Its a tough choice as all suggestions have their pros and cons. Sorry that I threw my suggestion in and caused more confusion, but I would definitely consider it if you want do get into nature on a budget, especially if you want to shoot birds.
If you have the money and want quality then Canon lenses all the way, but the trade-off is less flexibilty with a fixed focal length and swapping teleconverters.The Tamron has a long range and great zoom flexibilty, and very good performance for the price. I would a least take the Sigma out of the running and think of the Tamron instead.
All just my opinion of course, other people's suggestions are all valid.
Andy.