How big is the birding/ wildlife photography base for a high end crop camera? I know that the bird/ wildlife pro photographers trend toward 1DX, but there are some very well respected pros using 7D and the Big Whites. Amateurs with this interest are grouped into "money no object (already own a Big White)", "value for money, middling budget (using a Little White 400, 100-400, or a Tammy, planning on upgrading to Big White eventually)", and "bargain basement / don't plan to invest in a Big White, will stick with Little White". I am in the middle group and am a good sales target for a high end crop camera. The last group will be reluctant to pay a premium over the 70D for a higher frame rate. The first group? I have to say that I have not seen many 1DXs in the hands of amateur bird/wildlife photographers locally, with the exception of a very few tripod/blind shooters.
I can afford a Idx, and I would not buy one for birding even at half the price. I tried one and tried the 5D III. Those are not birding cameras IMO. Or at least not for me.
After the new big whites came out with the new FF cameras. My friends that I shoot with changed very quick. And just raved. I have been looking at there photos for the last two years. The detail and quality of there photos have gone downhill. And not just by a little. All of them also bought the new 600 to go with the new cameras.
If interested here is a quick informal test I did yesterday. Crop cameras are the best birding cameras IMO beating a FF pretty handily. Especially with the new crop sensors from sony.
Good thing in your post you put "IMO."