Ostriches are birds, but they can not fly. Their breastbones are flat, lacking the usual keel where flight muscles attach. Ostrich flight muscles are small, and they have fancy plumage instead of flight feathers. Ostriches, however, are great runners. Long legs raise their center of gravity high off the ground, like horses. An ostrich may stand more than 2 meters tall (6.5 feet) and run more than 60 kilometers per hour (38 miles/hour). Strong but lightweight pelvic bones anchor large muscles. An ostrich really catches its stride when chased by a predator such as a cheetah. It takes less energy to walk, though, and they spend most of the day walking around. Ostriches walk on their toes with a rolling motion of their flexible feet, leaving an even footprint. While they do not bury their heads in the sand, they do sometimes sit down and rest their heads on the ground.

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.