Merv
I have no experience of zooms except the 70 300mm VR
For birding I use a 600mm f4 a 300mm f2.8VR and the 300mm PF f4 with the 3 Nikon TCs
You never have enough reach especially with small birds
Budget - you need to get as near to 600mm as you can on your budget if this is the only birding lens that you will get - under £2k gets you a very good set up - 600mm zooms or 300PF + TC's .....after that you are into £5k + for the 400 f2.8, 500mm and 600mm f4
you do not need VR on a tripod
Then it is DX or FX
If you are using a DX .... 600mm gets you 800mm, (technically 900mm) without any loss of light - using a TC will lose you up to two stops of light, (but the 1,4TC only moves you from f4 to f5.6).... it may affect focusing in low light but the IQ at 1,4 will be good .... but when you get to x 2 . you will loose IQ
Getting near - unless you get near you may be disappointed with most of your shots
It is always a compromise ..... you really need a few lens (and bodies) for birding ....... but the main thing is getting near and finding good light ...... you can get lousy images at ISO 200 in bad light and very good images at ISO 4000 + in the "right" light
If you want lightweight, flexibility, v good build quality and IQ, plus VR at f4 or f5.6 get the 300PF ........................ if you want less expensive and 600mm in a big zoom, (there are no small 600mm lens, even zooms), with good IQ consider the Sigma or the Tamron 150 to 600mm ....... and remember zooms IQ degrades as you get to the longer end ...... I believe both produce very good results and the Sigma seems to be favoured, (maybe for it's build quality), but the Tamron sell for well under £1,000 ................... but Nikon have the 200 500mm zoom which needs serious consideration.
Also check focus breathing - some 600mm zooms are less than that because they just are - a 200mm Nikon maybe be 200mm but a 200mm Sony may only be 175mm