Transcript: Migration of the Monarch

NARRATOR: Some animals must be born knowing how and when to go to far-off places they have never seen before. It's late summer in these New England hills, and one of the longest of all annual insect migrations is quietly taking shape.

The key to this journey is the milkweed plant, and a homing instinct carried by the brilliantly colored caterpillars of the monarch butterfly that are feeding on it. The caterpillars, the eggs, the pupae, even the adult butterflies of other species can survive the harsh northern winter. But the monarch cannot.

After weeks of feeding and fast growth, the three-inch caterpillar begins the miraculous transformation that will allow it to escape the certain death that winter would bring. In a few hours, the caterpillar has shed its skin and the hard, protective casing of a chrysalis has formed. Inside, an utterly different creature is taking shape. And with it, the instinct to make a great migration, out of all proportion to its apparent fragility.

Using a long, slender tongue, it will now stoke up on nectar. Monarchs usually breed and then die within a few weeks, but not this generation. The temperature and day length may keep them from breeding, and that gives them eight more months of life than their parents

The autumnal equinox seems to trigger the migration. Very little is known about how they navigate, but somehow they will find their way to the only known place hey can survive the winter.

One theory holds that monarchs may steer by the sun, correcting for its movement across the sky like honeybees. The monarchs that start in New England and eastern Canada have the longest trip. The monarchs west of the Rockies move to the coast of California, but the vast majority of North American monarchs, those east of the Rockies, move toward Mexico. When they cross the tropic of Cancer, leaving cold temperatures far behind, they might be expected to disperse, but they don't. What they do instead is the greatest enigma of all.

Having crossed half a continent, tens of millions of butterflies converge toward a pinpoint on the map of Mexico, a 30-by-50-mile area of high mountains and Spanish Colonial towns.

Except to the people who live here, the wintering colonies of the monarch butterfly were unknown until 1974. Above the hillside farms in the thin, cool air at 10,000 feet, there is a habitat that may be unique in Central America.

Called the Transvolcanic Range, it is a rocky wilderness of ravines and fir forests. These forests combine the seasonal stability of the Tropics with the coolness and the moisture of the mountains. It's not warm enough to breed and lay eggs, so the monarchs are in reproductive cold storage until next spring.

There are known to be about 15 separate colonies in the Transvolcanic Range. Precisely how the monarchs find them is still a mystery. From December until March, they eat nothing, living only on fat reserves. But they must drink water, and a stream is crucial to the location of each of the colonies.