Flightless birs are birds which have lost the ability to fly, relying instead on their ability to run or swim, and are thought to have evolved from their flying ancestors. There are about forty species in existence today, the best known being the ostrich, emu, cassowary, rhea, kiwi, and penguins. It is believed by some that most flightless birds evolved in the absence of predators, on islands, and lost the power of flight because they had few enemies — although this is likely not the case for the ratites; the ostrich, emu and cassowary, as all have claws on their feet to use as a weapon against predators.