How about the 100-400 as a possible contender? I don't do that much bird shooting, other than in my own back yard but the 100-400 works well for me in good light. With the Xti you will get a 640mm 5.6 lens with pretty good optics, better than the 75-300 that you used in the past.


I would agree with Sean to use the 500 f4 to give you nice reach and better IQ, but if your thinking of using the lens hand-held at all, the 100-400 is much easier to deal with for any length of time. Also the flexibility of having a zoom will allow you to track the birds in flight a little easier, unless you are just shooting them while they are perched somewhere.


Good luck!