Circular Breathing
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You could describe our breathing as "tidal", since it flows in and out over the same path. Birds use a kind of circular breathing, where 9 air sacs act as bellows to maintain a constant, one-way flow of fresh air through their spongy, tube-like lungs. It's a little bit like the circular breathing of didgeridoo players, but the bird's purpose is to keep a continuous flow of air through their lungs, not out of their mouths.

Didgeridoo playing by David Hudson from the CD Proud to be Aborigine. The dawn chorus underlying my explanation of birds' circular breathing was recorded by Chris Tenney at the Los Padres National Forest.

I recommend looking at some lovely animated GIFs that TableTop Whale designed: "An animated guide to breathing," comparing what taking a breath looks like in a human, a bird, and a grasshopper.

There is a nice Shockwave animation of bird breathing at the San Diego State University College of Sciences website: Avian Respiration. You'll need the Shockwave plugin to play it. (There's a link on the SDSU site to download the player.) This animation is particularly good at letting you see how birds' circular breathing keeps a constant flow of fresh air flowing to their lungs.