While I can't say much on Sigma because I dont owned one, I recommend either the Canon EF100-400mm L IS USM or Canon EF400mm f/5.6 L. If you want flexibility go for zoom 100-400mm because of the zoom, IS and weathersealing, where is the prime 400mm will give you slightly sharper image and better bokeh and cheaper compare to the first. I used tripod for most of my bird photo when I used the prime.
But at the end of the day, which ever lens you use, you must learn to familiar your self with the lens. As for me, since I have both, the lens selection is very much depends when and where either lens will give me a better images.
Attached is sample photo using the Canon EF400mm f/5.6 (1/80, f/10, ISO 640) using Canon EOS 7D.