the cheapest option would be the sigma 70-300 APOMacro Super II. this lens is pretty good considering its 200USD price tag. just make sure you are getting the APO version. it will be sharper than anything comparable to its price from Canon.
if you are willing to spend a little more the sigma 135-400 will give you a much needed extra 100mm of reach. really this is about minimum if you plan on doing real bird photography. and this lens is really quite nice considering its relatively low price of 439USD. this should be your choice if you are serious about bird shots.