The life and migration of the Monarch butterfly is one of the most amazing animal stories of planet earth. It is the only butterfly in the western hemisphere to migrate as a strategy of survival. All other butterflies in this hemisphere die-out each year when the temperature grows too cold. Some of these have pupae that can overwinter, while others rely on siblings surviving further south to repopulate more northern areas the following spring.
The Monarch’s pupae only rarely survives winter, and then only if the winter is very mild. So the butterfly must travel south to survive. The irony is that once it accomplishes this, rather than enjoying the Mexican temperatures and sunshine, it heads to very high and quite cold altitudes where the insect essentially hibernates in mass colonies.
But no single Monarch butterfly does this: this is a community effort! Three to five generations of Monarch are necessary to accomplish the yearly cycle.
Each year between 100-150 million Monarchs undertake the migratory journey that spans two thousand miles. Their phenomenal voyage commences in southeast Canada with one generation of butterfly that will probably never see anything further south than the MidAtlantic States. This generation then breeds and in less than a month before winter sets in, a new generation of butterfly continues further south. Most of these second generation Monarchs – and some third generation groups — reach the over-wintering homes in the Oyamel forests of the Sierra Madre Oriental in central Mexico. The earlier generations all die in the growing cold.
Adult monarchs that reach Mexico live up to six months, in contrast to their non-migrating parents and grandparents that lived only nine to twelve days!
In Mexico, just 10 small patches of fir trees in the mountains are the winter roosting sites for these millions of butterflies. The density of the butterflies is so great that their weight can break off branches from the fir trees where they roost. The view as these forests are approached is hypnotic and the sights in the forest itself are surreal and intoxicating. Butterflies absorb it completely. They coat the fir trees in vast, fiery bunches, weighing down the branches by their vast numbers.
A deep, flaming orange carpet often covers the ground. Occasionally, sunlight breaks through and warms a cluster. Hundreds take flight with dramatic flurry. Rippling upward, they emit the otherworldly purr that is the sound of countless beating butterfly wings.
Sometime in March, the butterflies prepare for their journey north by mating. Many of the males never leave, dying in the Mexican reserves, but the fertile females begin a frantic northern flight path following a bountiful feast of flowers for distances hard to imagine for such frail creatures. A hundred miles a day is not uncommon; one tagged monarch covered 286 miles in 24 hours. Along their way the females lay their eggs primarily on milkweed plants.
Few of these over-wintering mothers get very far north. But a month after their eggs are laid, a fully developed new monarch emerges from its chrysalis and continues the migration north. Fanning out over the eastern two thirds of the U.S., from the slopes of the Rocky Mountains to the canyons of Wall Street, their great migration will carry many of them deep into Canada where they mate, create a new generation, then die.
In all, three to five generations occur each year between the butterflies which leave the over-wintering sites in Mexico and those that return in the fall, a miraculous journey repeated annually for perhaps the past 40,000 years and one that defies any definite scientific explanation. One unproved theory concerning this pinpoint accuracy is that, like some birds, the monarchs are sensitive to magnetism, and are guided towards large iron ore deposits in the Mexican mountains.
Sadly, the Monarch butterflies face several kinds of threat and their migration has been declared an endangered phenomenon. Urban sprawl is devastating their summer residences and their over-wintering habitats face destruction by logging. The Monarch caterpillar’s source of food, milkweed, is being steadily eliminated by herbicide from farmers and gardeners. Finally, climate change poses a threat to the Monarchs. The winter of 2002 destroyed as much as three-quarters of the entire population.