If you want a zoom, the suggestions above are great. I use a 300mm f/4 with a 1.4x teleconverter for a 420mm f/5.6 equivalent for most of my bird and wildlife stuff. If you are willing to forego VR and are not afraid to buy used, you can stay well under $2000 (USA) with this combo. It gives very sharp results, nice color and contrast, and isn't very heavy. It's certainly lighter than my Tamron 150-600. It's also a sharper lens. It autofocuses a little faster and more accurately, as well.
That said, the VR (or VC as Tamron calls it) of the Tamron 150-600 is a nice feature when things get really dark. Also, the convenience of a zoom can come in handy. Yesterday, using the Nikon set up, a Blue Heron walked up so close to me that I had to keep moving backwards to keep the bird in the frame.
