As others have said, the 7D will be the best camera. Fact is, the 7D has superior AF which can't really be touched by normal 9-Cross Focal points. Furthermore, the 7D is basically the 550Ds sensor, which is excellent, the 50Ds build, and then an AF system which is better than both the 50D and the 550/650D.
I've used the 550D for a year and a bit now, and while it's served me very well and was great for landscape photography, as I turned more towards birding, I started to feel it's short-comings.
In fact just today I opted to 'cross-grade' to a 50D and sell my 550D. The reasons for me were simple. The 550D has never worked with my 400mm 5.6 L perfectly, there has always been some softness issue and during controlled tests it was showing to front focus a lot, now having the 550D (or 650D) there is no option for me to correct this, I would need to send it in to get the body calibrated to the lens, which would then mean if my other lenses like my Sigma 10-20mm were syncing in the opposite direction on the body (back focusing), it would mean that the calibration may fix my 400mm, but it would make my backfocusing lenses worse. I think that MFA is an extremely valuable tool. I have seen people getting photos that are a world different after calibrating their lens to their body. The 50D and the 7D have MFA (micro focus adjust) abilities, where as the 550D and 650D do not.
Next was the issue of frame rate, I can't tell you how many times (using a class 10 SD card) I had a raptor in flight pass quickly overhead and it was only a couple of seconds before my camera stuck on the 3.7FPS and ended up spending 5 seconds writing to the card, giving one only 3 or if lucky 5 shots in a situation like that. Missing a lot of vital frames inbetween, this was another major reason for me opting for the 50D. The 7D has even BETTER FPS too.
The build was never something I worried about, but the 50D and 7D have great body builds I've heard.
The plus side to 650/550D as opposed to the 50D is the sensor, it has a better sensor which means you also get a bit less noise at high ISO, but this is something that one likely needs to worry about less if you're shooting primarily birds.
The 7D is still my desired camera, which I am currently saving for - and if it's an option for you, I'd definitely say it's the right one. It's essentially the best of both worlds. A semi-pro build, with MFA, amazing focus abilities, comfortable in hand, video capabilities and also with the same excellent low noise filter that comes with the 550D for example. It's by far the best cropped sensor Canon available and excellent for using with a 400mm 5.6 L.