Hello BillinBoston. I've been trying to shoot birds for just about 7 years now from my home mostly north of Houston. I've wanted the longest lens I could put my hands on. I recently got the Sigma 150-500 and find it a sweet piece of glass. I think Ken Rockwell has a review of it on his pages. Like all lenses in this category, it's heavy and can become a great pain to lug around. I honestly believe that with few exception, all the newest glass will give great results optically. I've posted four of my bird shots at https://images.nikonians.org/galleries/showgallery.php/cat/21549 with details of the lens used. All are sized for web in PS6 and the only processing is done in camera - fairly heavy I might add. My advise is to stay away from used equipment unless you have the skills to fully evaluate the hardware.