Which birds cannot fly?

A number of birds cannot fly, having evolved other ways of surviving. The ostrich, the largest living bird, cannot fly but can run at speeds of about 70 kilometres per hour. Emus and cassowaries are also large, ground-dwelling flightless birds. The small, nocturnal kiwi of New Zealand has tiny, almost useless wings. Penguins cannot fly through the air, but they are superb swimmers, effectively flying through the water with their flippers. Many flightless birds evolved on islands with few predators, where flight was not needed for escape.