The Canon 70-200 f/4L lens is many things to many people. It can be an excellent portrait lens. It takes fast and sharp close-up photos. In fact, I have taken many butterfly images with mine. It also is good for telephoto landscape shots.
The only thing it does not do well, in my opinion, is wide angle shots. The reason is obvious, with 70mm being the widest lens setting.
But personally, I love the bird photos I have taken with this lens. Now, you will not find Moose Peterson using this lens for birds (you won't find Moose using it for anything because he is a Nikon guy), but for a poor man's bird lens, this one is awesome.
So, here are just a few of the shots I have in my "birds" folder. These were all taken with my trusty Canon 70-200mm f/4.0L lens mounted on a Canon 30D.