I tried a pano crop here because I liked the splash at the far right. Curious to know if this works. This is 87% of full-width. See the next pane for an alternate crop.
D7000, 500f4 + 1.4, Auto ISO @ 1400, 1/1250s @ f/7.1, +1.0 EV, tripod.
Here's a more conventional 3:2 crop, this one at 66% of full-width. It lacks the interesting splash of the first pane, but has the advantage of making the bird larger in the frame. That's also a disadvantage since the IQ is less than perfect. (I underexposed, and lost some image quality when I brought it up in RAW.) Let me know which you prefer.
Bill, I am on my new ipad2 so hopefully I am seeing the frame correctly. Tend to like the larger image, the male mallard is so beautiful the closer the better regarding the eye, color and feather detail ,..let's see what others say. Also, he is the subject and I assume the reason for the image. Regarding the crop my rule of thumb is as long as the subject is at least 20% of the frame it should not have a material affect on IQ,..but you are correct it is important.
Bill - I like the shape of the splash, but I always enjoy the effects of the motion on water, water spray, droplets, etc. Either crop works, but I like the OP a bit more. I would move the bird down in the frame if it were me - crop from the bottom. TFS