I am in Ecuador right now and this lens just proved to be absolutely best in terms of versatility, no matter what kind of birds you shoot. Absolutely love it. Yes, it's heavy, but the F4 is very much needed for bird photography early mornings and late afternoons.
I have this lens and love it. For outdoor sports it's a very good range. I had a 400mm f2.8 IS before this and thought the 2.8 was nice, but this is so much more versatile.
