasted for about eighteen days!

As they reach their final destinations, the butterflies gather in tremendous concentrations along the Gulf Coast and in California from Pacific Grove along Monterey Bay southward nearly to Los Angeles. The butterfly flocks seem to prefer Pacific Grove; they arrive there late in October by the millions and cover acres of pine trees. Pacific Grove is proud to be the host to the monarchs and has become one of the very few insect sanctuaries in the world each year. The butterflies attract thousands of tourists who come from all over the world to see the "butterfly trees," and indeed it makes a spectacular sight. Street signs direct visitors to the butterfly centers. To celebrate the monarchs' return, schoolchildren stage colorful parades annually. In honor of their insect guests the seal of the Pacific Grove Chamber of Commerce depicts a monarch, while a city ordinance makes it a crime to harm a monarch butterfly.

However famous the Pacific Grove sanctuary, monarchs do winter elsewhere, particularly along the Gulf Coast from Texas to the swamps of Louisiana and parts of Florida. Some travel from British Columbia to the Gulf of Mexico to escape the cold, but nowhere is there such concentration as in Pacific Grove.

There are times when the great hordes of butterflies do not reach their destination. They have been observed by mariners well over 1,000 miles at sea. They were undoubtedly blown off course by strong winds and were winging their way to butterfly heaven.

In the spring the monarchs leave almost as suddenly as they arrived — not as successive waves of migrants but as individuals that will face the long trip north alone. The survivors arrive on tattered wings and with faded color and lay their eggs on milkweed alone. This is the sole diet of the caterpillars. In about two weeks the caterpillar encases itself in a pupa, and in about another two weeks a monarch butterfly emerges.

With the arrival of next September the new generations of butterflies will assemble as did their predecessors, and once again the sky will be brightened as the hordes of insects fill the air on their journey south. This cycle has gone on for many centuries and will doubtless continue for many ages to come.

From the book: 
Our Fascinating Earth