Also keep those shutter speeds up. Even birds that are perched it is best to stay above 1/800s so that you avoid blur due to their quick movements of the head. Birds in flight it is best to be faster than 1/1000s.
What type of birds are you shooting? Are they the size of songbirds or hawks or egrets?
50-100 feet is a long way away at 510mm (your 3oo/f4 + TC17) on an FX body. Even with the D800 I suspect that you are cropping deeper than the DX crop mode if the birds are smaller than a large hawk or an eagle.
>> “...I would like to know what contributes to sharp, distant bird feather detail that I see in the pics some of you have.” I have found that filling the frame with your subject (or in case of the D800 filling at least the DX crop frame) is the best way to get that great feather detail that you are looking for.
Work on getting closer, using feed or water to attract the birds and shooting from a vehicle or a blind.