Why can’t Chickens Fly?
Most chicken breeds are still able to fly short distances. For example, flying up into a tree (that’s where they would naturally roost), or to escape a predator.
They certainly are not good at flying, though. There are two reasons for that.
Chickens were bred from a wild species call the red jungle fowl. These jungle fowl are a little more adept at flying than chickens are now, but they are fundamentally more adapted for a ground-based life
All of their food is located on the ground, and they have an adapted beak to match. Their feet are adapted for walking, rather than perching. Its wings have become partially vestigial since the survival of an individual no longer relies heavily on flight; instead, natural selection has advanced those ground-oriented traits. So, to recap, chickens are bad at flying because their direct ancestor was bad at flying, because they’re adapted for spending time on the ground.
2. Selective Breeding by Humans
Chickens are not a natural species; they were created by breeding the red jungle fowl into a new organism. Since humans were responsible for the gene selection process (“artificial selection”, as opposed to natural selection), chickens were bred not for survivability traits, but to have great big tasty breast muscles. Chickens’ ability to fly has only worsened under human management because no breeder has prioritized that, opting instead for edibility and commercial traits.