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.
Birds typically live between 4 to 100 years depending on the species itself. Birds age differently than other animals as once they reach their adult plumage, they are almost impossible to age. It is believed that they accomplish this by the oxygen and protein process associated with their metabolism.
Birds sleep by finding either a secluded branch or tree cavity to sleep on. Before sleeping they fluff their down feathers beneath their outer feathers and turn their head to face backward. Then they tuck their beak into their back feathers and close their eyes to sleep.
Birds may migrate for long distances, medium distances, short distances, or not at all depending on their needs. Birds typically migrate to go from an area of low resources to another area of high resources. They often look for areas with food or nesting locations. These needs may also vary by season as birds migrate from the north to the south once the winter season begins.