You have probably bought a lens by now but if not, I would recommend the Tamron 150 – 600 G2. I am a Nikon user and have a 200 – 500 which is an excellent lens. Very sharp, has a good VR which allows me to hand hold it.
A friend gave me his 150 – 600 Tamron G2 to test and I found that I could tell the difference in bird images that I took with it and the Nikon. It is a great lens for bird photography.