Reading this thread with interest - I'm looking for a better bird photography setup than my 4/3 system, without getting too crazy heavy. I usually carry my Camera when walk-around birding, to get shots of opportunity, or to document uncommon birds. Auto-Focus speed and accuracy is the most important aspect for me, as the brief moment I often have for getting a bird shot often results in no pic or an out-of-focus pic with my current gear. Better low-light performance is also a goal.
Since I'm almost always at max zoom (and could use more), the 300 PF f/4 lens coupled with D500 or D7200 are my top contenders at the moment (along with a 1.4x or 1.7x TC when needed). Around 4lbs - twice the weight of my current setup, but still much lighter than the 80-400 or a Canon 7D MII / 100-400 setup.
I wonder how much better the AF is on the D500, specifically for birds, when they're not particularly close?