What a shame. I think it's just a case of the duck not being critically sharp. It seems to be purely a focus error without any slight camera shake being involved.
Which focus method were you using? If it was the one shot method don't forget that when you compose your shot and half press the shutter button, focus is locked as long as you keep the shutter button depressed. If you hang around too long before taking the shot it's very possible for even the slightest movement of the subject or yourself to mean that locked focus position is no longer accurate. For that reason, if I have a subject in the viewfinder and I'm waiting to actually take the shot, I'll keep releasing and half depressing the shutter to reconfirm the point of focus.
When you're using long lenses it's particularly easy for the slightest backwards or forwards movement of your body to cause you to lose critical focus from a locked focus position, which is why I like to release the button completely and do a complete smooth depression with only a slight pause halfway to confirm focus before actually taking the shot.
The reason I think this may have happened is that the focus point seems to be slightlly in front of your duck., and I've had this happen to me more than once.
*Waits for Doddy to say it was in servo mode*
