300 F4 1.4TC vs 100-400
If you need lens for birding I vote for 400mm, it is very sharp wide open, and has very fast AF (faster than 100-400 or 300 f/4 + 1.4 TC), and handles wonderfully. f/5.6 is not very fast aperture for a 400mm lens, so you will probably shoot wide open almost all the time, prime is sharper wide open than 100-400 or 300 f/4 + 1.4 TC.
here is my experience with it:
You are welcome to have a look at my gallery:
|_F0A5334-Edit_small by Dester Wallaboo|
from Open Air Fashion Photography
|Feed me, me, me, me, me by Denjw|
from Attention-Seekers in Nature