Inexpensive telephoto zooms are a good way to dip your toe into wildlife photography but chances are you'll notice their shortcomings very quickly and the yearning for better tools will likely start to be persistent if you continue to have opportunities to photograph wildlife. The advantages of these lens are clear: they are inexpensive, but also lightweight and their quality can be quite acceptable, particularly if you are using a sub 20 megapixel DSLR. You may have noticed that I'm on a bit of a roll for telephoto posts, and this for me is going back to the very beginning of my ambitions to photograph nature. The adage of buy cheap, buy twice has been true for me in this regard: I started with the very inexpensive Sigma 70-300mm DG Macro and it lasted two trips before I outgrew it. The below shots I captured with this lens are some of my favourites and the 6mp sensor of the Nikon D40 I paired it with was pretty forgiving of the optical limitations.
Focus with this lens is slow and clunky. IQ is good enough for what is today a primitive 6mp sensor but I'm sure pales on a modern DSLR. The limitations of the D40 and Sigma 70-300mm lens were clear to me such as in the photo below (shot at ISO 1600 and at 300mm f5.6). It was after this trip in Peru that I realised I enjoyed photography enough to invest in some better gear although its nice to look back on these images and remember this lens was not rubbish. It did the job at the time and got me some images I cherish. The quest for a better telephoto and more opportunities to photograph wildlife began.