Bats also fly but their numbers don't compare to that of birds. Flight takes special physical characteristics in living things to be possible. Only a very light animal can fly.
Birds have special adaptations that allow flight to be possible.
A bird has hollow bones that are both strong and light. A bird also has muscles that are thin and light. A bird uses a lot of oxygen and energy while in flight. They must eat frequently to replenish their energy reserves. It has a heart that pumps very fast to to keep the blood supply speeding throughout its body.