From what I have seen in this section, when cropping wildlife, most people center the subject instead of using the rule of thirds.
The rule is (and listen close) there ain't no rule

The reason you see so many shots in wildlife which have a very centred subject is because most people who shoot action based subjects in often overgrown or complex scenes tend to set the camera so that only the middle af point is active. That way they can point the camera direct at the subject they want to take the photo of and have the camera lock onto that - rather than let the camera pick the AF point because then its always going to pick the closest thing it can (that is how AF works) to focus on which might not be the thing you want it to.
Also (especailly the case for lower end camera bodies) the middle AF points tend to be the most accurate and fastest AF points with the outer ones being decent, but no where near as good. In the mid and higher range bodies the edge AF points are far more refined.
However after saying that there are no rules - well there are some guidlines you can follow - first one is to have frame in teh direction of travel or the subject or the direction of its attention (with most animals if they are moving their attention is also in that direction.) Its important to have frame for them to move into if they are moving or frame to look into if they are looking - if the viewers eye hits the edge of the frame when following that the shot feels constricted.
The rule of thirds is also a good guidline to follow when composing as well and certainly it gives a good basic starting point for being more creative with composure.
Of course composing without needing to crop in the field is a very good skill to master and certainly means better resulting images since you will no longer have to crop away and lose frame when finishing the shot.
If I take the example of your shot above if I were to crop it I would cut away the top areas above the bird and the ares to the left of the bird - leaving a little space to his left and above him (so as not to constrict the framing around the bird) but also leaving space to the right and the lower part of the shot. This is the area he is looking into and so it needs some framing to make it work. Though I might also crop away the green empty space below the branch as well.
though 95% of what I shoot is unusable, I'll blame my auto-focus, for now...
Camera: Canon 5D
Lens: Canon 300mm f/4L IS USM
ISO: 100
Exposure: 1/100 @ f/4.0
Thanks in advance for any constructive comments!
Bad photographer bad!
Ok whilst you have IS (and IS is great) I would really try to get your shutter speed up - your shooting on a 5D so noise is really nice for you. Myself I sit at ISO 200 as a base point to get that little bit of extra speed when shooting - in this case and with you camera I would have upped to ISO 400 and got a nice fast shutter speed. 1/100 is rather slow for a long lens and wildlife in general so try to get that speed up if you can. I know (bitter experience

) that its very easy to let the shutter speed drop low, but its also a harsh lesson that when one gets back that it results in soft shots.
The AF part might be helped by adjusting your AF method - looking at the framing I'm thinking that your not using your middle point and might be letting the camera choose - that is a bad move. If you do get a chance to frame offfcentre then do set the single edge AF point and use that - just get control over the AF system
