Owning the 100-400mm lens myself, I can say it is a superb lens to photograph birds with. It has some excellent IS, and supports a fairly wide range of focal lengths that will help you capture birds at a variety of distances and in a variety of scenes/poses. The push-pull zoom is handy as well, as it makes it easier (with practice) to zoom and focus simultaneously, something I find hard to do regardless of skill when you have two rings for those functions. The image stabilization of the 100-400 is dual-mode (full and panning), so it can help when shooting hand-held shots of perching birds, as well as gymbal or monopod supported panning shots of birds in flight.
In contrast with the 400mm f/5.6 L lens, the 100-400 has several advantages, while the 400mm has only a few. First off, the 400mm is a prime lens. While this will give it the edge in terms of image quality, and will make it smaller and lighter for hand-held operation, it limits your options. Second, the 400mm f/5.6 L lens has no IS. If your only interest is to birds in flight, its an excellent lens, however the lack of IS is definitely limiting, and will make it difficult to shoot perching birds at a distance. In most cases, the 100-400mm lens is a far more versatile lens, with the only real detriments being its slightly greater weight and slightly softer images at 400mm.
A better comparison would be between the 100-400mm and the 500mm f/4 L IS lens. While 400mm does give you a nice wight-to-reach balance, it often tends to be just not quite enough reach to really get you frame-filling bird shots without them scaring and flying away. If you are willing to spend the money on a decent prime lens that supports hand-held operation, the 500mm or 600mm L IS lenses are much better options. On a 7D body, the 500mm gives you an effective 810mm reach, while the 600mm gives you and effective 970mm reach. The wider aperture and IS make it a lot easier to get stable hand-held shots at a distance great enough not to scare off every bird you try to photograph.
With a 7D camera body, you could even use the 500/600mm lenses with the 1.4x extender, to provide even greater reach (1135/1360mm) without losing AF capability for all 19 points! For a bird photographer the ability to extend the focal length of those two lenses, keep full AF capability, AND have IS is about as good as it gets. Sadly, the 500mm and 600mm lenses come at significant cost, and are well out of range of most photographers. As such, it makes 100-400mm still the most versatile option for a bird photographer.