There are several routes you can take here. First, if nature is a priority I can suggest a Sigma 17-70/2.8-4.5 and a Canon 100-400 OR 400/5.6. The range of the
Sigma is outstanding and its ability to do 1:2 'macro' is very handy,
the only downside is its variable aperture. Second is the EF-S 17-55 and a 70-300IS--best overall image quality but you lose 100mm's on the long end which is crutial for wildlife. Last is the Tamron 17-50/2.8 and a Canon 100-400 OR 400/5.6 which is the best of both worlds pertaining to the subject matter you are interested in. Hope that helps.


-Matt