The 7D should be ok noise-wise up to around ISO 1600, but it depends on the subject and also tolerance of the user and/or viewer. However, the high pixel density of the sensor is unforgiving of even slight imperfections, so if your lens isn't correctly microadjusted or atmospheric conditions are less than perfect, then you will start to see reductions in image quality very quickly. I found that the traditional minimum shutterspeed of 1/focal length needed to be increased to get consistently sharp photos and for fast moving subjects, such as birds in flight, high shutterspeeds were needed to freeze the motion. This was a particular problem with the 100-400, because of it's narrow maximum aperture, as poor lighting conditions made it unusable very quickly. One thing I did find with the 7D, was that the AF didn't perform particularly well in high and low contrast scenes, somewhere in the middle it was very good, but less than perfect and it started to struggle.
If you want something to shoot birds with better image quality that the 7D, then the sugestions of the 6D and the D600 aren't really the best options. If you crop 1.6x, you'll be down to the 10-11 MP range and there will be situations where around 4-5 fps won't be fast enough. You either have to get the 7D or 60D or stretch the budget to a used 1D MKIV or 5D MkIII. Even if Canon released the 7D MkII soon and it was released quickly, the price would probably be pushing towards the 6D price (which, incidentally, is nearly £200 cheaper than the D600 in the UK).
