Thanks for the feedback, everyone.
The lens used for all shots is my FA*300 - my walk-around lens of choice. No TC used here, and I don't use filters.
The mallard was approx. 30 yards away. I didn't lead him by quite enough, resulting in less space ahead of him than I'd like and the resulting oddball crop size (cropped about 30% overall). If I make a print, I'll probably have to add more canvass to the left side and a tad at the top.
Jer- I try never to shoot birds at less than 1/1500, unless they're soaring lazily (never happens!) and my past experience as a hunter lets me get on the birds quickly.
Kent- I actually use a K100, not a K10.