The main feathers used in flight are the primary feathers, or primaries, and the secondary feathers, or secondaries. The primaries are firmly anchored to the carpals and metacarpals (the bones of the hand); the secondaries, to the ulna, or forearm. Most birds have 10 primaries and 20 secondaries on each wing.

Birds are capable of four types of flight: flapping, hovering, gliding, and soaring. Flapping flight is used for flying short distances and to gain altitude. It is also the main form of flight used during migrations. Flapping flight requires large amounts of energy but is very fast. Hovering flight is used to remain motionless over one spot. It requires more energy than flapping flight. Only a few species, such as hummingbirds, are capable of hovering flight. Gliding flight is the simplest form of flight. It requires little energy and is used to coast. Soaring flight is used by birds with large wingspans, such as eagles, vultures, and albatrosses, to maintain or increase altitude without flapping the wings. It is used over cliffs, large bodies of water, and hillsides where thermals (warm air currents) are deflected upward.

Whenever possible, birds take off into the wind. Many water birds, such as ducks and loons, paddle across the top of the water to gain momentum. Before landing, most birds glide to decrease their speed. They fan their tail, throw their body into an upright position, and beat their wings in front of their body. The legs absorb much of the impact of landing.

As you might guess, a flightless bird is a bird that can't fly. A penguin is a flightless bird. So is the ostrich. Other flightless birds include rheas (REE uhz), cassowaries (KAS uh wehr eez), kiwis (KEE weez), and the emu (EE myoo).

All birds have wings. But a flightless bird's wings are usually small for its body. Flapping small wings doesn't provide the “lift” needed to fly. Scientists believe that flightless birds used to be able to fly long ago, when their bodies were smaller. But over time, their bodies became larger even though their wings stayed the same size.

Not all flightless birds are closely related. But they do have much in common with each other and with other birds. All birds are vertebrates (VUR tuh brihtz), or animals with backbones. Birds are warm-blooded and hatch from eggs. All birds have feathers, and they have beaks instead of teeth.

Most flightless birds live in parts of the Southern Hemisphere. Emus are found in Australia. Rheas live in South America. Ostriches live in Africa—in both the Southern Hemisphere and the Northern Hemisphere. Some kinds of flightless birds live on open plains or in deserts. Others make their homes on islands far out at sea. Penguins spend much of their lives in the ocean.

There are 17 species, or kinds, of penguins. Different penguin species live in different places. Emperor penguins, for example, live along the icy coast of Antarctica. King penguins live along the rocky coastlines of South America and islands just north of Antarctica. Yellow-eyed penguins live off the coast of New Zealand, which is southeast of Australia. Galapagos (guh LAH puh gohs) penguins live farther north than all other penguins. They live along the shores of the Galapagos Islands, west of South America.