Shooting Wildlife photography, especially quick bird photography takes a long focal length with fast aperture lenses.
When you want to achieve this on Full Frame camera you are looking at over $10,000 for such a lens or 4x the price of the camera body. Shorter lenses can be adapted using tele-extenders to maintain auto focus and aperture control but at some loss of optical sharpness, loss of 2 F stops in the case of a 2x extender and quite often the auto focus system of the camera struggles under 1/4 of the amount of light reaching the sensor. Video thanks to Graham Houghton.
Take a look at the video below for a more detailed explanation.