I’d like to recommend a newsletter by E. J. Peiker http://www.ejphoto.com/newsletter.htm
His work is exceptional, he’s an expert birder and he gives advice on all kinds of cameras and equipment.
I did have the Tamron 150-600 lens on order but canceled it. The reviews are too mixed. Friends have mixed results at anything out over 400 mm, and this includes Canon and Nikon people. I go a lot by DP Review. If you don’t know this website, take a look. Unbiased analysis by experts. http://www.dpreview.com/
I love my new Nikon 80-400 – it is fast and exceptional and sharp. But sometimes I want more reach so I’m thinking about the new Sigma 150-600 – bigger and heavier than the Tamron, more glass and expected to be very good.