Flightless birds evolved from birds that could fly. Many flightless birds evolved on islands that had few predators from which the birds needed to escape. Others use fast swimming or running as an alternative to flying. Some flightless birds (like the ostrich) have developed effective defenses, like big toe claws.

The biggest flightless birds alive today are the ostrich, the northern cassowary, the emu, the greater rhea, and the emperor penguin.

Some differences between flightless birds and flying birds:

Feature	Flying Birds	Flightless Birds
Wings	Longer wings	Shorter wings
Cross-section Shape of Feathers	Asymmetrical feathers	Symmetrical feathers
Number of Feathers	Fewer feathers (large feathers only grow in feather tracks, to keep the weight down)	More feathers (growing all over the body)
Keel (the bone that anchors the muscles that move a bird's wings)	Well-developed keel	Greatly reduced keel
Breastbone	Rounded breastbone	Flat breastbone

Cassowary A huge, flightless bird from Australia with a helmet-like crest on its head.	Common Rhea The Common Rhea or Nandu is a large, flightless bird from South America.	Dinornis The largest bird that ever lived. This flightless bird is extinct, but it lived in New Zealand until the early 1800's.	Dodo The dodo is an extinct, flightless bird that lived on an island in the Indian Ocean near Africa.
Emperor Penguin The largest penguin; it lives on ice packs in Antarctica.	Emu The emu is a large, flightless Australian bird.	Jackass Penguin A small penguin from off the coast of southern Africa.	Kakapo A large, flightless, nocturnal, solitary parrot from New Zealand.
Kiwi A chicken-sized insectivore from New Zealand; the smallest ratite.	Moa A huge, extinct, flightless bird from New Zealand.	Nandu The Nandu or Common Rhea is a large, flightless bird from South America.	Ostrich The ostrich is the largest bird. It can't fly, but it is a fast runner.
Ostrich Printable Read and Answer Worksheet Read about the ostrich, then answer questions about it. Or go to the answers.	Penguin Penguins are birds that swim very well but cannot fly.	Penguin Printouts Printouts on these flightless birds from the Southern Hemisphere.	Rhea The Common Rhea or Nandu is a large, flightless bird from South America.
Rockhopper Penguin A small, aggressive, crested penguin.

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