400mm f/2.8 and 600mm f/4.
It must be something about having a piece of glass wider than the bird.
I don't have either of these lenses, but I do a lot of birding, and those are the two lenses I see most among those who do this really seriously.
The least expensive and lightest lens I see consistently great results from, from those who work really hard at getting great results at the limit of their equipment, is the 300mm f/4, sometimes with a tele extender, usually with DX cameras.
I use the 70-300 VR on my D200, but it's not for the kind of shots these guys take - it just lets me take the photo home for a little more thorough research on the ID. Before I bought the 70-300, I tested the 70-200 with extender and the 80-400. I decided that neither of these alternatives provided sufficiently better quality than the 70-300 to justify the price just for birding. If I was going to get serious, I decided I needed at least the 200-400 or the 300 f/2.8 with extender, and that even there I was stretching it.
For a lot less the 70-300 provided the info I needed for identification, and I decided I would let the pros take their time and their exotic lenses and get those great shots. I'd just buy the books.
It's lots of fun no matter what lens you use. Enjoy!
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Pat