Hello Chrissie I'm sorry for my late reply, I don't enter the forum much. I come from the world Nikon and Canon for nature, always with prime lenses. It is the first time I use a zoom for photography.
My experience with Nikon was very good with the duplex D800 + 300 f4 still with teleconverter (always talking about BIF) and absolutely disastrous with the D810. When I sold all my Nikon equipment, I switched to Sony but in nature I bought the 7D Mark II with the 400 f5.6. It gave me a very good result.
When I left the A7R3 I bought it, at first I used the 400 canon with a beautiful adapter (Metabones V) for birds perched but impossible for BIF.
The thing changed with the FE 100-400, I can focus BIF of birds that fly between trees, those that just take off, to track the ones that I already hooked flying, even with very low contrast. Always use AF-C with focus by zones (the second in the list) always at the center. Never shot less than 1/1600 because fast. The OSS is always on but I guess it will get better when it's off, I don't know and I'm not going to find out.
For me the secret of the BIF is to always focus one step before, keeping the bird in the back of the direction where one moves.