The ability to fly seems to define birds. But there are more than 50 species of flightless birds throughout the world — from the Ostrich and Kiwi to flightless rails, ducks, and this Humboldt Penguin. Why did they evolve the inability to fly? Many dwelt on islands. Others evolved until they were huge -- like the extinct 12-foot-tall Moas of New Zealand. And the penguins? Unlike most flightless birds, they still have the strong flight muscles and keeled breastbones of flying birds. They are supremely graceful flyers. But they do it under water. 

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