Best lense for bird shots

[wickidwombat]

I got a 600 fd f4.5 and edmika adapter shooting with a 5d mk2 is a great combo the fd lens takes full advantage of the 5d focus system :-) the lens was 1400 on ebay the adapter about 160

Could you post some shots with the FD 600/EdMika?

http://www.canonrumors.com/forum/index.php?topic=5103.0
Here are some I put up a while ago

[bdunbar79]

Just buy the 1200mm lens and slap a 7D on it and shoot all the wildlife photography you want from the comfort of your own home 

[bbasiaga]

Sigma makes a 150-500 that sells here for about $1100 USD.  I tried one at my local camera shop and liked it.  I dont know about AF speed for birds though.  I don't shoot that so I wouldn't have a frame of reference.

-Brian

[keithfullermusic]

I love my 100-400.  On my 50D, it's Ike 640mm, but I can't push the ISO enough sometimes.  I usually shoot with my 5Diii because the high ISO and the insane autofocus, and I love it. 

It seems like the answer to your question is obvious - 800mm, but I'm assuming you don't want to sell a kidney, in which case get a 100-400 or a 400 prime.