Even more important to excellent bird photography than having the right camera is having the right lens. There is no point in purchasing that dream camera body and crippling yourself with a cheap lens. In the 8 years I've been leading digital photo safaris I've had the chance to use or at least see used just about every likely lens made for Nikon and Canon camera bodies and have plenty of hopefully useful thoughts to share with you:
First, there is no free lunch. I can’t count the times that I hear “oh, I want to photograph birds but I don’t want to carry one of those big lenses or use a tripod.” Usually followed by a lot of griping about image issues. As I’m sure most of you have figured out, no matter how magical the camera, great glass really helps. If you have doubts about your lens or camera, rent a different one (borrowlenses is a cool option, especially for San Francisco Bay area folks for whom they are local) and see what happens.
That said, not everyone or every hike is suitable for a 500f/4 (let alone a 600f/4) and tripod, so there is nothing wrong with a mid-range zoom. The problem in this range is that the Nikon design (80-400) is _really_ old and has ancient VR technology and glacial AF—plus it is not cheap. The Canon (100-400) is a little newer, but some samples are soft at the long end (you need to test this on your own or any version you are thinking of buying). And it is push-pull, which many folks don’t like. It is certainly no speed daemon either.
For 2009 we had a loaner Sigma 120-400mm for Nikon on our safaris which was universally preferred by all the participants to the Canon or Nikon equivalents and is half the price. Both it and the Sigma 120-400mm for Canon are $899 at B&H. So if you don’t already own a telephoto zoom I’d seriously consider the Sigma. The Sigma 150-500 was also used by some folks and takes sharp images if used correctly, but is a little slower than the 120-400 so I’m not sure the extra length is really worth the greater difficulty focusing.
For big glass the 500f/4 (AF-S/USM) is clearly the most versatile for bird and some mammal photography. The Canon 500mm f/4 sells for $6140, while the Nikon version of the 500mm f/4 sells for an even heftier $8500 when you can find one. I haven't used the slightly slower Sigma 500mm f/4.5 but it also features focusing motors and would save you some money at $4700 for either Canon or Nikon mounts. It is not image stabilized though.
I love my 600f/4 AF-S, but it is huge, expensive, hard to travel with and not very versatile. The Nikon 600mm f/4 AF-S VR is a cool $10,000 if you can find one, while the Canon 600mm f/4L USM IS version is a relative bargain at $8,000. The Sigma 300-800 is even larger and heavier but offers a unique range. I haven’t used the new Canon EF 800mm f/5.6L IS USM lens but clearly it serves a similar function on a full frame camera that the 600f/4 does on a ‘cropped’ sensor (although at higher cost of nearly $11,000, albeit less weight).
Unique among all longish lenses is the Nikon 200-400mm f/4 G AF-S ED-IF VR lens. It has become even more expensive than it was originally (now just over $6,000) but it is still an amazing piece of glass if you want that range with a constant aperture & fairly quick AF with good VR. That said, it is a lot of money for not that much length if you are only going to photograph birds. It is probably the best general purpose “safari” lens for Africa, but if you’re only going to photograph birds the 500f/4 AF-S will focus quicker and give you more reach. A less expensive but almost equally capable lens is the Sigma 120-300 f/2.8 (you can use more extension on it so it has almost the same reach as the 200-400, but it does not have VR/IS so it is less useful from trucks or hand held). The Sigma 120-300mm f/2.8 EX APO IF HSM for Nikon sells for $2,999, as does the Sigma 120-300 version for Canon.
Another unique lens is the Canon EF 400mm f/4 DO IS USM (which sells for $5,800). Its smaller size and lighter weight than a 400mm f/2.8 or the Nikon 200-400mm make it an interesting option—although it is not really any cheaper than the Canon 500f/4. With a converter on a cropped sensor camera though it has good reach. I was frustrated by the lack of reach when I used one on a full frame camera for birds.
Other options include the Canon EF 70-200mm f/2.8 IS USM on sale for $1,699 (the brand new state of the art Nikon 70-200mm f/2.8 AF-S VR II is the pricey equivalent for Nikon shooters at $2,399 ). Either of these lenses have to be used with a Teleconverter for bird photography but have the advantage of giving you a great general purpose sports and action lens.
Similarly the Canon EF 300mm f/4L IS USM at $1,280 or the Nikon 300mm f/4D ED-IF AF-S lens at $1495 can be used with a Teleconverter as a less expensive alternative to longer glass. I wouldn’t purchase most of those combinations to only photograph birds (except for the 300 f/4, which is actually a great deal and a great flight lens as well as a good general bird photo lens when used with a teleconvter), but they are more versatile than only owning a long lens. Note that I generally do not recommend using a Teleconverter on a zoom that is slower than f4 (like the 80-400, 100-400, or 120-400). You’re better off with those lenses just cropping the image, IMO. I also don't suggest the excellent 70-300 lenses for bird photography as while they are small, quick and not too expensive their inability to work with a teleconverter leaves you pretty short on focal length. But if you are working close to your subject like in a blind they can be a good option.
So what do I use? When I photograph birds locally I lug my 600mm f/4 around (with a large Gitzo tripod & large Wimberly head). When I photograph in the blinds in Texas I use either my 200-400mm f/4 (often with a 1.4x or even 1.7x Teleconverter, on a more reasonably sized Gitzo 3541LS carbon fiber tripod with a Really Right Stuff BH-55 and smaller Wimberly Sidekick) [you don't need to spend quite as much as I have on tripod and heads, we'll be providing some less expensive options in a future GearGuide]. For other travel to bird photo locations it depends on whether the hassle of bringing the 600mm & larger tripod outweigh the increased reach I’ll get with them. Keep in mind that in blinds a zoom is extra valuable as it is hard to change your subject distance the way you can by walking forward or backwards when you're outside.
An important variable in your choice of lenses is your camera body—especially the sensor size (crop factor) and its low light capabilities. Smaller sensors mean you can get by with less focal length, but they typically also mean you have worse low light sensitivity so you need faster lenses. So for example a 300 f/2.8 on a cropped sensor (DX in Nikon terminology) is similar to a 400 f/4 on a full frame sensor. Not the same since AF speed is also faster with the f2.8 lens, but similar. See our GearGuide to Camera Bodies for Bird Photographers for thoughts on specific camera bodies and on body plus lens combinations.
Note on imported lenses: At many retailers you will have a choice of lenses marked USA or Imported. There is nothing illegal or truly wrong about buying imported lenses, but it will be much more difficult to get them serviced in the US (in some cases Nikon USA will not service these "gray market" lenses at all, for any price) and the price difference has become quite small. So it is almost always best to order the USA version if you reside in the United States.