Monarch butterflies have adapted a remarkable and complicated life cycle. Just like many migratory birds, Monarchs complete part of their life stages in the Eastern United States, and part of their life stages in Mexico. Why would they evolve to travel so far?
Monarchs begin to migrate south as soon as the weather starts to get cold, usually in October. In Mexico each year the new generation of Monarchs finds Oyamel Fir Trees to rest on all winter long. In the Spring, they have to return to the Eastern United States (as far North as Canada!) to lay eggs where their larval host plant, milkweed, is plentiful. Here a new crop of caterpillars turns into butterflies and prepares to migrate south once again.
If you’re paying special attention to the butterflies in your yard, attempt to differentiate the Viceroy, a Monarch mimic, from a real Monarch. Monarchs are poisonous to birds because the caterpillars feast on poisonous milkweeds. Their orange and black coloration serves as a warning to predators. Once birds taste one Monarch, they remember that coloration and never go near it again. Other butterfly species take advantage of their fellow butterfly’s defenses by evolving a very similar coloration. A careful observer can tell the difference between a Viceroy and a Monarch butterfly by looking for a black line across the bottom of the lower two wings. If it has a line, it’s a Viceroy. Other orange and black butterflies that are found on the east coast include the Baltimore Checkerspot, the Queen, and the Red Admiral- of which the Queen is also poisonous.
You can follow the migration of Monarch butterflies back to the Eastern United States on the website MonarchWatch.org, where citizens send reports when they spot their first Monarch of the season.