Focus Areas with K5/ 150-500

Some advice, please.

I use "spot" focus area for photographing birds.
This afternoon I was out taking snaps of a pied wagtail.
The closer ones are quite a lot sharper than others, the focus point having missed the target.
Could it be the case, do you think, that for quite a small target, it's difficult to judge whether the focus point is on-target, but when the target (subject) gets nearer &, therefore, larger in the frame, one can be more sure the subject's in focus.

If the answer to the above theory is "yes", what would be the better auto. alternative, or would one have to resort to manual?

Hope I've explained it clearly, any assistance would be most welcome.
(What do you do?)

Thanks,

Roger.

P.S., see the Gallery for an "in focus" one!)