Almost every part of a bird’s anatomy has evolved in some part to enhance flight. Birds must be lightweight to fly, so have evolved very lightweight hollow bones. The structure of their bones resembles honeycomb, making them very strong but also very light. For example, frigate birds have a wingspan of over two metres, but the skeleton weighs 113g. Birds also have fewer organs (e.g. only one ovary) and no teeth. Birds use a digestive organ called a gizzard to grind up food.