Well, somebody more talented than I could probably do some clone magic to replace the missing wingtip, but it ain't gonna happen in this lifetime from me!
All you do open the shot twice. IN one copy marquee the wingtip, then move your selection to the other copy. enlarge your canvas, add some more sky to the bottom.
Then take the piece of wing you added, flip vertically, flip horizontally, move it into place, free transform to get the fit right, and voila!
Or, yes, you could use the seagull filter, if you like the easy way out!