For Kingfishers and Pelicans I need to go to Botswana again ;-)
But I have an European Bee-Eaters colony 10min from my house and also Marsh Harriers. They will come in spring. I am alerady quite good in following bee-eaters becasue I had a lot of practice. They also have kind of approaching procedure. So, I will make tests when they are here.
Schooting in 120fps - interesting, never thought about that
And focus peaking is a good idea.
I also think that the camera slows everything; the initial focus as well.
The camera triggers and if the lens-response is slow then AF is really slow! By 500 PF the response time is very good. By the way I read somewhere that the cameras have kind of lens-database (this is why we get all those firmware updates) and the response time is actually there (along with other lens attributes) so, the software says how fast and how far to move. So, theoretically Nikon can favour own lens, who knows.
Sigma lens are very sharp and have a nice bokeh but slow in comparison to Nikon. However, it is not "hunting" and sometimes it is even an advantage.