400mm f/5.6L, the oldest lens still in the current lineup. Lightweight, well-balanced, fast AF, sharp, affordable - the perfect birding lens for beginners and those on a budget. ^ Yupp, I agree exactly with what she said!
How on earth can a telephoto lens without IS be described as a "perfect birding lens for beginners"? I would have missed 70-80% of my best photos of birds without IS. The lens' former greatest proponent Arthur Morris has long discarded it because lenses because it doesn't have IS.
I am not saying the lens is no good - it is excellent for birds in flight, and it is fine on a tripod. But, there are just so many opportunities, especially with small birds, where you have to be able to take shots at a 1/100 to 1/400s hand holding. The lens without IS is not "perfect" but limited in its usefulness and that is why we want a new one with 4 stops of IS.
I think of myself as a beginner. I have a 600d and the 400 5.6 is the only L-lense I own. Prior to the 600d I had a point and shoot. I bought the 400 it as a step up from the 55-250. Since having the lens I have switched to always shooting in manual, and I have a much better understanding processing because I have to pay more attention to iso. And I'd say that once the first few months were out of the way I don't miss much. OK maybe if a bird of prey is diving at speed or I'm in a rainforest then I don't expect too much. But for a beginner birder it's an awesome lens, not least because it's available second hand in terrific condition for so little money (I paid about the same for a mint condition 400 as I did for a new Sigma 35 1.4).
I guess it might depend on where you live. In and around where I am there's plenty of light and many of the small birds are curious, and it's quite possible to get reasonably close.
I'm not saying that it wouldn't be better with IS (there are times, for sure), but I would recommend it to anyone else starting out trying to photograph birds.