Most birds fly. Wing feathers are spread out to increase surface area and generate lift like airplane wings. Soft contour feathers overlap rigid flight feathers to make a streamlined but firm surface. Stiff tail feathers are used to steer and break. Unlike an airplane, a bird flaps its wings to generate thrust for forward movement. A typical bird body is adapted for flight. Hand bones are fused together to support the flight feathers. A downward extension from the breastbone provides a place to anchor large chest muscles that power the wings. Bird bodies are surprisingly light, thanks to thin-walled, hollow bones. For strength, the hollow channels are filled with bony struts like the scaffolding of a building. Many birds also spend a lot of time standing and walking. Their pelvic girdle is strong, supporting the bird's weight on just two hind limbs.

All modern birds have bills and no teeth. The shape of a bird's bill says a lot about what it eats, for example whether it specializes in seeds (stout , cracking bill), fish (pointy, spearing bill), or plants (wide, serrated bill). Birds swallow their food without chewing, so it travels to the stomach whole or in large pieces. Bird digestive tracts have some special features for digesting chunky food. A pouch in their throat (the crop), is used to store food to be digested later, or regurgitated to feed the young. An extra, muscular stomach (the gizzard) grinds food up. Birds are endotherms, using heat they make internally to keep warm. While a few species allow their body temperature to drop at night (torpor), a nearly constant body temperature is maintained by most birds most of the time. Continuously making heat requires fuel to burn, in the form of food. So, birds spend a lot of time eating.