Why birds like emu ostrich and penguin fly?

Emus, ostriches, and penguins are all flightless birds. Birds that can fly typically have large wings, large breast muscles, and a reduced weight. Emus have small, weak wings that would not be able to generate enough lift to fly. Ostriches have long legs and use them to run at speeds of up to 50 mph. Penguins have wings that are modified for swimming rather than flying. They use their wings to propel themselves through the water.