Hi Karl, I really like this and one of the best posted to date of this subject IMHO.
I think you did well as light wasn't on your side, but I feel even with the MKIV you can push the ISO for better SS. As an option, if you have it, then a tad more above so you get the feeling of the weight coming down if you know what I mean. A classic pose and nice sharpness & clarity where it counts. If light was on your side then more DoF, but as presented it works for me. The part I'm not a lover of is sadly the perch, especially the notch at the bottom, if you can resolve that, then I feel you will be onto a winner with better light in the future. Just watch the patchy parts in the BKG at the bottom.
Perfect Nuthatch pose, nice bg and fabulous detail. I see Steve's point, but as an alternative I might wish for more room at the bottom since that's the way he's headed, depending on whether you have it and what the trunk looks like. You might also consider warming it up a bit to get the whites close to neutral.
I have a real soft spot for Nutties (albeit we don't get the white breasted over here), and this shows the classic Nutty pose. I think that maybe the head could take a little extra sharpening, but overall it's a nice shot
Excellent shot I to would go for a slightly looser crop. I agree the perch on reflection could be better but having said that I find it entirely acceptable, it doesn't keep taking my eye from the subject. The depth of focus in the eye area is a little limited, was this actually your focus point?? If your nuthatches are like ours they are hardly still for a second so I can imagine it would not be easy to get the point exactly where it is optimal.