Thousands of monarch butterflies winter in California (and Mexico). When the temperatures reach 55F, they begin their migration towards the Pacific north west, spreading out from southern Canada to the rocky mountains, often flying 60-100 miles per day and laying their eggs on milkweed on the way. Those that eventually return to their winter location are the great, great, great grandchildren of those that set off. Milkweed is toxic and by ingesting these toxins, they become unattractive to avian predators. I am told that corvids often nibble first at the edge of a butterfly wing to check its toxicity before they attempt a meal.