Sigma 170 500
Canon 450D with Sigma 170-500?

I have Canon 450D with Sigma 70-300 APO'm thinking about buying new lenses for bird photography. we know that we have have at least 400 or 500 long to get some good shots and if I go with canon 500 mm lens that is completely out of budget, so I am considering Sigma 170-500 mm F5 6.3 DG lens. I wonder if anyone has this lens or using this lens are kindly advised to buy.

Will better than a 50-500 or Sigma 150-500 and Tamron 200-500. Personally I have not used the 170-500, but has received many negative comments about the sharpness of AF and open. Moreover, the 170-500 is an old model. The 50-500, which I have, however, is a great lens all the way up to 400 mm and a deterioration of the quality very mild 500 mm, with auto focus also great. The same happens with the Tamron 200-500. The latest 150-500 also has image stabilizer. Moreover, 50-500 is much more versatile 170-500. There is also a very good lens Tokina 80-400mm with excellent reviews. The 50-500 goes for about $ 900 and the 170-500 for about $ 700. If you need something in the range of $ 700, my advice is to get the Tokina 80-400 on the 170-500, 200-500 or maybe the Tamron is somwhere in between that and the 50-500.