Following the Monarchs
1A monarch passing over a field may be making more than a short trip between flowers. Each fall, many monarchs from eastern North America travel south to mountain forests in central Mexico. Some begin their journeys in the northern United States or southern Canada, far from their winter destination. This article follows that eastern migration; monarchs in other regions may follow different routes or remain in one area.
2The journey north unfolds differently from the fall trip south. After winter, surviving adults leave Mexico and fly north, laying eggs on milkweed along the way. Their offspring continue the northward movement, and later generations spread farther across the breeding range. Thus, the monarchs arriving in northern areas in summer are generally descendants of the butterflies that left Mexico, rather than those same individuals.
3Long-distance travel requires fuel, even for an insect that can glide on rising air. Adult monarchs drink nectar from flowers, including flowers they encounter during migration. A flowering patch beside a road or in an open field can therefore serve as a feeding stop between distant destinations. Milkweed has a different role: monarch caterpillars eat its leaves, so breeding females need places where it grows.
4A southbound monarch cannot follow an older butterfly that remembers making the entire trip before. The fall travelers belong to a generation that has not previously visited the wintering grounds. Research shows that migrating monarchs use the sun, together with an internal clock, to help maintain their direction as the sun changes position. Scientists have learned important parts of this navigation system, but they have not fully explained how monarchs locate particular wintering areas.
5In Mexico's mountain forests, monarchs gather in large groups on trees. Cool conditions help them use their stored energy slowly during the winter months. However, severe cold combined with wet conditions can threaten their survival. Forest cover helps shelter the butterflies, making the condition of the wintering habitat important even when the insects are not traveling.
6Researchers study migration through observations, counts, and the recovery of tiny numbered tags attached to some butterflies. A tag recovered far from its starting point connects two locations visited by one individual. However, those two locations do not reveal every place the butterfly stopped between them. By combining many records rather than treating one recovery as a complete travel diary, researchers build a fuller picture of the migration.