"That shot was among the first two dozen shots that I took with it, ..."
I do not offer praise lightly as it can reinforce bad habits. If you are taking that good of photos from the start, you are well above what most can do. People often forget that besides magnifying the subject 10 times (100mm = 2 x) a 500mm lens magnifies all the problems associated with lenses the same amount. That is vibration and camera movement, etc. Not to mention more critical focus.
"The IS system on the Sigma 150-500 works fairly well ..."
IS or OS as Sigma calls it, is not an on or off tool. It is variable. It works best when things are best or in other words when you need it least. It never stops working, persay, it just gets less effective. It will be at its best at one stop below 'idea' settings. A little less helpful at 2 stops and so on. However, some people can get good use at far below what others can. This, perhaps, is you! You must have a pretty steady stance.
"... the AF system If you're trying to lock focus while moving the camera suddenly."
Now of course, focus. A lens of this type is going to have more issues trying to get good focus. This is not your 50mm f1.4. It is darker for one thing. It has father to travel and consequently is going to be slower. It has less contrast to work with, too.
Botton line is, don't be discouraged if all your photos don't come out as nice as the bird. It is the way it is. You give to get.
EB
EOS 1D, EOS 1D MK IIn, EOS 1D MK III, EOS 1Ds MK III, EOS 1D MK IV and EOS 1DX and many lenses.
Whatever works.
