- Heavy bones: Birds have hollow bones, which makes them lighter and allows them to fly. Heavy bones would make flying more difficult.
- Cold-bloodedness: Birds are warm-blooded, which allows them to maintain a constant body temperature and stay active in cold environments. Cold-blooded animals would have difficulty flying in cold conditions.
- Teeth: Birds do not have teeth, which reduces their weight and makes them more aerodynamic. Teeth would add unnecessary weight and could interfere with flying.
- Fins: Birds have wings, not fins. Wings are specifically adapted for flying, while fins are used for swimming.
- Long tails: Some birds have long tails, but these are not specifically adapted for flight. Long tails can be used for balance, steering, and display, but they do not provide lift or propulsion.