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.