When photographing birds, the ultimate goal is to get that "frame filling" shot, where the bird (or birds) cover the bulk of the image. To capture such shots, you need a lot of reach, however even with reach, you still need to get pretty close. As a general rule, 400mm is the minimum necessary to get good bird shots without having to get so close that you scare your subject off.
Canon offers several lenses that fit the bill. One of the most popular bird and wildlife lenses is the EF 100-400mm f/4.5-5.6 L IS USM lens. The nice thing about this lens is it comes with image stabilization, which helps offset its tighter aperture a bit, allowing better hand-held performance at lower shutter speeds. I use this lens myself, and it is a great lens overall. It truly shines when used with a camera body that offers good high-ISO performance (i.e. 5D, 7D, 550D). This lens goes for about $1500-$1600 street, $1800 list. It is probably the best bang for the buck as far as 400mm telephoto goes, and my top recommendation. I would avoid using this lens with a teleconverter, as it reduces the long-end aperture below the AF limit for many camera bodies, and greatly darkens the viewfinder. Your maximum aperture at 400mm shrinks as well to f/6.3 or so, which is much too slow to be effective as a birding lens.
Canon also offers the EF 400mm f/5.6 L USM Lens, which is one of their cheapest telephoto primes. It lists for around $1200, and can sometimes be found for around $1000. It is a great lens, but a tad slow. Optically, this lens is excellent, and provides better, sharper images than the 100-400mm. It is a favorite of a lot of Colorado bird photographers, and I have seen it paired with the 7D quite frequently lately. It is best paired with a high-ISO body. I wouldn't really recommend it for the Canon 450D, although it would probably be fine on a 550D, 7D, or 5D. An f/4 aperture would be more ideal, however for telephoto primes, the price quickly rises past $4000 for wider apertures than f/5.6.
As an alternative to the 100-400mm zoom, you could also get the Canon EF 70-200mm f/2.8 L II IS USM lens, and combine it with a teleconverter. Canon offers 1.4x and 2.0x teleconverters, which can extend the focal length of your lenses. The 70-200mm becomes a 140-400mm lens, but looses some aperture (it would drop to about
f/4, maybe f/4.5 f/5.6 [note: this aperture makes this option not much better than the 100-400mm, and a lot more expensive]). Such a combination would make for a very ideal zoom lens setup, with a great telephoto range and a good, wide, constant aperture at f/4. This setup would cost more, some $2500 or so, but between the lens and the teleconverter(s), would cover a focal range from 70mm through 400mm, greatly expanding the usefulness of the setup beyond just bird photography.
There are some great third-party vendors who make good telephoto lenses with a Canon mount. One of the more popular is the Sigma 50-500mm f/4.5-6.3 lens, often called the "Sigma Bigma" for its huge telephoto range and 500mm long end. Optically, it does not perform as well as, say, the Canon 100-400mm, and certainly not as good as a 400mm or 500mm prime lens. At the 500mm end, an aperture of f/6.3 is not ideal, so higher ISO performance would be best. For the price of $2400, it is a pretty good deal, and it can make a good bird lens, as well as a general purpose lens that covers pretty much all useful focal lengths outside of wide-angle. I would bump the camera's sharpness setting up, or add some sharpness in post processing if you use RAW.
Any 500mm or longer lens with an f/5.6 or wider aperture will offer better results for bird photography. Sadly, once you hit this range, prices rise dramatically, for both on- and off-brand lenses. Usually, such lenses can be found starting for around $4000, and moving on up to $10,000 or more.