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.