Welcome to the forum!
To answer your question properly, I'd have to know what your budget is for a lens. To be quite honest, it's going to be pretty expensive - I have the Nikkor-Zoom 70-200mm f/2.8 IF-ED VR lens with a 2x TCII teleconverter, making the lens effectively a 400mm @ f/5.6, and it's not necessarily enough for some wildlife.