I am trying to choose between the Tamron 150-600mm, Sigma 150-500mm and the Nikon AF 80-400 D. I am mainly into wildlife and birds. Any suggestions?
The AF-D 80-400 is not a good bird lens, AF is very very slow. It's old technology that was released back in 2000 if I remember correctly. It doesn't have a focus motor so can only be used on a camera body that has a focus drive and even then it's just too slow for anything moving quickly. The AF-S on the other hand is great, but is a lot more.
I've not used either the Tamron or Sigma, but have seen fairly good results from both.