We see them dropping out of the sky by ones and twos. Surfing thermals, they've been spotted by pilots at 12,000 feet. Born above the Canadian border in fields of milkweed, these tropical insects sense their way south, and in all of North America, migrate to just two winter shelters, the Central Coast of California and central Mexico.

Monarch butterflies are extraordinary navigators. They possess ancestral memory and a perplexing set of aspiring, heroic forces that compel each tiny creature to migrate to specific sites they've never been to before. Complex cues guide them, including light, genetics, electromagnetism and circadian rhythms. We don't know how they do it. Neuroscientists study them for answers to long-distance migration and because of how our brains respond to time and space. Amazingly, some of their genes resemble those found in humans.

The winter migration is divided into two groups by the Rocky Mountains. Five percent of the population heads to California sea-level sanctuaries, a more challenging but shorter journey of 1,000 miles. To the east, 95 percent of the population makes a three-month journey of up to 2,500 miles, to a dozen 10,000-foot mountaintops within 300 square miles west of Mexico City. And yet, the two microclimates are similar.

In Pacific Grove, it's 55 degrees on an early morning that promises to be an unseasonably warm day. I'm sitting on a log waiting for temperatures to rise. I'm patient. The monarchs are sleeping; wings are closed, camouflaged. With slight warmth, they stir, a flutter here, another there. And then a flurry as sun filters through the sheltering trees. They lift brilliant wings and start to fill the sky. I'm transported, optimistic, enthralled.

From egg through caterpillar, monarchs feast only on milkweed, poisonous to many creatures. Emerging as butterflies, their brightly colored wings are a toxic warning to predators. Each weighs the same as a paper clip, but they can fly 30 mph, and farther than any other insect.

While they are remarkably evolved, driven and resilient, these critical pollinators are vulnerable. Their milkweed breeding regions and sensitive wintering microclimates are threatened by human development and climate change.

When they reach their destinations, they gather in vast, pendulous clusters for warmth, by the thousands in California and by the many millions in Mexico. Come spring, with rising temperatures, they explode in a frenzy of reproduction, and they all head north. While this generation has lived for nine months, their offspring and the next two generations will live just weeks – mating, laying 700 eggs each, one by one on milkweed, dying, repeating and working their way north, generation by generation. What took one migratory generation south to California or Mexico, now takes three to four generations to return to Canada.

I identify with the monarch compulsion and persistence to find my origins and pursue perfect conditions for survival. The magnitude of their audacious achievement is inspiring. We need to know where we come from and what we must do to adapt to our environment and thrive.

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