Lens focus issues?

[Always Looking]

I purchased a 60d about 1.5 years ago and have gotten into wildlife and bird photography (also shoot my son's high school baseball). I purchased a new 400mm 5.6 about a month ago and am getting used to it. I've got more birding keeper photos with the 400 in the last month than I have in the last eighteen months. I used to use my 70-200 f4 (non-is), occasionally with a 1.4II.

Most of my roughly 1000 photos with the 400mm have been in focus if I hit my target (center point), often aimed at the birds eye. This morning I went to a new spot and found a few good opportunities at waterfowl I have not seen yet. I took about 100 shots, mostly of swimming ducks and a few in flight. It was drizzling on and off all morning, overcast and not ideal by any means. While shooting I thought to myself these might be my best photos yet. When I got home, I was very disappointed. It appears that most of the shots were out of focus, despite the fact that I hit my target (eyes) on many occasions.

Can a lens work fine for a month then have problems with auto focusing for no apparent reason? I assume I cannot do anything to correct this and may have to send the lens to Canon if the problem persists. Anyone have any ideas how a lens can appear fine for about a thousand photos then act up out of the blue? I will try another lens tomorrow and see if the problem is the lens or the body for starters.

Please share your thoughts.

[HDNitehawk]

It was drizzling on and off all morning, overcast and not ideal by any means.

Perhaps this had something to do with this.

[Always Looking]

#1#2#3#4

All shutter speeds are manual, all fstops set manualy, all iso on auto. I did not do any processing on these, not sure I wil even bother with how bad they look.

#1 1/400 5.6 iso400 This one of the sharpest shots I've gotten so far, all of these are from this morning.
#2 1/1000 5.6 iso250
#3 1/640 5.6 iso400
#4 1/500 8.0 iso500 (the blurry vertical lines are from the weeds, I don't think they were in front of the target though)

I added the data since I don't know how to include EXIF data (yet).

[andnowimbroke]

To answer your question about the lens: Yes, even if you had a 50D and used the microadjustment feature to get it "dialed in", there is still enough slop in the mechanism to get soft shots. Even the normal use of a lens wears down components let alone the accidental drop.
What focus mode were you in though? Assuming the camera could lock on to small detail on a dark face in not so good weather, those reeds could play havoc in servo mode.
Don't give up just yet. Tomorrow is always a better day.

[Always Looking]

I guess it makes sense. In the four photos above, the three with the lousy focus the lighting was gloomy at best. With the gull photo (#1), the clouds broke and things got a lot better. I was in one shot focus mode.

Thanks for the information, obviously I still have a lot to learn.