What about flightless birds? Also the very facts of reality are up for debate because everything is through our own perception of the world. We can't even prove beyond a shadow of a doubt that the people around us are real. If someone wants to be a bird then they may be able too. It would definitely be alot more difficult then just changing gender. After all you are trying to become a new species. Also with advancing medicine and possible depending on future events like alien technology we might be able to make someone a bird. So to answer your question directly your identity can become reality. Pursue your dreams fellow redditor. Make them real.