If you're after crisp detail, don't rely on image stabilisation even if your camera has it !!! My camera has it, and it will allow me to record a recognizeable bird when hand-holding the camera, but will not allow sharp detail when hand holding - even in good light. Though I always have image stabilisation turned on, I almost always use a good quality, heavy duty tripod or some other very solid form of camera support. There is no other way to get good shots -- no matter what kind of camera you have! Image stabilisation is for getting candid shots of your friends at the bar. Don't rely on it for very much else.
For these owl shots, the camera was supported by a very large, custom shaped bean bag with 5 lbs of beans that I always keep in my van ready to use. The bag is draped over the door with the window rolled down. I put the camera on the bean bag. Then, on top of the camera lens, I place a couple of fairly heavy ankle weights normally used for exercising. It makes for a very stable setup that is always close at hand in my vehicle. (And that's just my setup for a couple of quick grab shots!) There are a couple of caveats to this setup. Shooting through the camera's viewfinder would be very awkward because the camera is positioned very low relative to my head height sitting in the seat of the van. My camera has a flip down display that allows me to compose the shot without having to contort myself into shooting position. Also, the subject has to be beside the van - which is not always the case. You do, after all, usually have to keep your vehicle more or less parallel to the road.
If there is time to get out of the vehicle, I am always carrying my heavy duty tripod in the back seat, so the first thing I do is grab that. It has a quick release head which allows me to set up quickly. Shooting from the tripod gives me far more flexibility in shooting position and angle. It also allows the camera to acquire focus lock much faster with the telephoto lens compared to hand-holding, since the subject is not bouncing around all over the place in the camera's field of view. I use my tripod almost always - even for scenic shots of old barns with no telephoto zoom. One of the advantages of doing this, aside from sharper photographs of course, is that it allows me to pay more attention to the finer points of composition - getting exactly the framing I want so as to make the strongest visual statement. The only time I decide not to use my tripod is when I am trying to shoot birds in flight. A good tripod is the most valuable and most under-rated piece of photographic equipment in a photogrpher's camera kit. If you want to take better shots, start shooting everything from a good heavy tripod.