I currently use the 5d3 for birding and I love this camera and I find now I cannot go back to crop (7d). My particular love in birding is capturing bird in flight and action (behavioral).
What I have noticed is my camera is slow (buffer, I am guessing when I shoot continuous) - my card speed is 60mbs (sandisk). I spoke with a very good birder who has both cameras and said it was my card and to try the 1000x (150 mbs). I was on the verge of buying the 1dx, but I decided to ask him some questions before I hit the buy button. So I ordered the card and hope to have next week.
Anyway, I went to a camera shop today to use a voucher I had won for cards. So, I was asking for fastest cards and he said it was my 5d3 that was slow and that the 1dx was way better. He said that the 60mbs was the maximum that the 5d3 could cope with.
I know that the 1dx does double fps that the 5d3, but I am reasonably happy with the 5fps of the 5d3, but finding it really noticeable the lag and sometimes it seems ages before I can shoot again.
What are your opinions on this, as I thought the technology in the 5d3 is pretty close to the 1dx.