I used a 400 5.6 with & without a 1.4 for 2 years. I really wanted to upgrade to
a better lens. I rented a 300 for a week & shot it daily. When it was said & done
I decided the 300 f 2.8 was not much of an improvement(to short) & continued to save, a few months later I managed to get a 500. I am so glad I waited.
If birds are your thing keep your 400 f 5.6 & get the 500 when you can, anything less is a half step.
JMHO
