You might also try using Wide (L or S) as your primary AF point and, when you see the camera has recognized the subject, have another button set to engage 3D. I've been trying that and it works well for me,
I am using shutter release AF with Wide (L) selected as my AF area. When I press AF-On, it switches to 3D and picks up using the AF point the camera was using (AF persistence setting is on). That way, my initial lock is pretty easy - but the area not too large - and once I'm on the subject and can see subject recognition at work, I just press AF-On and can track all over the frame. I think it's easier than trying to get the small 3D area on the bird and I don't have to trust Auto not to pick the wrong thing
Still testing and playing though. I'm FAR from convinced my current method is the "best" method, but it might be something to try.
Going out Saturday and going to give this a try....though I'm not sure if my fingers are going to be responsive since I've been using back button for so long... I may end up assigning a different button if I can and keep using AF-On as programmed....