When autumn arrives in the United States and Canada, you can see one of nature's greatest scenes—the yearly migration(迁徙) of the monarch butterflies(帝王蝶).
No other butterfly in the world migrates like the orange and black monarch of North America. They can not live for long periods in cold weather. So each autumn around October, the butterflies travel up to 3,000 kilometers south to spend winter. Some even fly to the mountains of Mexico.
It usually takes the butterflies two months to reach Mexico. When it is warm, they start to back north in the spring. But not one butterfly finishes the whole trip. They lay eggs along the way and die. It might take the monarchs as many as four to five generations(代) to complete the journey all the way back up to Canada, says Sarina Jepsen, who directs the endangered species program for the Xerces Society.
The most amazing thing about these butterflies is that they return to the very same trees in Mexico that their great-great-grandparents used the winter before.
However, the number of these colorful butterflies has dropped in recent years. The US Fish and Wildlife Sen ice said that nearly a billion monarchs have disappeared from overwintering sites(越冬地点) since 1990, the Washington Post reported, one of the main reasons, experts say, is that farmers have used herbicides(除草剂) on the milkweed(马利筋) plants that these butterflies feed on.
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