AF-C d51 is what you should be using. This give you a single point to establish focus and then the widest area under which the focus system will continue to track it. Using just a single point requires you keep that point on the object or lose focus.
From there it's critical to adjust your focus tracking settings so they best allow you to follow the object without losing focus should something else (branch, tree, another bird) come into the focus area. I use a D500 for birds so my settings are a little different - you need to experiment to find out what works best for you.
This question gets asked about every 2-3 months here and every time I do I remind folks of the search feature and then (re)post this video.
As was mentioned, shutter speed is critical here, as is the proper aperture for clarity (Shutter Priority will usually have you go to wide open which can hurt clarity). I shoot in Manual Mode setting shutter (1/1600+ though I may go lower depending on the light), usually around f7.1 on a 300mm f4 PF with 1.4x, and Auto ISO set with a max at 4000-6400 depending on the situation.