If you get a chance, try it stopped down. Even 1/3 stop seems to help a bit.
I think your images were very close to what you probably want or expect. Somewhere around f/9 my lens (with TC17) gets about as good as it is going to get. So I would stop down by thirds to about f/10 and see what you like.
Light is important. Your ducks were shot not to far below sunny 16 but I guess the light was diffused, and that can soften the look of feathers.
Support is not all or nothing. For $100 or less you can get a monopod, and that is good for 2 stops. That would help make up for stopping down. You may need a head too (that is debatable I guess), but Manfrotto makes one for about $35 to get you started.
It takes time to master that lens. More than a day or two.
Here's another image shot wide open, but at ISO 3200 1/2000s. Don't ask why ISO 3200; I had some sort of plan . The D700 did a decent job.
Reviewing the embedded original JPGS, I could see a little improvement as I progressively stopped down (and dropped to ISO 1600), and some of my f/6.7 images I did not like, especially a couple shot at a longer distance that could not be "keepers". But it sharpened up nicely and responded well to PM's added post-downsizing sharpening. This is actually sharpened more than I like with some haloing. But the image has to be sharp for that to happen. It is moderately cropped.
Compare to the shot above with the berry in the beak.
Edit: keep in mind this is 12 mpx FX, not 16 mpx DX, which is probably tougher on the lens, especially wide open but perhaps depending on how much you cropped your images.