First I'll discuss the 'starter' bird lenses.  These range in price from about $1000-$1300 in price

Canon 100-400 f/4.5-5.6 IS L Zoom lens -  This is a lens I used for approx. 2 years and it made a very good lens for capturing birds and other wildlife. The general problem for birds is that unless you are taking photos of relatively large and/or tame birds you will find yourself using this lens at the 400mm end most of the time.

Pros: Relatively lightweight, convenient zoom, Has IS, good optical quality for a zoom.

Cons: Doesn't take a extender very well, has slow 5.6 aperture on 400mm end, is a little soft at the 400mm end where used most for birding, has a slower auto focus  speed compared to other prime lenses in this article.

Recommended for bird photogs who: Want to use the lens for other larger wildlife or purposes or are primarily using it for larger or tamer birds that can be approached closely.   This is a entry level lens or a addition to a serious bird photographers kit.

Canon 400mm 5.6 L lens - This lens is currently in my arsenal and I use it mainly now for a birds in flight lens on a second camera body.  This is generally the lens I recommend to most beginner bird photographers.

Pros: Very sharp lens with one of the fastest auto focus speeds around.  Small for it's 400mm reach.  This lens can take a 1.4XTC resulting in 560mm and maintain great sharpness and focus speed ** . This is a excellent bird in flight lens which can be kept if you graduate to longer glass in the future.

Cons: Doesn't have IS so this lens needs good light or will need to be tripod mounted for best results.  With a 5.6 aperture you will be limited to slow shutter speeds or high ISO's in less than ideal light.

Recommended for bird photogs who: Are starting out and want super image quality and auto focus speed in a small package that won't break the bank.  Generally if you want one lens just for birds than this is it.

Canon Super Tele Lenses - These lenses are the bread and butter of serious bird photographers but are very pricey and range from $4000-$7000.

Canon 300mm 2.8 IS L lens - This lens is a excellent all around performer and can be pressed into service as a 420mm or 600mm with 1.4 and 2.0 Teleconverters.  It is currently my preferred lens when travel space is at a premium and I want a versatile lens.

Pros:  Takes both the 1.4 and 2.0 extenders VERY well and has the fastest auto focus speed of any Canon lens.  This is also one of the sharpest lenses made.  It is also the smallest format to get a useable 600mm focal length. 

Cons: Generally for bird photography 300mm doesn't cut it so this lens will be used almost entirely with extenders for bird photography. 

Recommended for bird photogs who: Are serious about their bird photography and want one of the most versatile and travel friendly lenses in the Canon arsenal which provides the very best image quality.

Canon 500mm and 600mm f/4 IS L lenses - These are the bread and butter lenses of all professional bird photographers.  These lenses currently make up the majority of my bird photos.  The real consideration between these 2 lenses is portability and size.  The 600mm is difficult to travel on airlines with because of the size of the bag required to hold it.  The 500mm makes a much better airline travel lens.  While both lenses can be handheld by a well bodied man for a few shot really neither lens is at home on anything less than a good solid tripod.  The 600mm gives you a image that has 44% more pixels on target (20 % horiz x 20% vert).  Both lenses shine on a gimbal type head such as a Wimberley.  In general I would recommend the 500mm for a great all around wildlife lens with a emphasis on birds.  For specializing in small birds lof warbler size or birds on impoundments in the distance the 600mm is much preferred.  Realistically for carrying into the field mounted on a tripod the difference between the 2 lenses is not of primary concern.  The weight difference of the 2 lenses is approx. 3 pounds.  When carried into the field on a 25-30 pound pro rig the weight difference is only about 10-15%.

Pros:  The best 2 Canon bird lenses made. Perfect image quality and super fast auto focus speed.  Both lenses take the 1.4 extender very well and both take the 2.0 extender ** very well on a 1 series pro camera body.

Cons: High cost, weight and portability.

Recommended for bird photogs who: Want the best pro quality lenses on the market.

** Canon cameras such as the 20D, 30D, Rebels and 5D will lose auto focus ability on apertures greater than f/5.6.  Canon 1 series bodies can still auto focus at apertures of f/8 using the center focus point only.