The main characteristic of the flying mammals is that they are all species of bats. Bats are the only flying mammal. Some of the characteristics of bats are that they fly almost exclusively at night, they can't walk very well, and they have excellent hearing.
Cute question! Birds like to get high. Feathers allow birds to fly.
Flightless birds have a small keel and wings.
Wings, feathers, and light bones.
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Their bones are hollow which makes the bird light. With its lightness it is able to fly.
It is not only birds that fly. Bats fly and they are not birds.
Ducks fly, lay eggs, have feathers, molt, migrate - all of which are bird characteristics.
Contour feathers. Wait: I was wrong. The answer is remiges.
They have bones in their wings that allow them to fly, if they did not have bones, they would be a lump of skin and organs.
Birds are able to fly in the rain because their feathers are designed to repel water, keeping them lightweight and aerodynamic. Additionally, birds have strong muscles and efficient respiratory systems that allow them to maintain flight even in wet conditions.
Adapted bones in birds are lightweight and strong, aiding in flight by reducing overall body weight. Feathers provide lift and propulsion, helping birds generate the necessary lift force and thrust to stay aloft. Together, these adaptations allow birds to fly efficiently and effectively.
Just because penguins, ostriches, emus and kiwis don't fly doesn't automatically make them not birds. They still have the same characteristics that make them birds like all other birds have: a beak, feathers, porous bones, etc. The ability to fly is not part of the definition of what makes a bird.