Can all birds fly?

The majority of birds fly. They can only fly for brief periods of time when molting (naturally shed their old feathers for new ones). Several birds, however, do not fly, including the African ostrich, the South American rhea, and the Australian emu, kiwi, and cassowary. The Southern Hemisphere penguins are likewise incapable of flying. They have feathers and insulation for breeding, but they move in a different way: their sleek bodies glide across the ocean thanks to their flipper-like wings. All of these flightless birds have wings, but they have lost the capacity to fly through millions of years of evolution, despite the fact that they are likely descended from flying birds.

These species' ability to fly may have been lost due to the slow deterioration of their wings. Perhaps they were isolated on oceanic islands where there were no predators, thus they had no need to fly to avoid danger. Another hypothesis is that food became abundant, removing the need to travel large distances in pursuit of it.