I think the 3541 is way overkill for a 50D and a 100-400 for most uses. I have a 3531 and an RRS BH55, and while it's a great tripod combo, it's not something I want to carry in the field unless I am using something 300/2.8 or larger. This combo is rock solid, big, and while it's lighter than an aluminum tripod, it's not really all that light.
For birding, I use a Manfrotto 190CX3 with a Manfrotto fluid video head for my spotting scope. When I want to switch to a camera, I put a mid-size Giottos ballhead on it. (However, I am very interested in downsizing to a smaller, lighter head for photo use. Maybe a BH25?) This combo is fine for my 40D and 300/4 lens (and a 1.4x converter), and it's terrific for the spotting scope. For critical work you'll want to hang a camera bag from the tripod, and use mirror lockup and all that, but that's standard form for almost any tripod.
Now, if you are talking about serious bird photography, with a Wimberly head and a big serious telephoto, then you'll need the bigger tripod. But for a spotting scope and a lightweight 400mm lens, the mid-size 'pods are fine.