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Why have ostriches, emus, penguins, and other flightless birds lost the ability to fly?

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A zebra finch (Taeniopygia guttata) with a bright orange beak, gray head, orange cheek patch, and black-and-white striped tail perches on a branch. — Bird A brightly colored zebra finch (*Taeniopygia guttata*) perched on a branch.

Flightless birds, including ostriches, emus, and penguins, are descended from ancestors that were capable of flight. Their current flightless state is a secondary, specialized condition that evolved as these birds adapted to new environments.12

One primary hypothesis suggests that flight became unnecessary for birds that colonized habitats lacking predators, as they no longer required the ability to fly as an escape mechanism. Over time, natural selection favored other traits, such as increased body size, which allowed larger birds like ratites to defend themselves against predators on the ground.213

Physical adaptations vary by group:
* Ratites: These birds lack the keel on their sternum, which is necessary for the attachment of flight muscles. They have evolved heavy leg bones and thick feet suited for running rather than flight.2
* Penguins: These birds retained the keel but evolved their forelimbs into paddlelike structures for swimming, adapting to an aquatic existence.2

While these adaptations were successful in their original environments, many flightless species became vulnerable to extinction when humans introduced predators like rats, dogs, and pigs to their habitats.2