I'm not sure that sharpness is the problem here. The eye looks sharp on my screen, and the movement in the wing adds something for me that does not detract from the image. But, there is something odd with the look of the bird's plain grey chest that makes me wonder if there is some noise reduction going on? If so, Alexander, I would process the image again. If that is not noise reduction, maybe do something counter intuitive and add a little grain to it for a bit of texture. Or, perhaps downsizing has added the strange look to the plain area?
As i was there with Alex, and i do remember that afternoon well.....(hard to forget 5 Cuckoo's flying around us..!!)
I also remember how hard it was to shoot this scene as there was wind and a bit of rain and on top of that the Cuckoo was balancing on this very small perch...So the answer to this is slight motion blur....
I got the same shot with some twigs in front of the Cuckoo due to the wind, so i really think this is quite an achievement!
Well done and hope we can get these guys in front of our lens again next season!