I have nikon D60 with 18-55mm kit lens and sigma 18-200mm lens.I want to start bird photography.I noticed that resolution of bird pic taken by telephoto end of sigma lens is not good at all.Will it be a better idea to buy canon powershot sx40 hs or equivalent from nikon(nikon p510 bridge)or i will invest in nikon 55-300mm lens.If there is any other option please suggest me.