Anyone getting started in wildlife photography is likely to get the best results from a long telephoto lens, which will allow them to fill the frame with a relatively distant subject. Most such lenses are zooms, which tend to be much more affordable as well as more versatile.
All of the long telephoto zooms in the Sigma line-up have very fast AF that is compatible with the latest subject tracking technology, and image quality is exceptional for maximum detail capture. Fully zoomed in, they have a maximum aperture of F6.3 for fast shutter speeds and blurry backgrounds, and all have an Optical Stabilizer for keeping the lens still when working hand-held low light.