I used the R6 with the adaptor and 400 DO and 1.4x converter for all the bird and insect images on the trip -Results WHEN it picks something up in flight it sticks with it and the AF seems almost infallible but the problem is Focus Acquisition particularly against the sky - for some reason it seems to just not want to find even big birds in a white sky and I have missed several raptor images because of it but when it does lock on then its great - I took 22 shots of a Hen Harrier approach, side on and going away and all were sharp but the second problem is the viewfinder lag and with a fast moving bird it simply cannot keep up at 12 frames per second and you end up with the bird in one corner of the frame or at the top right or bottom left and have to wait for the viewfinder to catch up and get it centred again; image quality is very good, not sure if its up to the 5D4 but not tried it on any sort of real high ISO yet. Battery life is poor and I find even on limited use I have to change a battery a day whereas one often lasted a week on the same level of usage on the 5D4. So you are between a rock and a hard place; better AF when it locks on but you miss sometimes vital shots v lower success rate with the 5D4 but better focus acquisition against the sky, oddly with limited contrast the R6 seems to find things better against cluttered backgrounds and the real result is that you cannot be using both cameras at the same time so as usual everything is a trade off and currently I am sticking with the R6 - lets see if it can find Woodcock and Nightjars tonight!
Included below a full frame shot of one of the later cropped Little Terns to show the way it can pick up small in the frame birds and attain sharp focus immediately and image quality even after big crops is still good - all processed in ACR and Photoshop CC plus Topaz Denoise A1
