This is one of Canon's most popular lenses, which is a favourite for wildlife photography and aircraft photography, amongst other uses - it has the length you need at 400mm, but the zoom makes it flexible. As you'd expect it's very sharp, even at its longest length where it's comparable to Canon's 400mm primes. The zoom action is a push-pull movement, rather than twisting the zoom ring, which may take a little while to get used to if you've not encountered it before, but it's worth persevering - with a bit of practice it becomes almost telepathic, and your left hand is always in the correct position supporting the lens. The IS gives you 3 stops of compensation for camera shake, so with reasonable technique you can shoot with shutter speeds of around 1/50th instead of 1/400th.

This lens can be used with a Canon Extender (teleconverter). The 1.4x Extender makes it a 140-560mm f/6.3-8 zoom, which will still autofocus across the entire zoom range on EOS 1D-series cameras.