You'll need to tell us what settings you used (aperture, shutter speed and ISO) and how you processed them, especially how much you cropped, if any. Also what AF method did you use?
I'm assuming you cropped those pictures? I wouldn't have cropped so much. With pics like #2,3,4 I prefer the bird to be slightly off centre and looking into space (like
this I took the other day. Just an example to show my point. I'm not saying it's a great photo or anything).
If they are uncropped then 600mm may be too much reach for tits etc up close, a 100-400 or similar may be better (or 300 f2.8 if you have the cash).
I can't see the EXIF data but for birds up so close that aren't sideways one you may need to step down to f8 or lower. #4 is sharper as it is sideways on perhaps.
Anyway I'm not an expert, just my thoughts.