March, 2009:
After realising the advantage of the high resolution of the 50D (ie it captures more details compared to 40D) buying the 21MP 5DkII for birds was an easy decision. This camera must be able to capture great details. The added bonus of video was something I wanted to be able to capture the behaviour of the birds, which sometimes is impossible with still pictures only.

After using the 5DmkII for a couple of months I can only confirm that my predictions were correct. Indeed this camera captures great details. It has a similar pixel size as the 40D, which means that the 40D and 5DmkII captures the same amount of details from a given point. So what's the point of using a 5DmkII then? Let me explain: Getting feather details to show is no problem with the 40D. You have to be fairly close to the bird of course, usually no problem when using a hide. The big problem with the 40D is that you will cut off the head and the tail since the image is cropped. This will force you to move back to capture the whole bird and then you loose the details. With the 5DmkII you don't have any cropping so the head and tail and some background are there, making it a full picture.

Compared to the 50D though it's a tighter race. Still the 5DmkII wins but you have to move in closer to beat the 50D.

The video turned out to be almost addictive. I'm using it much more than I expected and sometimes I even forget to shot the stills. The video feature doesn't offer much settings, only exposure compensation. Shutter, aperture and ISO are all automatic. Works in most cases for birds but for creative video shooting at least aperture control would be desirable. Anyway I now have tons of videos.

My videos are only documentary and I'm not doing any advanced video editing. However if you want to be serious about HD video you will need:
1) A lot of disk space
2) A fast computer with at least 4GB of memory (mine is too slow for HD editing)
3) A good piece of video editing software (I haven't found any yet)
4) Time

Currently I'm using MPEG Streamclip v1.2 which is a free and simple application converting from quicktime to other formats. I have posted my videos on http://www.vimeo.com/user1569901/albums.

My conclusions so far:
- Low light AF is not at good as the 50D
- The thinner DOF of a full frame camera means you must stop down more to cover the bird (quite often I use f10-f11)
- Unbelievable details, but you have to be closer compared to the 50D
- You can shoot up to ISO1600 with good results, which is a fstop better than 50D.
- The builtin noise reduction will smear the details at ISO800 and 1600. I will turn NR off and do everything in post processing.
- Turn off settings for Auto Lightning Optimiser and Highlight Tone Priority (for RAW shooting).