Out of interest, why do so many of you use/want a zoom lens for bird photography?
I bought a 300mm and a 1.4x converter, AND an 80-200mm zoom, and I have never, ever wanted to use the zoom for bird photography. In fact, I kinda wish I'd not bothered with the zoom at all, and used the money to buy an even longer, faster tele.
I guess it sounds like a versatile instrument, but it'd be interesting to hear from all of those who bought (for example) a 100-400 zoom, how many shots they take at the 100mm end of the range?
It seems to me that the zoom feature reduces the speed and quality of the lens, and increases the bulk and weight, for little practical benefit (in bird photography).
To answer the original poster's question, I reckon it depends on how you organise your days out. If you're setting out specifically to take pictures, then get the longest, fastest lens you can afford, and the best tripod to go along with it. Spend any change on hiring a porter to carry it all for you.
If, like me, you normally go out birding but would like to take some pictures of anything interesting that you see along the way, then size & weight are a big consideration, and I'd recommend the 300 f4 (IS) +1.4x conv. It's a great compromise between bulk, performance and usability. The IS means that you CAN use it hand-held in good light. I also find that the perspective & magnification is approximately the same as my 8x32 bins, so I know that if I see something well through the bins, I'll get a reasonable photo.
I'd also get a Lowepro 'Minitrekker' backpack (around £50 I think). Scope, beanbag, eyepieces, bins, camera body, 300mm+1.4x, bits&bobs AND lunch will all fit in easily, and it's easy to carry around.
I'm also experimenting with a good monopod at the moment, rather than carrying a bulky tripod. Seems to work well - I use a beanbag in hides, and the monopod outside.
To be honest, I know that shots taken in anything less than bright sunlight are never really that interesting - they never have the 'wow factor' that everyone keeps banging on about. It doesn't seem worth the extra THOUSANDS of pounds to buy equipment that allows shots to be taken in dull or flat light.
Cheers,
Al