Steen: I know, the 400/2.8 is a beast. It weighs almost 3 lbs. more then the 200-400/4. I think the 200-400mm/f performs well at both ends of it's range. I've also used it with the TC-E14 successfully, IMHO. The attached photo was taken a few weeks after I got the camera and lens (just learning). And, it should work with the new 200-400/4.
The sweet spot for birds, seems to me, to be in the 400-500mm range, and you need a moderately fast lens/camera to shoot a 1000s +. I had a 500mm f/4 VR, but, it was too unwieldy, handheld (weighs 2 lbs. less then the 400/2.8, though). OK on a gimbal head tripod.
I've even tried a couple of the __-500mm Sigma's, but, always returned them because of unsatisfactory IQ. Nikon glass spoils you.
D3; 200-400mm f/4 w/TC-E14; 550mm; 1/1000a @ f/8; ISO 3200
Cheers, Matt
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