For bird photography, at least 200mm usually are needed. Of course, more millimeters are better, because you can photograph birds from a larger distance and get more blurred background. My personal preference is 400mm f/5.6 lens. Why? Because I get as much millimeters as possible, not too small aperture, and I am still mobile. Of course, sometimes you might prefer longer focal length or/and larger aperture to mobility. Actually, I would use 300mm f/2,8 and 500mm f/4 lenses along with my 100-400mm lens if I could afford them. So, here you will find my Canon lens recommendations for various needs.
These are all Canon L lenses, so the build quality of all the lenses is very well. Actually, all the lenses which are mentioned below are excellent, but you will achieve better results if you choose the lens which is designed for your needs.
It is very popular and it offers a wide focal length range. Moreover, the optical quality is decent, almost as good as Canon 400mm f/5.6L USM. It has an image stabilization, which helps much in low-light conditions. Actually, it is not easy to write pros and cons because it has good but not the best optical quality, pretty fast but not the fastest auto-focus, and the minimal focusing distance is also in the middle of these three lenses.
Pros: wide focal length range, IS, good optical quality, 1.8m minimal focusing distance, pretty fast AF.
Cons: the heaviest and most expensive ~400mm f/5.6 lens. However, it becomes featherweight and relatively cheap when compared with something from the last group (e.g. 500mm f/4 - more lens below).
It is slightly sharper than 100-400mm zoom and has faster AF (auto-focus), which makes it an excellent lens for bird in flight photography. However, it requires much light because it does not have an image stabilization. This is the best ~400mm f/5,6 lens if you do not need zoom and always use a tripod when there is not much light.
Pros: very sharp, quite lightweight, fast AF, cheapest (although not cheap) among ~400mm f/5.6 lenses.
Cons: no IS, 3.5m minimal focusing distance, fixed focal length.
It is a 300mm lens, so it is usually used with 1.4x extender for bird photography. Then you get a 420mm f/5.6 lens with IS, still good AF speed, 1.5m minimal focusing distance, and still good optical quality.
Pros: f/4 without extender, lightweight, fast AF (slower with extender), very sharp without extender, 1.5m minimal focusing distance.
Cons: fixed focal length, optical quality with 1.4x extender is worse than 400mm f/5.6L USM and only slightly worse than Canon 100-400mm f/4.5-5.6L IS USM (in reality the difference is very little).
I have heard many good reviews about Canon EF 70-200mm f/4 L USM. Of course, 200mm is not perfect for bird photography, but with 1.4x extender you get 280mm with f/5.6 aperture. Even with 1.4x extender it is a very sharp lens. And because it is a Canon L lens, it offers excellent build quality. However, before buying this one, I recommend to consider saving for a ~400mm lens instead.
Canon EF 300mm f/2,8 IS USM (often used with 1,4x or 2x extenders and can be used without a tripod or monopod)
Canon EF 500mm f/4 L IS USM (in my opinion, this is the best lens for bird photography - you get 500mm without or 700mm with 1,4x extender, plus it is lighter and less expensive than the next two)
Canon EF 600mm f/4 L IS USM (you pay extra cost for extra reach, plus it is bigger and heavier - if you can afford this one, you should also consider buying 800mm instead)
The new versions (II) of these three lenses have a strong advantage - they are less heavy, especially the 600mm f/4 lens. However, you need to pay extra cost for this advantage. I advise you to compare weight - the difference is not always huge.
Canon EF 800mm f/5.6 L IS USM (the best choice if you need 800mm)
I haven't tried these 4 lens enough for detailed reviews, so I can only say that they all are excellent. Personally, I would choose the 300mm f/2,8 lens and the 500mm f/4 lens from these. However, everyone's needs are different, so you have to decide what focal length, aperture, weight, and price combination is best for you. I recommend to visit the-digital-picture.com and read detailed reviews before you buy.