Somnath, I have been shooting a D7000 for three years. Mostly nature but with a heavy emphasis on birds in flight. I found the AF speed very frustrating. I recently gave up waiting for a pro DX and upgraded to a D750 (then Nikon's best AF system).
My success rate at capturing birds in flight has increased massively. Even on static subjects the AF is a lot more accurate.
If the D500's AF system is as good as it looks on paper, this camera may well be a game changer for nature and BIF.