> you were absolutely right...(and why shouldn't you be)
Lots of reasons, actually. For example, one possible reason would be that you have a defective lens or camera, and the correctly indicated focus is not in fact correctly focused. Or perhaps because you might have been shooting AF-S intead of AF-C and while the camera locked on, the bird may have moved a bit. Or ...
But in this case, probably not.
> I'm wondering why not. I must be doing something wrong?
Pay attention to the rangefinder dot in the finder. If it's showing the dot, release the shutter. It's telling you something. Note also that while you're in the field, I'm pretty sure that there is an option in the menus to have it tell you what focus point was locked on.
One possible answer: the AF targets are now pretty small (they're especially so on my D3). It doesn't take much of an aiming wiggle to get a different distance under the AF target, and in AF-C the camera is set to Release priority, so you can easily get an out of focus frame. This is the flip side of the "my camera won't release the shutter" problem - and quite possibly in the same conditions, but with different "error messages."
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!