英语听力：自然百科 大迁徙 Great Migrations—11

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In the Mara River, another migration² is culminating. Crocodiles have memorized when and where the wildebeests will try to cross. They have gathered in massive numbers, waiting, watching and hungering for what's to come. 

 

While the wildebeest migration has converged³ on a small crossroad of terror versus⁴ hunger, the monarch⁵ butterfly migration has spread to fill a continent. The second, third generations of monarchs⁶ have been moving progressively north. They have fanned out across North America, and the migration has reached its tipping point. It's August, time for the super generation to be born and to take the species all the way back to the Mexico. Tattered⁷ females alight, their brilliance⁸ dimmed by the arduous⁹ flight. One by one, the female lays her 200 eggs. She deposits them on the leaves of milkweed, ensuring the hatchlings will have a ready supply of food to fuel the marathon ahead. These new travelers will be different, living 10 times longer than their parents. Only longevity¹⁰ and uncanny endurance can take them back to Mexico, this time, in a single generation.

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