I came across this video on Youtube with a very interesting comparison between the Tamron 150-600 f/5-f/6.3 with other lenses such as Canon EF 100-400mm f/4.5-5.6 L & 400mm f/5.6 prime.  I have both the Canon lenses but I find myself using the zoom lens a lot more since I need to go wide to locate the bird in the sky or a tree, then zoom in on him.  The prime lens doesn’t allow for that.  I really have to admire the physical strength of the photographer in this video, that Tamron weighs 1951 g (4.30 lb)!  By comparison, the Canon 100-400 zoom weighs 1380 g (3.04 lb) and it’s already hard enough for me to hold more than a few seconds!  More specs on this lens can be found in DPreview.