A flightless bird is a bird that is unable to fly, either due to physical limitations or the lack of ability to sustain flight. Some of the synonyms for flightless birds include the ostrich, emu, penguin, kiwi, and cassowary. Ostriches are the largest flightless birds and can run up to 70 km/h. Emus are native to Australia and are known for their long necks and powerful legs. Penguins are an interesting species of flightless birds, living in cold environments and using their wings for swimming. Kiwis are unique because they have hair-like feathers and are the smallest of all flightless birds. The cassowary also hails from Australia and is known for its colorful appearance.