[Jon]

Canon 100-400L IS lens - without a doubt your best bet - flexible in use and high quality. Even works after you drop it - though I don't recommend it! This lens has transformed my photography.
Perhaps a little expensive but it will last. If you don't want all the advantages of a zoom then I would suggest the Canon 400mm f5.6.
Disadvantages of zoom - have not found any other than poor performance with a 1.4TC.
Apart from birds and mammals you can use it for Dragonflies and butterflies - you may need extension tubes for some.
I think you will see all the owners just love the lens - have a look at Arthur Morris's (google: birds as art.com)images taken with the 100-400L - they will blow you away! Trust me I am a scientist!

I am just waiting for Canon to produce an updated lens perhaps 200-500mm f4 with latest IS system, now that should work a treat with a TC!
Jon