Bird Flight Adaptations
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Birds combine traits supporting flight: hollow, lightweight bones, lift-giving feathers, a keeled sternum anchoring flight muscle, and efficient lungs.
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A bird's keeled sternum anchors flight muscles unusually large relative to body size, generating the force flight demands.
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Not every bird flies — a penguin retains feathers and a keeled sternum for swimming, but has dense, non-pneumatic bones instead of hollow ones.