The only problem with the 28-135 is you lose the wide end due to the crop factor. I have the 17-85 S IS and am very happy with it as long as you remember there is some barrel distortion at the wide end. When you have your new setup you can contemplate taking out a mortgage for the new 800 f5.6 IS Canon have announced will fairly soon be in production!
That will sort out your birding pictures!