Bsmooth, a fixed length prime telephoto will outperform a zoom. You're at 520mm/f5.6 and nearly 3kg now.
You'll have to sort out the FX vs DX body question yourself. What you gain in reach, you lose in high ISO ability (2000+). It's a matter of striking a happy medium for the type of shooting you mostly do. The body is nearly double the weight. Yours now, in between the two. Whichever set up and lens you ultimately choose, you should be able to walk around with it on a sling set up.
To my mind there's only two lenses you should be looking at - the 300mm/f2.8 IS II, or the 400mm/f4 DO IS II. Both are light enough to hand hold and use for BIF.
The 300mm/f2.8 MkII is 2.35kg but beautifully sharp and AF's well with TC's. Pointing it for BIF is just a matter of practice. There is a noticeable difference in handling for me between the Mk1 and the MkII 300mm/f2.8 IS. You wouldn't think ~600grams would make that much difference, but I find that it does for BIF. You might use it with a 1.4xTC on your IDMkIV (for 545mm/f4) or a 7D II (for 670mm/f4), or go to 2xTC on the FX IDX II (600mm/f5.6). A friend uses it with 2xTC on a 7D II for 960mm/f5.6 and is still reach limited at times.
The 400mm/f4 DO IS II is a bit lighter at around 2kg, and could be used bare on your ID MkIV for the same 520mm at f4, or use the 1.4xTC for 730mm/f5.6. (560mm/f5.6 on FX). On the 7D II it would be 640mm/f4 bare, or 900mm/f5.6 with the 1.4x.
As far as shooting setup goes, I use Auto ISO. I set the lower ISO level to the camera's minimum, the upper level to 2500, and specify a shutter speed of 1/2000th sec wide open. I then adjust exposure compensation depending on sun angles and black or white birds. Some advanced photographers poo-poo this, but while you are spending a good year or so practising pointing and maintaining AF, I find it's just a whole lot less to think about in the heat of the moment.
As always, the best advice is going to be to see if you can try out some of the above camera/lens combo's yourself and see how you find shooting with them, and the results. Good luck! :t:
Chosun :gh: