Monarchs of Monterey. Why they come is no secret, but how do they find their way here?

THIS autumn, as in every autumn, the monarch butterflies have returned to Monterey Bay. One of the most remarkable and least-known or understood migrations in nature has completed another cycle. Now, with the cool, wet winter closing in, many of the monarchs have settled down. Seemingly dead branches hang from the trees. Only when there is a puff of wind, or the gentle breeze of a passing butterfly, will the branches burst into color. Huddling together against the coming cold, the dormant monarchs form long fronds along the bare branches.

All the monarch butterflies west of the Rockies winter along the California coast, ranging from Big Sur as far north as Mendocino. The monarchs east of the Rockies winter in Mexico. The world's largest colony is in the mountains of central Mexico, where some 14 million butterflies congregate each year.

Most of the western monarchs come to Monterey, and the largest colony -- the second largest in the world -- winters at Natural Bridges State Park, just north of here.

Ranger Charles Gordon estimates that between 150,000 to 200,000 butterflies are currently at Natural Bridges.

What makes the migration exceptional, besides its beauty and the fact no other butterfly makes such a sojourn, is that not one of the millions of monarchs has ever been to the wintering ground before. Each was born during the summer somewhere between Monterey and Canada. How they find their way to the same wintering ground as their ancestors, sometimes even to the same trees, is one of nature's great puzzles.

``Maybe they leave a scent,'' ranger Gordon speculates. ``They may leave a patch for the next migration to follow.'' It is known that female monarchs produce a pheromone, a scent that lures males to mate, and one theory is that this scent marks the way to Monterey.

However, Dr. Fred Urquhart of the University of Toronto, generally considered the leading expert on the monarch, thinks the path is hidden in the butterflies' genes. Like spawning salmon going upstream, butterflies are genetically programmed to return to Monterey, he contends.

During the winter, the monarchs do little except survive. They cannot fly in temperatures below about 55 degrees F. When the thermometer falls into the 40s or 30s they cannot even hold onto the trees, but fall to the ground to be eaten by other insects or animals.

The wintering generation is the longest-lived, however. During the summer, when the butterflies follow their food supply north, they survive only six weeks, Gordon says.

Monarch butterflies lay their eggs only on milkweed. When the eggs hatch into caterpillars, the insects spend all their time eating the plant, eventually turning into chrysalides. Metamorphosis begins a week later, and the green chrysalides turn into black and orange monarchs.

These insects continue their travels, following the milkweed up as far north as western Canada and then south again as the milkweed dies off, reproducing as they go, eventually creating a total of eight to 10 generations in a cycle.

The last generation undergoes a hormonal change that ends their reproductive rambling. They devote themselves instead to sipping nectar to fatten themselves up for the winter, and this is the generation that finds its way to Monterey.

The round trip for the species can be as much as 4,000 miles, Dr. Urquhart believes.

Urquhart, who has placed tags on monarchs' wings, believes they frequently travel as much as 40 miles a day on air currents. He has plotted some that have covered as many as 208 miles daily.

The monarchs have probably been making this migration for millions of years. Contrary to popular opinion, they come to an area for its climate, not because of a preference for a specific species of tree.

Many winter now in eucalyptus, which was introduced to California only 130 years ago. Before that, pines and redwoods probably sufficed.

Although the monarchs are not endangered, human development may some day put them in that category. The colony in Pacific Grove has been shrinking, probably because of nearby construction. It was once thought if developers left the butterfly trees alone, the insects would be content. Apparently, however, the overall environment is what is crucial.

Gordon says the population of the Natural Bridges colony dropped one year, alarming environmentalists, but returned to normal the following year. Environmentalists unsuccessfully fought construction of a semiconductor plant near the park, fearing fumes would harm the butterflies.

``It turned out that one passing car was more destructive than if that whole plant had blown up,'' ranger Gordon said. The plant was built and the colony, harbored in a park, was unharmed.

The monarchs will begin their northward migration in March. By the first week of April they will all be gone. When they return next September, the event will be celebrated the way only Californians can celebrate something like this.

The state park ranger will hoist a butterfly flag to signal their arrival. Schoolchildren, many dressed in wings of orange and black, will parade through the streets of coastal towns. Poets will be invited to read verses extolling the miracle at butterfly celebrations. Folk musicians will play songs of praise.

The forest and the butterflies seem to gentle even boisterous children, who gape at the dangling winged fronds. Usually, the only sound is the click of long-lensed cameras.

And if it is a warm day, and the breeze is gentle, the grove is full of silent, beautiful butterflies. . . .