Birds are a vertebrate class, like mammals, however they are a class adapted for flight that evolved alongside dinosaurs in the Jurassic period. Birds are distinguished by their feathers, toothless beaked jaws, laying of eggs, a high metabolic rate, four chambered heart, and a strong lightweight skeleton. Most of these characteristics were a result of years of evolution for birds to become the optimal flying machines they are today. Some of these evolutionary adaptations for birds include: forearms that evolved into wings, the shape of the wing developing for the most lift and glide (the shape which is the standard model for airplane wings), and the incubation of their young outside the womb to avoid excess weight while flying. A few species of birds, like the penguin and ostrich, have lost their ability to fly, but have retained their wings.