An important factor is bird lungs, which essentially have a mechanism which keeps the lungs perpetually inflated. Air flows through. This makes it possible to p

An important factor is bird lungs, which essentially have a mechanism which keeps the lungs perpetually inflated. Air flows through. This makes it possible to provide way more oxygen to the body than our own inefficient lungs. Which again makes it possible for birds to fly over the Himalayas and still get enough oxygen to drive those power-hungry mitochondria.