Lovely photos Chris, you must have steady hands and a good technique to shoot the 400mm at 1/250 and obtain that kind of repeatability.
I’ve noticed that with the very shallow DOF on the 400mm af F5.6, unless you place the AF point on the eye to force the focus there, the camera may “choose” focus on another high contrast feature closer to the lens, such as the wing feathers, thus leaving the eye slightly out of focus.
I didn’t find these birds at all difficult, in the large, walk-in aviary at Currumbin!
Cheers
Dennis