Monarch butterflies migrate to the eucalyptus trees of coastal California and the Oyamel Fir Forests in Mexico during late summer or autumn, and return to the northern region during the spring when their primary food source, milkweed is abundant. They will huddle together in the treetops for warmth and safety. The butterflies use the same trees each and every year when they migrate even though they are a different generation. During its caterpillar stage, a monarch survives on milkweed plants alone. They lay their eggs on milkweed plants so the tiny caterpillars will have access to food as soon as they hatch. The full migration spans 3-4 generations. The first three generations will go through a four-stage life cycle over a 2-6 week period - egg, caterpillar or larva, chrysalis or pupa, and finally adult. Their built-in compass will lead the butterflies over 2,500 miles to warmer climate each year. The second generation emerges in May/June, and the third is born in July/August. The fourth generation is unique. Born in September/October, the monarchs will go through the same life cycle over a 6-8 month period, laying their eggs in February or March.