Gabi, I really like the image overall but the comp isn't the strongest IMO. Reason being - his eye is almost dead center in the image frame and it feels too long vertically for how small and "squat" the bird is. If you're dead set on leaving the image this thin then I'd take a chunk off the top so his eye is lowered closer to the ROT's position within the frame.
Make sense?
Lovely BG and awesome detail on the bird. You did a great job of controlling the blacks and whites on the head. There is a small spike on the black end of the histogram so try bringing those dark tones up just a little and you might recover even more detail in the head feathers.
Would be better if more room at the bottom. Negative space is not necessarily a bad thing ;) So either you get closer/crop tighter, or give it more room.
I'd give the bird a little more head room and, as suggested above, a little more bottom.
This may sound trivial, but I found your copyright is a little distracting. It might work better if it was vertical, or vertical, white, and in the tree bark.
(EDIT: Forgot to mention that this is a great shot plus a wonderful and unique pose.)