We are a Canon household and have been using Sigma lenses. We have the equivalent Sigma lens (
http://www.amazon.com/Sigma-70-300mm...9531982&sr=1-1) and really like it. If all you're going to do it take pictures of birds, the sigma will save you about $100 to put into a few add-ons like a filter to protect the lens, a pouch for the lens, and maybe a harness to wear the camera so it's always within your reach. At the end of the day both lens are very good. The Sigma is 1/2oz lighter. With the Canon you're paying for the name. Both lenses have excellent reviews. My partner has been a professional photographer in the past. She's used Sigma lenses longer than any other brand and has never been unhappy with any of them. Consider saving the money and getting the Sigma.