Chapter three looks at the muscles, skeleton and nervous system of birds and shows how they are similar or different from that of mammals and other groups of animals. Bird skeletons are hollow, but still strong, with a very flexible neck and interesting skull. The muscles are organized differently for flight. Their feet, too, differ based on their lifestyle. Birds' brains are organized differently from humans too, and can change depending on the season (for breeding or migrating, for example). Their sensory organs include a sensitive bill and eyes that perceive the world differently from us.