The magnificent march highway

As it does every year, one of the great wildlife spectacles on the planet is taking place right now — in our very own backyards.  The delicate and beautiful monarch butterfly has begun its annual migration southward.

Each autumn, the last and northernmost generation of monarchs to hatch in North America takes off in the fall for warmer climates.  From most of the continent, this migrating generation makes a complete and uninterrupted journey of more than 2,500 miles to wintering grounds in southwestern Mexico.  (Monarchs west of the Rockies make a shorter flight to the coasts of California, Baja, and northwestern Mexico.)

The monarch is the only insect to undertake such a long-distance migration and the only butterfly that migrates both southward and northward in a cycle comparable to that of birds.  Northbound monarchs in spring, however, make a “leapfrog†migration that covers several generations.  The overwintering individuals begin the movement as the weather warms.  Each successive generation makes a shorter leg of the journey and then breeds and dies.

Southbound monarchs live up to nine months.  From now until spring they physiologically lack the ability to breed, which helps them save energy for fall migration and winter survival.  They are built for hardiness and for flight.  Migrants use a combination of tailwinds from the north and thermals — rising currents of warm air — to propel themselves southward with as little effort as possible.  The latter strategy is the same one used by hawks that migrate long distances.  Gliding from thermal to thermal requires almost no powered flight.

Here on the East Coast, we are fortunate to live along one of the major flyways for these amazing orange-and-black fliers.  As we move into mid-September, when migration peaks, we can see migrating monarchs floating by nearly everywhere we look.

My own admittedly unscientific observation is that monarchs have started on the move in good numbers already.  One can certainly hope that this bodes well for the continued survival of one of nature’s wonders.

That survival is perpetually jeopardized.  The most critical problem is that the entire monarch population (except for the West Coast monarchs) overwinters in just 12 locations in a single region of Mexico. That’s right, just 12 places hold all the wintering monarchs!  The butterflies require a specialized winter habitat: cool, damp mountain conifer forests with a particular kind of fir tree called the oyamel. Here they gather in the hundreds of thousands, covering the trees like Christmas ornaments and crowding together for warmth.

Some of the Mexican forests have been set aside as reserves, but many are still subject to logging and other activities that destroy the habitat.  Global warming also threatens the wintering butterflies, because they need a very narrow range of temperatures and humidity to survive.  In addition, the monarch’s home region is affected by the violence of Mexico’s drug wars.

Protecting monarchs’ North American breeding grounds and migratory stopovers is also a high priority.  Coastal habitats are particularly important. Readers who want an idea of the true magnitude of this magnificent migration spectacle should spend a day at the shore, where hundreds or thousands of monarchs can fly by in a single day.

Fred Baumgarten is a writer and naturalist living in Sharon. He can be reached at fredb58@sbcglobal.net.