I have tried many systems for flying bird. The video focus tracking capability (speed and accuracy) is very critical for this purpose. I haven't found the one I like yet, but I suspect something like Canon 7D or 5DIII is required.
I have concluded that no EVF will satisfy my need due to the time-lag. So Any Sony (Alphas, NEX, or A7/A7R), M43 (both Olympus and Panasonic), and Fuji X are out. I like Lumix FZ200, but it is still not good enough.
I also concluded that the raw low light capability is critical too. The boosted, post-processed, high ISO does not count. The pixel count and the pixel size must be balanced for the optimal result.
The lens does matter, not all lenses are equal. Even with, say 7D, I need to choose the right lens. This is where I have the most problem. It's hard to find a right lens with a right price.
Before I look into Canon and Nikon, I found Panasonic GH2 or GH3 the best in the video mode and in the focus tracking. They are fine for slow moving objects.