The 400mm f/5.6 is Canon's most affordable "L" super-telephoto. It was built to be very light, focus extremely quickly, and be stupidly sharp. It does what it does to perfection and is a superb lens for photographing birds - especially in flight. The 300mm f/4 L IS with a 1.4x Extender might seem to be a more attractive choice, which also offers Image Stabilisation. But try this lens once and you'll immediately find out why people love it so much.
This lens can be used with a Canon Extender (teleconverter). With the 1.4x Extender it makes a 560mm f/8 lens, which will autofocus on 1D-series cameras. You can also use the 2x Extender to get an 800mm f/11 lens, but you'll have to focus it manually.