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why canít non-flying birds fly?
I canít give a very complete answer, but parts of the answer are pretty obvious. Lotís of animals (us, for example) canít fly. Flying requires special adaptation- delicate bones, etc- which are not good for other ways of life. Some birds have found ways to live that require other strengths, like good running, swimming, or just staying warm at the South Pole (like penguins), so theyíve evolved features that get in the way of flying, and these new features must have outweighed the flying feature in how they help the bird to survive and reproduce.
Changes like that arenít all that rare. Think of the various mammals that adapted to life back in the sea (whales, seals, dolphins) and lost the ability to get around well on land.
(published on 09/25/06)
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