I am using my EF 70-300 is lens at present, with my 20D trying to capture wild bird images, but I am finding it a little limited. I am thinking of maybe the 100-400 zoom or the EF 400 5.6 usm any advise would be very much apprietiated. Mick.
There's a huge number of 100-400mm vs 400mm prime threads on here, Mick - all you get is zoom fans saying "get the zoom", and prime users saying "get the prime"!

I'm a 100-400mm user myself.
I'm happy to put my pics up against pretty much anybody's for sharpness, and - personally - I zoom the lens a hell of a lot (some folk reckon that you'll be at 400mm all the time "so why bother with a zoom?", but just today I was down to under 200mm a few times).
The prime is faster focusing, but the zoom has a much shorter minimum focusing distance.
The prime is a lot lighter than the zoom of course, but the zoom has IS.
Bottom line for me is that if I thought it was a good idea I could easily sell my 100-400 and get enough for it to buy the prime.
Ain't going to happen..!

Are these sharp enough for you?
Thank you keith for your reply, i have looked at some of the archive threads, very interesting, I have a lot to learn and read.