Sorry about that Alan, you're right! I apparently didn't check the focal length of the owl shot before uploading, I have plenty of 400mm examples that are of excellent sharpness to choose from as well. The image of the Cardinal above the Owl is at 400mm, with the same lens...100-400mm is really irritating on the 7d too... (sarc., implied )
I downloaded the photo to check the exif data. It is at f = 190mm, the sweet spot of the lens, not the full 400mm, the weakest length. The lens is very sharp at ~200mm and f/5.6-8. I have had some great shots under those conditions, as well as 400mm when I could fill much of the frame.
You must have been very close to the owl to get it to fill so much of the frame. And, that is the way to get the most from any lens. Here is the lens with 100% crop on 7D at 100mm of terns feeding (is0 400, f/5, 1/1600 s).
When I first got my 100-400mm several years ago, I wasn't totally thrilled with the sharpness. After spending hours tweaking the MFA for that lens, I was blown away with excellent sharpness at both ends of the lens as well as the sweet spot around 200mm. Since then, the first thing I do with a new lens is MFA... Kind of a pita as it were, but totally worth it every time!
Thanks for pointing out the focal length on that image, as we were discussing maximum focal length for the lens, not just overall quality and/or sharpness.. Additionally, I may point out that it was handheld as well. Yes, relatively close as the Owl flew and landed nearby me. Also, this is a cropped image. I was probably in the 75'- 85' range from the subject and eventually worked my way quite a bit closer...
All the best,