Which birds cannot fly?

Not all birds can fly. Flightless species of birds include ostrich, the rhea of South America, the emu of Australia and a few others. Large breast muscles are needed to fly but big birds do not have room for such muscles. Birds which can’t fly, however, can run very fast. Ostriches can run 45 to 60 km per hour. Penguins too cannot fly but they have wings which can be used as flippers to help them swim fast.