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段落1

NARRATOR

Now listen to part of a lecture from a biology class.

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段落2

MALE PROFESSOR

All right, so I-I've got a good example of this.

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There's a bird, a species of crow, that lives near the water,

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and it feeds on a type of shellfish that has a hard outer shell.

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In order to eat this shellfish,

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the bird has to crack open its hard shell.

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So when this bird feeds,

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what it does is it dives down out of the air into shallow water,

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grabs a shellfish in its mouth, then carries the shellfish up in the air....

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It then drops the shellfish

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lets it fall onto the rocks below.

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When the shellfish hits the rocks, its shell cracks and splits open,

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and the bird can eat it.

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段落3

Now this bird, this crow, doesn't just swoop down, grab the first shellfish it sees, then fly up to any height and let it fall.

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Instead, it does two things:

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first, it carefully selects only the biggest shellfish;

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that means it's going to get the biggest possible meal for its efforts. and drops it from there.

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If the bird dropped the shellfish from a lower height,

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it would have to pick it up and drop it too many times in order to break the shell.

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On the other hand, if the bird carried the shellfish up to a higher altitude,

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an altitude higher than is necessary to crack the shell,

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it'd be wasting energy.

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So this bird expends just the right amount of energy-no more, no less-

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that it needs to obtain just the right kind of food.

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