If there is one excellent feature among the new genre of super telephoto lenses, i.e., the Canon 500mm in this case, it is the Image Stabilization technology. In many situations, I end up photographing without a tripod. It is counter-conventional wisdom for many, but when an effective method and style is learned, there is a tremendous amount of flexibility and speed gained in a quick response, free from a tripod and the intrinsic necessary limitations. I might also add, that it is the reason I have recently developed biceps tendinitis, but that is another story.
Moments are fleeting in the natural world, and the ability to respond quickly is often less emphasized than all the necessary gear one is supposed to have. My advice to anyone with camera gear is: get to know it, learn how to use it quickly and with good form that translates into the right balance between capturing the moment and achieving technical excellence. Of course, it is not always advisable or possible to hand hold big lenses, but explore the liberties available to you, given your ability.
I took this photo from a moving boat (although in calm waters), with an extender pushing the focal length to a 700mm equivalent The shutter speed, at 1/400 sec resulted in a super sharp image. Image Stabilization technology declares that with excellent form, one can gain 2 -3 stops of stability. This varies certainly, but anything helps. The one other thing that helps is: practice. And after that: practice