Academic Reading: 4

Monarch Butterfly Migration

Each autumn, millions of monarch butterflies leave their summer homes across the eastern United States and Canada and fly south toward central Mexico, covering distances of up to 4,000 kilometers. This journey is remarkable not only because of its length but also because monarchs are far lighter and more fragile than most other long-distance migrating animals; a single butterfly weighs less than a gram, yet it can travel roughly the same distance as many migrating birds.

What makes the monarch migration especially unusual is that no individual butterfly completes the entire round trip. The butterflies that arrive in Mexico each winter are not the same ones that flew north the previous spring; in fact, they are usually the great-grandchildren of those butterflies. A monarch typically lives for only a few weeks during the summer breeding season, so the northward journey back to Canada and the northern United States is completed gradually, over several generations, as each new generation is born, matures, and continues the journey a little further before laying eggs and dying. Only the generation born in late summer, sometimes called the "super generation," lives for several months and is capable of flying all the way to Mexico.

Monarch populations have declined sharply over the past few decades, and researchers point to several interconnected causes. In the northern part of their range, the widespread use of herbicides on farmland has reduced the amount of milkweed, the only plant on which monarch caterpillars can feed. In Mexico, illegal logging has damaged parts of the oyamel fir forests where the butterflies cluster together by the millions to survive the winter cold. Conservation groups in both countries have responded by planting milkweed along roadsides and in gardens, and by working with local communities in Mexico to protect the remaining forest. While these efforts have slowed the decline in some areas, scientists caution that monarch populations remain far below the levels recorded just a few decades ago.

Practice questions

  1. According to the passage, what makes the monarch's migration distance especially remarkable?

    • Monarchs are unusually light and fragile compared with other long-distance migrators
    • Monarchs migrate at night only
    • Monarchs travel in large groups for protection
    • Monarchs fly faster than any other migrating insect

  2. According to the passage, which butterflies actually complete the entire journey from Mexico back to the northern United States or Canada?

    • Butterflies that were born in Mexico
    • No single butterfly; the journey is completed across several generations
    • Only the largest and strongest butterflies
    • The same individual butterflies that flew south

  3. According to the passage, what distinguishes the "super generation" from other generations of monarchs?

    • It is physically larger than other generations
    • It lives for several months and flies all the way to Mexico
    • It never leaves the northern breeding grounds
    • It reproduces more often than other generations

  4. According to the passage, why has herbicide use on farmland harmed monarch populations?

    • It has destroyed the oyamel fir forests in Mexico
    • It has reduced the amount of milkweed, the only food for monarch caterpillars
    • It has directly poisoned adult butterflies
    • It has changed the migration route of the butterflies

  5. According to the passage, what have conservation groups done in response to declining monarch populations?

    • Banned all use of milkweed near farmland
    • Planted milkweed and worked to protect forest habitat in Mexico
    • Bred a new, hardier species of monarch
    • Relocated monarch populations to new countries

IELTS 5.5をめざす: related lessons