I'm surprised that no one mentioned the Canon 70-300mm f /4.5-5.6. It is lightweight, costs less than half as much as Canon's 100-400, and (to my eyes, at least) is tack-sharp if you stay below f /11.
One problem with shooting birds these days with the really high-power zooms (300 and up) is that the horizontal format of a 16:9 screen forces you to back off top zoom if you want to get the whole bird, sitting in a sort of upright posture. Thus the practical difference between 300 and 400 mm becomes less important than it used to be shooting for a 4:3 screen.

Steve. F11 !! Oh if only I lived somewhere I could get an aperture like that, in the UK, fast glass in needed oh too often! I had the 100-300 Canon (F4.5-5.6) and while it was very portable the focus ring was very tricky and when you put it side by side with the simga F4...well its obvious what all that extra glass get you.