How do Birds Breathe - a brilliant article on Avian Respiration.
Flying takes a lot of energy. More so the bird needs plenty of oxygen to keep it in flight. So how do birds, most of whom are so much smaller than humans manage to breathe. We found this extremely well written explanation on http://people.eku.edu/ritchisong/birdrespiration.html - with plenty of images. There are also animated gifs that demonstrate and compare the respiratory action in humans, birds, insects. Other diagrams show the similarity between dinosaurs and birds.