What really makes this difficult is the head angle Loong. It should be at least side on or turned a few degrees towards the viewer. Also, I feel the bill is the sharpest part of the image. You could probably add some sharpening around the eye as perceived sharpness in that area will generally give the feel of overall sharpness. I like the colors though and you composed with a nice BG that enhances the overall image! Thanks for sharing. :)
Nice species and super colours. Maybe you should consider opening up your aperture and winning some more shutter speed. 1/125th seems pretty slow with a 600mm lens. The European Bee-Eaters are really fast moving, so I guess these are going to be the same!!
Top marks on composition but I agree a better head angle is needed and point of focus must be the eye. I too use a manual lens, so I know how challenging this can be! This is such a beautiful species so please keep trying.
One thing that might help is to do your noise reduction on the background after you sharpen. That way the noise reduction will have a sharper line to work against. That will help to eliminate the blur you see on top of the head.