You need a lens with blazing fast autofocus response (usually a prime)... if you want to catch birds in flight. For sitting birds, need a large aperture. So to summarize... the most expensive lens. Also a (very) sturdy tripod and probably a gimball head.
And if not bought the D500 already a good (superzoom) point and shoot is more versatile.
I (usually) don't enter competitions, so got binoculars instead
Way more fun... and waaaaay cheaper! Also better for tracking and studying wildlife. [But then I also don't have a social life, so YMMV]