Based on my experience,

6m-infinity would be noticably faster to focus than 1.8m-infinity.  I don't own 100-400, but I have the 400mm f5.6 and the 600mm.  They all have similar focus limiter.  This is especially important if you shoot bird in flight, where you need all the focusing speed you can get.

 

Another thing I do is to pre-focus.  If you are waiting for a bird for some action (such as taking off, or fighting with another bird), focus it before hand; when the action happens, the lens is already pretty much focused

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• Very good tip! Thanks! -xialibaren- ♂ (0 bytes) () 12/28/2012 postreply 13:54:59