It's often said that humans stand atop the evolutionary ladder. We walk upright. We fashion tools out of iron. We can get cash at any hour from an ATM. But if our brains, compared to those of other creatures, have no equal, it's the birds that best us when it comes to breathing.
In most mammals -- humans, too -- the process of breathing is fairly straightforward: air enters the lungs through the nose and/or the mouth, and the same air, minus much of its oxygen, is expelled a moment later via the same route. Thus it's only when we inhale that we have oxygen-rich air in our lungs. A bird, however, can have such air in its lungs at all times, which makes for much more efficient breathing.
Skeptical? Well, there's a simple explanation. In addition to its two small lungs, the bird possesses a system of tiny air sacs throughout its body; even some of its bones are hollow and can be filled with air. So when air enters the bird through the nostrils in its beak, the air fills both the lungs of the bird and these sacs. Then, as the air in the lungs is expelled, the oxygen-rich air stored in the sacs replenishes it. In other words, the sacs act as a temporary air reservoir.
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