Can all birds fly?

Most birds fly. They are only incapable of flight during short periods while they molt, or naturally shed their old feathers for new ones. There are, however, several birds that do not fly, including the African ostrich, the South American rhea, and the emu, kiwi, and cassowary of Australia. The penguins of the Southern Hemisphere are also incapable of air flight. They have feathers and insulation for breeding purposes, but use a different form of motion: their sleek bodies “fly” through the ocean using flipper-like wings. All of these flightless birds have wings, but over millions of years of evolution they have lost the ability to fly, even though they probably descended from flying birds. These species may have lost their ability to fly through the gradual disuse of their wings. Perhaps they became isolated on oceanic islands and had no predators; therefore, they had no need to fly and escape danger. Another possibility is that food became plentiful, eliminating the need to fly long distances in search of food.

Ostriches are the biggest bird on the planet. While they can run very fast with their powerful legs, they cannot fly.