Not only are Monarch butterflies striking to look at, but they are the champion distance travelers of the butterfly world. During the late summer and fall, millions of Monarchs spend up to ten weeks migrating as many as three thousand miles to warm winter roosting spots! Monarch butterflies that live west of the Rocky Mountains in the United States and Canada migrate to the California coast. Most Monarchs that live east of the Rockies fly to the mountains of the Mexican state of Michoacan.
Monarch migration is full of mystery and wonder. The butterflies that migrate to their winter homes are not the same butterflies that spent their winter in California and Mexico the previous year. It is the great-grandchildren and great-great-grandchildren that make the journey! The adults that leave their winter homes lay eggs during their spring migration. The eggs hatch, caterpillars emerge, the caterpillars feed and form chrysalises, and from the chrysalises, adults emerge and continue the northward migration. This cycle occurs several more times during the summer, until after three or four generations, the descendents of the original overwintering butterflies are ready to make their fall journey to warm winter roosts.
No one really knows how Monarchs find their way back to the winter roosts. (The offspring of other migratory animals, like whales and caribou, have parents to guide them.) Scientists have offered the following answers to questions about how the butterflies know it is time to migrate and which direction they must travel:
- Shorter hours of daylight and cooler temperatures tell the butterflies when to begin their fall migration.
- Monarchs use a combination of their internal body clocks and the sun to figure out the direction they need to migrate.
But knowing “when” and “in which direction” doesn’t tell us how they “navigate” (how they find the exact location of where their great-grandparents and great-great-grandparents spent the winter). The question of “navigation” remains a mystery!
People have been counting and tagging Monarchs for many years to help solve this mystery. “Tagging” refers to attaching a tiny tag to a butterfly’s wing to help identify individuals when they are caught. Suppose your teacher gives you the following information:
- On average, 60 Monarchs will migrate past your school each hour for the next 30 days.
- The migration will last 8 hours each day.
- Your class will catch and tag Monarchs for 2 hours each day
- In all, your class will tag 1% of the Monarchs that migrate past your school during the time your class is catching and tagging Monarchs
Estimate how many Monarchs will migrate past your school over the next 30 days. Of these, how many Monarch butterflies will your class tag?