英语听力：探索发现 2012-11-04 美洲大平原 American Serengeti—4

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These giant potholes¹ were left behind by blocks of buried ice that melted, leaving hollows that later filled with water.

 

For thousands of years since then, they've been vital pit stops for migrating birds, the millions of cranes, geese and ducks that follow ancient routes across the plains.

 

These rolling hills are made of debris² dumped by the retreating ice. lt also left rich grasslands³ able to support hundreds of millions of bison, vast herds⁴ drifting with the seasons, always searching for fresh grazing.

 

Pronghorn antelopes⁵ are unique to North America and among the toughest creatures on the plains. Surviving year round in the open, they must cope with freezing winter temperatures and scorching⁶ summer heat.

 

Every spring these open spaces echo to the sounds of courting sage⁷ grouse⁸, as males compete to win a mate. Migrating Sandhill cranes strut⁹ and dance for their partners before heading further north towards their nesting grounds.

 

Small birds breed here, after an epic¹⁰ journey from their winter home. These cliff swallows have flown thousands of miles from South America.

 

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