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Why are some birds unable to fly despite having wings?

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Illustration showing examples of vertebrates: a turtle for reptiles, a songbird for birds, a bluegill fish, a salamander for amphibians, and two mammals-a whale and a groundhog. — Vertebrate The major groups of vertebrates include fishes, amphibians, reptiles, birds, and mammals.

Birds that are unable to fly, despite possessing wings, have lost the power of flight through evolution as they adapted to new environments. Scientists generally agree that these flightless birds are descended from ancestors that could fly.12

The loss of flight often occurred in environments where it was no longer necessary for survival, particularly on isolated islands that lacked mammalian predators. In such habitats, the energy-intensive adaptation of flight provided no survival advantage, and natural selection favored other traits.12

Physically, these birds have undergone significant changes:
* Musculature and Sternum: Many flightless birds, such as ratites (e.g., ostriches, emus), lack a keel on the sternum, which is the essential attachment site for the muscles required for flight.2
* Skeletal Structure: Unlike flying birds, which have light, pneumatic skeletons, flightless birds often have heavier bodies supported by robust leg bones adapted for running.2
* Specialization: Some birds, like penguins, have retained the keel but evolved their wings into paddlelike structures for swimming rather than flight.2

While flightlessness was an effective adaptation in predator-free environments, many species became vulnerable or extinct when humans introduced predators like rats, dogs, and pigs to their habitats.2