This is a historial post from Hiking the California Trail, a 1998/2002 book set by Bob Lorentz and Richard Nichols. Where possible an update has been provided.
The fragile and beautiful monarch butterfly, brightly orange and black with white spots, makes a journey as long as 3000 miles from the eastern seaboard to wintering sites in central Mexico where they escape harsh winters. The west coast population doesn’t have to make it all the way to Mexico in winter because the central California coast’s mild Mediterranean winters suit the monarch just fine.
In spring the monarchs individually migrate north as far as Canada to lay eggs on milkweed plants. The eggs hatch into larval caterpillars with bright yellow, black and white vertical bands, feeding on the milkweed before metamorphosing into butterflies. At least four generations will live and die before autumn.Then the monarchs rise en mass and head south to find the same trees their ancestors visited in previous years. There they pack together by the thousands, hanging from branches in a state of dormancy. These winter adults live up to eight months. Certain eucalyptus, cypress and pine trees are so thickly populated that they take on the orange hue of the monarchs.
Although monarchs congregate in their favored groves along the California coast from Bodega Bay to Santa Monica, one perfect place to see this monarch butterfly display is Natural Bridges State Beach. From October through February the monarch winters here in a grove of eucalyptus. You can also see monarchs wintering in Pacific Grove on the Monterey Peninsula, where they inhabit trees throughout the town and are protected by city ordinances. The monarchs have natural protection against bird predation because many milkweed species are poisonous, making the butterflies and their larva toxic to birds. The viceroy and queen butterflies not only share the monarch’s coloration, they’re also poisonous to birds. This mimicry increases the degree of protection for each species. How the tiny one-ounce monarchs make the long journey to arrive precisely at their ancestral home was long a mystery, but recent scientific breakthroughs have illuminated, if not fully answered the puzzle. Not only do the monarch’s wings contain traces of magnetite that somehow act as an internal compass, the butterflies also home in on the chemical odor of billions of fallen butterfly scales at their ancestral destination.
—Originally Published in Hiking the California Coastal Trail: Guide to Walking the Golden State's Beaches and Bluff from Border to Border - Volume One: Oregon to Monterey (2nd Edition) by Bob Lorentzen and Richard Nichols