Willie again I agree with Ken above. I have the 500mm f/4 and think it is a dream lens. I know it is a bit on the heavy side, and I always use a tripod with mine which is another item to carry. If you are seriously into birding I think that you may regret selling it, as I always feel that bit of extra reach is always welcome, and the shots it is capable of producing are well renown.
regards: Gordon