Light but strong, flexible but tough, feathers keep birds warm and dry, apart from helping them to shape their wings. Birds have between 1,000 and 25,000 feathers, made from a substance called keratin. Though they look quite solid, the spines of these feathers are hollow. This helps to keep the bird’s body weight down for flight. Every part of a bird is designed to provide heavy-duty power. It has fewer bones than other animals and the bones are very light. In fact, some birds have bones that weigh even less than their feathers. Many of the bones, including those in the skull, are ‘filled with air’. Its skeleton comprises only about five per cent of its total body weight. Also, a bird’s lungs works double time and makes sure that it never runs out of air. What a well-planned flying machine!