I would go with the tamron 18-270 or the sigma 18-250. Since you shoot birds the great range is helpful over the 18-135. You will give up IQ a bit, but there is a time and place for megazoom lenses. Just sharpen them up in PP.
Super Frequent Flyer, no joke. Ex Patriot and loving it.
Canon Eos 60D, T1i/500D, Eos1, Eos 630, Olympus EPL-1, and a part time Pentax K-X shooter.