Golden clouds flutter above the fir-tree forests of Mexico. That is where most monarch butterflies spend the winter. “You hear a gentle whooshing sound of thousands of monarchs flying all around you,” scientist Ernest Williams told TFK.

The monarch is the only butterfly known to make a two-way migration. Every fall, millions of monarchs leave Canada and the northern United States and head south to spend the winter hibernating in Mexico and California. In early spring, the monarchs head north. Females lay their eggs on milkweed plants along the way. New generations of butterflies continue the journey north.

Williams teaches biology at Hamilton College, in Clinton, New York. For a recent study, he observed monarchs in Mexico. Williams and other scientists found that over the past 17 years, the number of monarchs in Mexico has dropped. “The data shows a distinct and sad decline,” says Williams.

Protecting Butterflies

Monarchs face three main threats: habitat loss, plant-killing chemicals and extreme weather. In Mexico, people illegally cut down trees, destroying the butterflies’ homes. Logging also thins the forest. This causes temperature changes that make it hard for the monarchs to survive.

In areas where monarchs lay their eggs, the use of chemicals is a big problem, says Williams. These chemicals kill milkweed plants. Female monarchs lay their eggs on the plants’ leaves. Caterpillars eat the leaves. To help monarchs, Williams urges people to plant milkweeds.

Some experts are not convinced monarch numbers are falling. They feel more research is needed. But all experts agree that the annual migration is a natural wonder that shouldn’t be lost. “Different generations fly in two directions. How do they know which way to go? How do they end up on the same mountaintops in central Mexico?” says Williams. “It’s simply amazing.”