Shooting flying birds and fast subjects.

For BIF the 4005/6 is ideal. Fast focus (I'm talking acquiring focus not just tracking) light and sharp. I originally used mine on a 20D and it's even better now on 1D2 and 7D. I believe that the relatively long minimum focus distance has a lot to do with the focus speed, less distance to travel from end to end. I never found the need to preset the focus. A great lightweight birdiing lens for travel.

The 300/4 IS and 1.4x isn't fast enough focus to be considered a contender for BIF. I love it for perched birds and dragonflies etc though.