Wildlife photography usually requires quality lenses in the 500mm to 600mm range. Sure backyard birds and some of the tamer mammals can be shot with 300mm lenses. But, unless you are somewhere really exotic, longer lenses are needed for birds in flight and wild animals.
The two long lenses I have owned are the EF 400mm f/5.6L USM and the EF 300mm f/4L IS USM. As I have mentioned, I sold the former when I purchased the latter figuring that the shorter minimum focusing distance and Image Stabilization of the latter lens would render it more useful than the former. Plus, the 300mm lens fits in camera bags better.
In general, I am happy with that decision. I have made lots of photos with a Canon 1.4X extender attached to the 300mm lens which creates a 420mm f/5.6 lens. But sometimes you need something longer. I used to attach the 1.4X extender to the 400mm lens which, with a 1.6 crop EOS 50D, rendered a field of view equivalent to an 896mm lens on a 35mm SLR. And while that setup wouldn't autofocus normally, I could use live view to focus on static subjects like wading shore birds.
Currently, my wildlife/sports camera body is an EOS 1D Mark III and while its crop factor is 1.3 rather than 1.6 due to its larger sensor, it will autofocus normally attached to a lens with a minimum aperture of f/8. So, I can use a 2X extender with my 300mm f/4 lens which creates a 600mm f/8 lens. This results in a lens/body combination with a field of view equivalent to a 780mm lens on a 35mm SLR and it can be focused normally and, with IS, handheld to a certain extent.
In practice, this combination operates as well as the 400mm f/5.6 lens alone on the 50D and is half again as long although with the larger sensor the apparent difference is only between 640mm and 780mm.
Below is a shot of a White-tailed Deer from about 120 yards away with the 1D Mark III, 300mm f/4 lens and 2X extender. The first photo is uncropped to show the field of view and the second photo is a 100% crop to show image quality. Exposure information is f/8, 1/500 sec., ISO 400.