Cheers Andy - indeed I am a birder with (or actually right now without) a camera rather than a photographer. I just don't want to shell out a lot of money on a camera which is too complex for me to use. Also I am wondering if the 600mm lens is enough for me if I compare to cheaper camera's with bigger zoom.
Diffraction limits the effective resolution of the ultra zoom/small sensor offerings, so that a cropped image of a lesser zoom on a larger sensor can be very comparable. Sadly there are no miracle fixes in optics, just trade offs.