听力原文

People have been visiting this cave for generations. The cave floor is covered in guano, so plentiful that ten minutes' work can fill these farmers' baskets.
数代的人持续受到洞穴的恩泽，洞穴地底覆盖了满满鸟粪。仅仅十分钟时间就能让农人满载而归。
It's used as a valuable source of fertiliser. A clue to the source of the guano can be heard above the noise of the river.
这是一种宝贵的天然肥料。鸟粪的源头能听到河水上空的鸟鸣。
The sound originates high up in the roof of the cave.
噪音因为山洞而被放大。
The entrance is full of swifts. They're very sociable birds. More than 200,000 of them share this cave in southern Guizhou Province, the biggest swift colony in China.
入口处聚满了雨燕。他们是社交性动物。约200000多的雨燕共享贵州南部的洞穴。这里是中国最大的雨燕栖息地。
These days, Chinese house swifts mostly nest in the roofs of buildings, but rock crevices like these were their original home, long before houses were invented.
如今，中国的家燕多数将巢穴建在建筑物的屋顶，但其实在房屋被发明出来之前，这样的岩缝才是他们原本安家的地方。

重点讲解

重点讲解：
1. be covered in
盖满，覆满，充满；
eg. Her face was covered in wasp stings.
她脸上满是黄蜂蜇过的伤痕。
eg. This sofa is covered in real leather.
这沙发是真皮的。
2. be full of
有大量…的；满是…的；
eg. His songs are full of dirge.
他的歌充满了忧伤。
eg. Jem was full of beans after a long sleep.
杰姆好好睡了一觉之后又变得精力充沛了。
3. more than
超过；超出；
eg. For safety's sake, don't drive more than 30 kilometers per hour in the city.
为了安全起见，在市内开车时速不要超过三十公里。
eg. This city has a population of more than 1,000,000.
这个城市人口超过一百万。