The front legs of birds act as "wings" covered with feathers. Most birds can freely fly in the air and themuscle between breastbones and wings has developed greatly in order to do so. Wings are covered with feathers of different shapes and sizes. Although they are large, they are quite light. Their bones are empty and light and only accounts for 5 to 10% of their total weight. There are approximately 10,000 species of known birds which is more than double the number of mammal species.