Monarch butterflies are probably the best-known species of butterfly in North America. They are famous for their long migrations south before the northern winter frosts kill their eggs and adults. These migrations are one of the most amazing in nature and individuals can travel up to 2,000 miles from home. In North America, the migration patterns differ depending on where the butterfly lives. Monarchs living in the western part of the US migrate to a small number of sites scattered along the coast of California. One of these sites is close to where I live in San Francisco, called Pacific Grove. The most famous travelers come from the eastern US where an estimated 100 million butterflies head south to Michoacan in central Mexico.
These journeys are very tough on the Monarch and most of them do not make the whole journey. However, many stop to breed along the way and the butterflies that make the reverse migration in spring are often five generations removed from those that originally migrated in autumn. Along these migrations, Monarchs lay eggs on milkweed, which is the only plant their caterpillars will eat. It is from this milkweed diet that the butterfly gets its toxic defense. Milkweed contains poisonous cardiac glycosides that the Monarch sequesters when the caterpillar eats the plant. These glycosides cause severe vomiting in most animals that eats either a monarch caterpillar or the adult butterfly.