I am getting into birding too, especially keen on capturing birds in flight. It is a challenge....especially with PD tremor. However, the 5d mk3 has been a revelation for me. I have been using it (hand held) in combo with the 70-300L Canon lens......but I have a hankering for something longer AND prime! Been looking at the older FD Canon fit 500L. It means an adapter and manual focus. The could be disastrous as a purchase if I don't get on with the manual focus. I can only do it on a tripod. However, for tracking flight of a fast bird it would be cumbersome on 3 legs rather than hand held. Have also looked at the Sigma 150-600 but not sure of quality.