You have several possible solutions here.

a 8*10 means you need to frame the bird correctly within this space, this is strictly a matter of taste.

You could also use a 1:1 crop ratio to get e 8*8 or 10*10. but there again the position of the bird will make or destroy the capture. The rotation of the bird (by 45 degree) will completely change the vision...

In term of colors you also need to be careful. As you can observe you have many options offered already. Your best bet is to give a feedback so that one can follow your idea(s) and produce something you can live with.

Head to the right or to the left, flying over (ordinary)...
Head to the top right, flying away (safe)...
Head to the bottom left, flying toward (a bit threatening)...

My experience with that lens is that IS seems to slow focus acquisition down a little but doesn't seem to affect tracking once it is locked on. You should be using IS selection 2 for panning as that ignores the horizontal motion of the camera and only stabilizes it in the vertical axis. I also tend to shoot at f/8 for the extra DOF and I think that lens is a little sharper there.

At 120mm it appears that the GBH was quite close to you. I say this because it doesn't look like you've cropped at all. At that near distance, the relative motion of the bird in the frame is quite fast and that may have been the biggest factor. There appears to be a slight forward and upward motion of the head, consistent with the natural motion of this bird in flight, that was not frozen by the 1/1600 shutter speed. I regularly shoot at 1/2000 and up and still can see some subject motion. And I am not referring to wing tip blur but rather the same type of motion I see in you shot.

The truth is these are pretty good shots. You've kept them in the frame and It appears that you have focused on the body of the bird which should still give you sufficient DOF to keep the head/eye in focus. Depending on the print size and the viewing distance, these could be quite acceptable shots.

Keep practicing and have fun!