Bill it's a tough call, each have there own pluses and minuses,.
IMHO if birding is your intention, both 5D and 50D (and 40D for that matter) are second choices behind the 1 Series AF system which is the ingredient that sets it apart as the Birding body (particularly IF birds in flight is your forte)
Everyone elses seems to have answered the question as best it can be answered already IMHO.. 1D would be best,
I am sure your intentions were good in this reply, but as the local glee club for POTN I have to take personal umbrage to the basis of your reply..
1st, POTN has a number of fabulous birders quite capable of answering Bill's question, one need not go elsewhere to get an informed reply to his question..
that said, yes there are other forums with many knowledgeable birders,. add Naturescapes.net to your list.
As to the second part, here is where you've really drawn me to reply,
Arty Morris is indeed one of the great bird photographers of our time, he has in a crowded field managed to stand out as unique, and this is due to his excellent work. He is a genius! He is not however, THE authority on Birds in flight as your reply would have it. Arty himself would tell you that he is not the best at flight photography, in fact he would likely list a number of names, including Jim Nieger, a regular contributor here at POTN, as the "authorities" on Birds in flight. Jim literally wrote the book on it and teaches classes in it..
O-kay, sorry to come off strong, but again, there are of course dozens of excellent sources for answers to Bill's simple question.. but do not assume that POTN is not one of the best sources. Why tell anyone to walk to another lake to drink when he asked the question at the lake in which you are standing?