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 Canon EF L IS
200-400mm f/2.8 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. It
is relatively small and light when compared to a super-telephoto. But
if you want a light lens I would consider the 70-200 f/4 version. 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 larger
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 only 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.