Why can't all birds fly?

Answer 1

the weight of an bird
because of the way some birds are built their body structure wont allow them to fly

Answer 2

The main reasons involve the feathers and the skeleton. Special feathers are required for flight and the "Flightless Birds" have none of these special feathers. There is a unique part of the breastbone called the ridge. An extended part in the center of the breastbone is where the stronger muscles used for the wing movement are anchored. Finally the flightless birds have solid bones to transfer blood and oxygen through the bones themselves. Flight able birds have hollow bones to actually lighten the internal structure and weight of the bird. They also have much heavier blood vessels to transfer blood and oxygen to the limbs.

Answer 3

Not all birds have the physical adaptations necessary for sustained flight, such as large wings or strong breast muscles. Some birds have evolved to be flightless due to their habitat or lifestyle, such as penguins or ostriches. Flight requires a significant amount of energy and resources, so for some birds, being flightless is more efficient for survival.

Answer 4

2 reasons - one is the same as why you can't sit in a vacuum, the other is that to fly anything needs support from air.