Birds have evolved numerous adaptations for flight, including wings, powerful muscles, hollow bones, and, most distinctively, feathers. They are a group of endothermic vertebrates, characterized by feathers, a beak with no teeth, the laying of hard-shelled eggs, a high metabolic rate, a four-chambered heart, and a lightweight but strong skeleton. Birds live worldwide. Some species, such as ostriches, penguins and kakapos, have lost the power of flight. All birds reproduce by laying eggs. There a