Most birds will give a signal, a consistent motion, when they plan on taking off. That doesn’t mean it doesn’t happen really fast, but at least you have a fighting chance of getting the shot.
The bird above is alert, but that’s probably a good thing considering where he lives.
And then it’s a quick bend. Great Blues tend do drop lower than other wading birds. Maybe it’s the big wing span. It makes for an extra moment to get the shot.
Most times bird’s prefer to take off into the wind, it helps with lift off.
For a bird like this all it takes is 2 or 3 flaps with the large wings. After that, the photograph is the classic bird butt shot. Of which I have taken thousands.
And yes, gear matters. Without a long lens to zoom along with the motion, and 10 frames a second, I can’t get these. Even with that I don’t catch these often enough.