The lens provides much greater reach with precision and affords more light with its F4 opening than the Sigma 150-500 which starts at f6.3 at 500mm. HOWEVER, for limited budgets I would suggest keeping away from exotic BIG glass which is hardly handy and expensive.
Our area is not a national park and many birds are shot high up in the sky, cropped and doctored with resulting noise, please excuse the quality. My purpose is to identify the variety of birds in this Provence backcountry location. In Apil 2011 bought the new D300s fearing a stock drop with the Senday earthquake and the Nikon TC1.4 which replaces my Kenco Tc1.4.