英语听力文摘 English Digest 500、鸟儿如何睡觉

（单词翻译）

 How do birds sleep? 

How do birds hang onto trees when they’re asleep? Why don’t they fall off?

That's easy. It's because of the hallux toe. Hallux toe? Is that the toe on some birds that points backwards¹ instead of forwards?  
That's the one. We already mentioned the hallux toe once on the show, when talking about how well-adapted some bird feet are for perching on branches.

But you say it holds the bird in place even when it’s sleeping. Absolutely! It’s because of the way the tendons work in a perching bird’s leg. When the bird hunkers down for sleep, it bends its leg at the knee and the heel. Two thin wires, called flexor tendons, run down the leg around the knee and heel, and attach to the toes.

The bending of the bird’s legs when it perches² tightens³ up these tendons, and, like a puppet’s strings⁴, curls the toes. The neat thing is the bird doesn’t even have to think about it to make it happen.

鸟儿如何睡觉？

鸟儿睡觉时是如何挂在树上的呢？它们为什么掉不下来？

很简单！那是因为它们有大趾。大趾？就是某些鸟儿身上指向后方而不是前方的趾头吗？对，就是那个。我们曾在节目中谈论适应性强的鸟儿的脚是如何帮助其栖息于树枝上时，提到过“大趾”。

但是你刚刚说大趾能在鸟儿睡觉时助其保持平衡。正是！这与栖息的鸟儿腿里鸟腱运作的方式有关。当鸟儿蹲下睡觉时，鸟腱使鸟腿在膝盖和脚踝弯曲。两根纤细的屈肌腱使腿在膝盖和脚踝处弯曲，然后就与脚趾连接在一起。

鸟儿栖息时，鸟腿的弯曲拉紧了这些屈肌腱，就像是拉动木偶的绳子一样，使得脚趾卷曲起来。有趣的是，鸟儿根本不用去想怎么让其发生。