Question: Why can't all birds fly?

Rupert Marshall answered on 16 Mar 2017:

Some birds don’t need to fly. If they live somewhere where they are not chased by predators then they might not need to escape by flying. As years go by the birds with big wings might suffer more by catching them on branches or by spending too much energy on them – birds with smaller wings might survive better. So, gradually a species’ wings might get smaller until they can’t fly even if they wanted to. This happened in New Zealand. There were no predators and the birds stopped flying. Now there are predators, introduced by humans, and the birds can’t fly away – birds like Kiwis are now very endangered. Some birds do very well without flying – penguins are brilliant swimmers.