Arthur, I've been thinking a lot about this flying thing and how to represent it. so FWIW here are some things I'm musing over for creating images.
•Certain wing positions give a "flight" message more than others
•Hard lines and edges stop motion, conversely softened edges will recede, and even more softened edges will let the viewer imagine it moving. Blurred edges in the right places will suggest motion.
•Using lighter values where the motion is suggested, and darker ones where the eye is to focus will give the feeling of movement and more weightless mass to the eye. For example in the drawing above where the wings are outstretched, if the tips are lighter in value than the head, and the line is slightly softened there at the tips, the wings will suggest movements that the eye perceives to be flightlike, as this is how it sees in real life...
I know this kind of comment is not usual here, so if I'm out of line, let me know and I'll shut up.
I come from a little different place, as my orientation is from a complex art background and art history study and only recently to birds. If you want to show the correct delineation and illustration of the bird itself, then my comments are not very useful for that.