Flying is tougher than walking or running, right? So how come birds don't get winded and run out of breath?

Well, in terms of distance per unit energy, flying is pretty efficient. Plus birds have a super efficient respiratory system. While a bird's respiratory system takes up about one fifth of its body, its lungs are relatively rigid and small, and don't fully deflate. Instead, birds have a complex network of air sacs that work like bellows to pump a constant stream of air through the lungs. What's more, air only goes through the lungs in one direction, which means it has a higher oxygen content than the air in human lungs.

When a bird inhales, the air goes into the rear air sacs, and when it exhales, that air moves forward to the lungs, where oxygen is exchanged for carbon dioxide waste. Then, when the bird inhales again, the air is pushed out of the lungs and into the front air sacs. When it exhales a second time, the air leaves the body.

It takes two breaths to process a packet of air. Plus the air sacs mean that birds weigh less than you'd expect, which makes flying less costly in terms of energy. Unfortunately, scientists still don't fully understand how birds breathe. Birds fly over Mount Everest at altitudes where the air is so thin humans can't function, and no one knows how they do it.