EL ROSARIO, MEXICO -- Silent except for the dry whir of their wings, millions of orange and black butterflies swarmed in a brilliant canopy over this fir-studded mountainside in one of nature's strangest and most beautiful spectacles.

Some spread in gaudy carpets over a brook as it dribbled from the woods into small cornfields. Countless others zig-zagged in flight, seeming to frolic in the winter sun. Higher up the slope, they gathered in shimmering colonies on the boughs, turning entire trees orange in an ageless and still mysterious rite.

The annual winter congregation of monarch butterflies has begun here in the mountains of Mexico's Michoacan state, about 100 miles west of Mexico City.

The clouds of delicate insects -- estimated at 35 million just on the mountainside rising from El Rosario -- flew in here in small groups from breeding grounds in the northern United States or southern Canada beginning in November. Millions more alighted on the other side of the Campanario Ridge, at Chincua, or to the south at a site called Nicolas Romero.

In all, scientists say, the butterflies have selected a dozen winter hideouts in the mountains. As they do every year, about 100 million of them will spend five months basking in the pure air and relative warmth before heading back north to complete their extraordinary life cycle.

Although seasonal migration is relatively frequent for birds, the monarch is one of the few butterflies known to make such a trip. Smaller groups of monarchs spend the winter along the California or Florida coasts. But for reasons known only to the pre-Columbian spirits who are said to inhabit these mountains, most monarch butterflies, those who begin their travels in the northeastern United States and southeastern Canada, unerringly come to southwestern Mexico.

No one knows for sure how they do it. Their trip from southern Canada has been measured at up to 2,500 miles. Scientists have tracked some monarch butterflies at more than 3,000 feet in the air, gliding along atop thermal currents and unfailingly adjusting course toward a landing in El Rosario and the other nearby sites.

Strangest of all, the butterflies that show up every November are three or four generations downstream from those that left the previous spring.

Few of the insects survive the entire migratory cycle. They reproduce in the southern states on their way north and die. Their progeny resume the flight, producing roughly one generation a month. Those that survive into the fall head back to the wintering sites that only their ancestors knew.

The navigational system built into their minuscule brains apparently is passed along genetically. But no one understands why one generation of monarch butterflies along the U.S.-Canadian border suddenly decides to head for the Michoacan mountains at the first sign of a fall chill or why that generation lives 10 times as long -- to cover the entire flight south -- as its immediate forebears that migrated north in several generations.

U.S. and Canadian scientists, seeking the answers to these questions, discovered the butterflies' winter refuge here only 16 years ago. But for Jose Reyes, who was born in this mountain hamlet, it was not really a discovery. He has watched the butterflies come and go for most of his 58 years.

The butterflies that sparkle in the sunlight for almost half the year in a silent display of beauty have transformed Reyes' life and that of El Rosario's several hundred other inhabitants.

Once settled in for the winter, he told a visitor, the butterflies spend their nights huddled on the branches of fir-like trees called oyamel locally. The trees, native to this region only, are called abies religiosos scientifically, because their boughs were used in pre-Columbian worship.

The butterflies begin to stir with the first light of dawn, Reyes said. Breaking away from the security and comfort of their colonies, they dart into sunbeams that filter through the branches and ride them down toward the plateau to drink in the creekbed and feed on the violet flowers of milkweed.

Researchers have told Reyes the milkweed honey contains a substance that, once in the butterflies' system, protects them from predatory birds. Any bird that eats a monarch butterfly vomits it back up, they have found.

The central government's Secretariat of Urban Development and Ecology has declared the butterflies' winter grounds a natural preserve, barring Reyes and other villagers from logging in the forests where they live.

In return, the villagers have been blessed with a growing stream of visitors who marvel at the butterflies. A string of food shops has sprung up in El Rosario. Two parking lots and a camping site have been prepared nearby. Villagers have cut steps into the steep slopes to enable city folk to make the climb. Most important, a pay shack has been erected where each visitor is charged $3.45.

Mayor Jose Arismendi of the town of Angangueo, just down the mountainside, said up to 1.5 million visitors a year pass through to see the spectacle.

Arismendi said Angangueo's 11,000 inhabitants have made plans to improve the dirt road up to El Rosario to facilitate visits. The town's only hotel, the 30-room Don Bruno Monarca, has been augmented by 80 guest rooms in local homes, he added, in an effort to boost the area's sagging economy. Until recently, the area had depended on silver mining.

Some scientists and nature lovers have expressed fears that expanded tourism will spoil the beauty and eventually drive out the butterflies. To prevent this, the Secretariat of Urban Development and Ecology has restricted visits to some of the sites, allowing in only scientists with permits. Its officials also have impressed on residents the need to preserve the natural setting necessary for the butterflies' annual stay.