Wildlife photography isn't just about long lenses. Putting aside the differences in image quality between crop and full frame, you will get better quality shots on a 500 mm than an 800 mm on a regular basis, due to presence of atmospheric pollutants. Then you have the added effect of extenders, especially the 2x, not just on image quality, but also AF speed. In perfect conditions, it will be the sensor and glass that will be the determining factor on IQ, but those days are limited. Even the slightest bit of moisture in the air has an effect, so you need to be as close as possible to reduce that. It then comes down to field craft, so that you can get closer to the subject without disturbing it. Obviously there are times when that isn't possible (either the terrain prevents it or the subject is especially sensitive), but otherwise, a shorter lens is often preferable. Also, something that tends to be more advanced in Europe than the US, is showing the animal in its natural habitat and there are more and more wildlife photographers using short telephoto or even wideangle lenses. If you can't get close enough, think about how else you might be able to display the subject.