A Regal Year for the Monarch At least 150 million of the butterflies, the most in two decades, are expected to make their mysterious migration.

As autumn begins stretching into the American heartland, the cooler days are stirring what may be the largest migration of monarch butterflies in two decades.

At least 150 million of the fragile orange-and-black butterflies are expected in coming weeks to fly in a living plume stretching from Canada to the mountains west of Mexico City, where in some roosting spots the fir trees each year are draped so heavily with butterflies that branches crack under the weight.

Experts say the unusually large migration, fostered by favorable weather in early August, signals a dramatic resurgence of the elegant monarchs on both sides of the Continental Divide.

There are two major populations of monarch butterflies, and both are on the move. Every year, monarchs west of the Rocky Mountains migrate out of their summer home in the Pacific Northwest to California coastal groves, from just north of San Francisco to just south of Los Angeles.

At the same time, the majority of the monarchs in North America fly more than 2,000 miles from their summer breeding grounds around the Great Lakes to Mexico, borne about 60 miles a day on the seasonal winds.

As one measure of the large number taking flight this year, butterfly watchers in Colorado said the number of monarchs between Colorado Springs and Fort Collins was up to 20 times normal. Equally large flights also are forming in Minnesota, Michigan, Wisconsin and Ontario, experts said. In Kansas, butterfly watchers said they expect a monarch “stampede.”

“This year we will have great numbers of monarchs,” said David Marriott, executive director of the Monarch Program in Encinitas, Calif., which monitors the butterflies in Western states.

In California alone, the monarch migration may reach a peak of a million winged insects, Marriott said. In contrast, the past three years have been the worst in history for monarchs in the state--a period when sites that commonly were the winter home to 50,000 monarchs had, mysteriously, no butterflies at all.

The annual national spectacle poses provocative questions for scientists trying to understand how such a diminutive insect--weighing barely one-hundredth of an ounce--can find its way so many thousands of miles across the central United States to winter roosts in the highlands west of Mexico City.

Many migratory creatures, such as salmon, caribou, sea turtles and swallows, may learn their annual migration routes from more experienced fellow travelers who have made the same journey in previous years. When those animals migrate, they usually are retracing a route to the place of their birth, guided by subtle clues from the sun, the Earth’s magnetic field or the scent of particular sea currents.

But researchers have yet to find a clue to the monarch migration’s most puzzling aspect: How do butterflies that are several generations removed from any direct contact with their ancestral winter home in Mexico have any idea where to go?

The life span of the monarch is so short--several months--that the millions of butterflies making their way south today have never seen their winter roosts before and none of the insects that made the journey north last spring survive to guide them.

“Each butterfly has to get to Mexico, [but] they are three or four generations removed from the butterflies that moved north in the spring,” said insect ecologist Orley R. Taylor Jr. at the University of Kansas in Lawrence.

“The real question is how does a butterfly in Maine know where it is, as opposed to a butterfly in Minnesota?” Taylor said. “How do they sense in an approximate way where they are?”

Taylor and his colleagues recently demonstrated for the first time that the butterflies rely heavily on the position of the sun to orient their flight patterns as they travel long distances.

“They use a sun compass,” Taylor said. “They, in effect, have a physiological mechanism that can track the position of the sun and they use this as a guidance system. But that can’t be the entire mechanism by which they get to Mexico.”

Other researchers suspect that the butterflies also are able to sense the lines of force of the planet’s magnetic field and follow them as invisible lane markers along their journey. Some scientists believe that the butterflies also are able to take cues from the changing length of days.

But no one knows just how nature packed so much precision navigation gear into such a tiny insect.

Not until 1976 did anyone even know where most of the monarchs went each winter. That was the year that Lincoln Brower, a University of Florida entomologist, hiked into the Sierra Chincua west of Mexico City and discovered the winter home of the tens of millions of monarch butterflies that live for the rest of the year in North America east of the Rockies.

Now, to better understand the biology of monarchs, scientists each year enlist thousands of students across the United States to help tag the butterflies.

As director of Monarch Watch, a collaborative program of the University of Minnesota, the Texas Monarch Watch and the entomology department at Kansas, Orley last year helped tag 90,000 monarchs. The Monarch Program near San Diego last year tagged about 6,000. Only a handful of the tagged butterflies have ever been recovered, but those few are testament to the stamina and navigation skills of the monarchs.

Earlier this year, researchers found one tagged monarch butterfly in Mexico that had flown from Algiers, La., where it started its journey at the Alice M. Harte Elementary School. There, last fall, students had raised it along with dozens of other monarchs from tiny eggs.

In 1994, researchers recovered a tagged monarch that had traveled 2,210 miles to Angangueo, Mexico, from Spottswood Elementary School in Fredericksburg, Va., where students had raised it from an egg collected on a milkweed plant.