Thank you for your opinions! They're really appreciated.
Nevertheless at the moment I have no idea what to change exactly as you all mentioned different things. Eduard, on my monitor the sky doesn't look turquoise at all (and I believe that I have calibrated my monitor correctly). I agree to Peter's statement about the overexposure and Codeshare's one about the dark wing. I will rescan the slide and try to improve it.
Btw, Codeshare, how would you crop this shot in a different way?