Wildlife photography is not easy and the more help your camera can give you the better. I rate high ISO performance as the most important element when selecting a camera for this type of work. For over 30 years I used the old Kodachrome slide films of ASA (ISO) 64 and 25. I was lucky to get 1/30th second exposures and learnt to hold a long lens verrrrry steadily and the exposure latitude was pants, about 2.5 stops.. Nowadays I can use ISO 1000 as standard and get excellent results even with cluttered backgrounds. Here's a couple of pics of a Nuthatch to prove the point. The first is taken at ISO 6400!! The second at ISO 800. How things have come on these last few years. Full frame sensors rule for me.