I haven't put more than perhaps a day or two worth of hours into bird photography, but even from that limited experience, it's a genre I highly respect. With wildlife photography, there are two sides to its difficulty: the understanding of wildlife and the understanding of your camera. You could be a technical master, but without knowledge of behavioral patterns and hotspots for the wildlife, you'll come out empty-handed more often than not. Similarly, you could be a wildlife expert, but if you don't understand what your camera needs from you to get the shot, they'll inevitably be blurry or underwhelming.
For many years, I'd say the camera side was more difficult. However, with the advent of powerful new tools, that has shifted. As Jan Wegener says in this video, animal Eye AF is incredible for nailing focus, which is no easy task, even for a fast autofocus, when the bird is in mid-flight. With focus under control, you then just have to worry about exposure with a quick shutter speed. No problem, right?