In late August, monarch butterflies (Danaidae plexippus) begin their migration from Canada to central Mexico. In Fairfax County, monarchs start moving by mid-September and continue through October. Millions of these winged jewels will erratically fly a course that they have never previously traveled. Unlike migratory birds, these sojourners do not have the benefit of older brethren showing them the way.
Monarchs as far west as the Rockies join the northern and eastern population to head for an 18-by-36 square-mile area 80 miles northwest of Mexico City. Along the way, there are no common landmarks to guide them. So how do they do it?
Scientists recently have developed a theory that monarchs have two systems that aid them in their migration. One is the ability to use the sun as a compass. In scientific studies, monarchs tricked with artificial lights into believing that the time is off just by a few hours will alter the direction of their flight upon release, according to where the sun "should" be.
Yet many scientists believe that, since monarchs can travel 90 miles or more in a day, they must be able to fly at night, without the benefit of the sun to orient them. Hence they believe that monarchs also fly by following magnetic fields. The diet of monarch larvae is almost exclusively milkweed plant, from which the caterpillars biosynthesize magnetite. This magnetite sensitizes the caterpillar to fly in certain directions.
Amazingly, each generation has a different chemical adjustment than its parents' generation. One new theory is that monarchs have a circular migration. The monarchs leaving Mexico in the spring mate, and the females continue into Texas and the Gulf coast states to lay their eggs. The bulk then continues up through the Midwest to the Great Lakes and Canada. The next generation begins the journey south; the final generation makes the return trip to Mexico. Each of the 4-to-5 generations during the year is heading a different way, so their magnetic alignment must be different as well.
Not only do monarchs' flights change with the alignment, but also there are other changes in the generations. The final generation born in late summer is physiologically different from the previous few generations. Instead of being sexually mature, territorial and short-lived (about one month), these are the migrating adults that become sexually mature only after diapause, a state of rest, in their overwintering grounds in Mexico.
The typical temperature in this oyamel fir forests in winter is about 55 degrees Fahrenheit, just below the temperature the monarch needs to fly. Logging of these forests has raised temperatures and threatened the diapause phase (see Mexican Protection Efforts for more on logging). Should the temperature rise too high, the monarchs will fly and use up much of their energy reserve stored as fat. If too cold, the monarchs will die. Several years ago a freak snowstorm resulted in the deaths of millions of monarchs in one night. A layer of monarchs was two feet deep on the forest floor. Birds and mice are common predators of fallen monarchs.
There are several disturbing aspects concerning the monarchs' migration future. In 1999 the number of overwintering monarchs was 70 percent less than in previous years. The reasons offered include weather events and forest fires outside the sanctuary. The smoke from these fires can disturb the monarchs, causing them to flee. Other concerns are that monarchs are not returning to protect sanctuaries but alighting elsewhere nearby where logging is still permitted.
As well, even with all of its buddies for support, many monarchs do not successfully complete migration. Whether it is from starvation, weather conditions or predators, estimates that from the 100 million to half a billion monarchs that begin the journey, only 55 million to 300 million arrive to overwinter. In 1998 55-to-66 million overwintered. In 1999, 96-to-108 million were estimated to visit the sanctuaries, according to Monarch Watch.
Research and efforts to "tag" butterflies (see accompanying article, Tagging Monarchs) to learn more about their migration are the best hope for discovering the dynamics that affect successful migration. Fairfax County Park Authority staff are active in these efforts.