A quick note about terminology for Canon cameras only, unless Nikon is indicated.

Nikon has an equivalent lens for most the Canon lenses described below. I am providing Amazon links to these lenses, not because Amazon will have the absolute lowest price (maybe bobs-discount-camera.com would beat them), but because you get a very good price and can expect a safe problem-free shopping experience.

I went through a progression of telephoto lenses, from a 300mm lens with no stabilization like the Canon EF 100-300mm f/5.6, to the Canon IS 100-300mm USM f5.6 lens, to the Canon EF L IS f/2.8 70-200mm lens, which I sometimes use with a teleconverter to get more reach. All of these lenses are good lenses for their prices, which are about $200, $600, and $1600. The next step might be a buy a super-telephoto lens like the popular Canon EF L IS 500mm f/4 lens, or the Nikon 200-400 f/4 VR AF-S lens. You can see many razor sharp photos with the first lens on this site, by Mdf. Both lenses cost around $5000.

First I want to talk about the lens I have and why I would never part with it. I bought the Canon 70-200mm lens not because I expected great results with the extender (which gives me 400mm) but because I knew I would need this general purpose lens even if I bought a super-telephoto. I sold my first two lenses, but I'll never part with the 200mm lens, and expect I will always use it most of the time. But I wrote this when I was photographing nature in general and not just birds, and now I use a 300mm f/4 for my general purpose lens. Both lenses are relatively small and light when compared to a super-telephoto. But if you want a really light lens I would consider the 70-200 f/4 version. The rest of the paragraph refers to the 200mm, but the 300mm f/4 is in the same class of lens and most comments apply to both lenses. My lens weighs 5.6 pounds, which is very heavy compared to an ordinary camera (I was once told as a joke that I couldn't enter a place because bazookas were not allowed). But I can take it with me traveling and hiking, and can take super-sharp hand-held shots. Even if a bird is far away, I can hope to take a picture where I can crop the bird and acheive Internet-quality photo. I only have a 10 megapixel camera, so this would be even more true for someone with a better camera. The same principles apply for photographing birds in flight. Maybe some would argue they can do these things with their larger lens, but I still believe most of these people own the 200mm lens as well. And some would say even my 200mm lens at 5.6 is way too heavy and bulky for carrying around, and they might have a point.

For 400mm and greater, I really think you need a tripod, although a beanbag might suffice only when used in specific situations like a boardwalk. When I wrote I would always use my 200mm lens the most, it is because I like to visit many different places, and like traveling and walking/hiking. I live on Fort Lauderdale beach, and the most interesting thing here is a Pelican. There are people who have nature in the back yards, and for them pointing their 500mm lens at their bird-feeder might be their nly thing they need. So the point is you need to choose your lens based on how you are going to use it.

Maybe best now is to do a comparison, which I will try and present in the context of bird photography.